"What is an earthquake?"
This can be something of a confusing question, because the answer can be either one of two directly related phenomena, and both involve movements in the Earth's crust. An earthquake is the ground motion you experience when a part of the Earth's crust shifts along a surface, a sort of fracture in the crust, known as a fault. This shift produces vibrations in the rocks and soil beneath us. These vibrations are called seismic waves, and if they are large enough, they can be felt, just the same way that you can feel a bell vibrate when it is rung. So when you feel that kind of shaking of the ground, you could say "That was an earthquake!", and you'd be correct.
On the other hand, not all shifting along faults results in seismic waves strong enough for a person to feel. The smallest of shifts often aren't even recorded by sensitive instruments. Yet every shift is fundamentally the same, no matter how small, so a more technical definition of an earthquake is that an earthquake is the sudden slip of part of the Earth's crust, relative to another part, along a fault surface.
Of course, these answers don't begin to explain why, where, when or even how earthquakes occur. Instead, we're left with even more questions.
"What causes sudden slip in the Earth's interior?"
The sudden slip that is an earthquake results from a gradual build-up of stress inside the Earth. Mechanical stress -- not the kind of stress (emotional or mental stress) we often hear about and encounter in the hassles of day-to-day life -- is what seismologists are talking about when they use the word "stress". If you've ever bent and snapped a twig between your fingers, or lost part of a corn chip in a dip or salsa, you've seen how a material responds to an excess of mechanical stress. Basically, the rocks that make up the outer layers of the Earth are no different than the examples of wood or vegetable matter given above. They too, if subjected to sufficient force, can be brought to a "breaking point". It is even easier for them to snap if certain places are already weakened, much the way a sheet of paper will tear more readily along a sharp crease. When the stress in a particular location is great enough to overcome the forces holding together the rocks below us, something, effectively, "breaks" or "gives way", and an earthquake begins.
The forces needed to cause this stress and move such large masses of rock are, as you might imagine, immense. It is quite natural to wonder, then, "What causes these forces?"
The answer can be found in the theory of plate tectonics. This theory -- a radical concept when first suggested which has since gained acceptance as an overwhelming amount of supporting data was gathered by researchers in several fields -- states that the Earth's crust is sectioned into great slabs, known as plates. These plates drift very slowly and steadily with relation to each other as they "float" on the more fluid material (the mantle) beneath them. At their edges, they may be colliding, separating, or moving laterally past each other. The nature of these plate-plate boundaries can have a tremendous effect on the geology and types of volcanic and seismic activity found along the edges (margins) of each plate.
What, exactly, is a fault?"
Since earthquakes are produced by sudden slip along faults, it follows that understanding the nature of faults is a key to understanding the nature of earthquakes. A fault, in the geologic sense, is a roughly planar fracture in the Earth's crust along which slip -- the relative offset of the two sides -- has occurred. Faults can be active, meaning that they currently hold the potential for producing earthquakes, or inactive, meaning that although they once slipped and produced earthquakes, they are now "frozen" solid. If the tectonic environment of an area changes, however, inactive faults can sometimes be reactivated.
In terms of size, faults can be anywhere from less than a meter to over a thousand kilometers in length, with a width of a similar scale. The depth of very large faults is constrained by the thickness of that portion of the Earth's crust and lithosphere in which brittle fracture can occur. In southern California, this depth is roughly 15 to 25 kilometers. The kind of faults seismologists study are generally at least a square kilometer in area, and typically more than a hundred square kilometers in area. Faults of this size or greater can "break", or rupture, violently enough to produce significant earthquakes. There are approximately 200 faults in southern California that are considered major faults, capable of producing damaging earthquakes. Smaller faults -- of which there are countless thousands, even millions, in southern California -- will only produce very minor tremors. Large faults can also produce minor earthquakes, if they rupture only in part, and not along their entire length.
This is one of the most basic connections between faults and the earthquakes they generate. With few exceptions, the size of the fault rupture area is directly proportional to the size of the earthquake produced by the slip along that area. In other words, the greater the fault area that slips, the greater the earthquake produced. Keep in mind, though, that the actual rupture area of an earthquake is not always equal to the total surface area of the fault that ruptures -- often, only a small fraction of that total (potential) area actually slips.
"In what manner does slip along faults occur?"
It has been put forward here that earthquakes occur as sudden slip along a fault, but what does that mean? In what manner does slip occur along faults? Contrary to many preconceived notions, and to the block models employed for simplicity, the entire length of a fault does not experience slip simultaneously; instead, slip begins at a particular point on a fault and then propagates, or travels, along the surface of the fault.
Because of the scale involved, demonstrating the correct method of slip, or an accurate analogy, is not simple. We will look at some common examples in the activity below, and determine which analogies are the most accurate, and why.
Hypocenters vs. Epicenters
A point can be used to locate the hypocenter of an earthquake (also known as the focus), which is, in fact, a true point -- that point at which the slip of an earthquake begins. Hypocenters are therefore always located at some depth underground. For general purposes, it is more common to refer to the location of an earthquake by plotting its epicenter on a map. An epicenter has no physical meaning; it is simply the point on the Earth's surface directly above the hypocenter. Because of this standard way of mapping the location of an earthquake, large earthquakes are often named after the city, town, or geographic location nearest the epicenter.
The difference between epicenters and hypocenters explains why the epicenter of an earthquake which occurs on a particular mapped fault may not be plotted on, or even very near, the surface trace ("fault line"). A hypocenter located at substantial depth on a fault not oriented vertically with respect to the Earth's surface would not appear along the trace of that fault, but at some distance away from the trace. Similarly, in later sections, when we review the effects of earthquakes, we will show that sometimes the strongest shaking is not along the "fault line".
Hypocenters -- The Beginnings of Ruptures
Though epicenters are a useful tool for visualizing seismicity more easily (by a simple map plot), it is the position of the hypocenter that matters to those studying fault rupture at depth. While the error in the position of a hypocenter of a small earthquake can be larger than the fault area which ruptured to produce that event, for larger earthquakes, we can determine not only the hypocenter, but additional information about the fault area which ruptured at depth. This picture of what happens beneath the earth when a fault ruptures can be assembled by means of careful and detailed observations of an earthquake.
"What causes the rupture to come to a stop?"
The answer to that question is currently being researched. While it may seem a simple question, the factors that control the size of a fault rupture, and thus determine where it stops, are complex and do not easily lend themselves to being reproduced in laboratory experiments. Obtaining the answer would give us great insight into the processes at work in fault ruptures, and might provide more understanding of what leads to the onset of rupture and the earthquakes which result.
Since laboratory experiments fall short of duplicating the natural proces of fault rupture, researchers must use data from actual earthquakes in their studies. Southern California is an excellent place to obtain such data. Not only are earthquakes common here, but the area is under constant surveillance by a large network of seismic monitoring equipment. These instruments are shown on the map at right; each green box represents the location of an instrument collecting data, and the red lines represent the surface traces of major faults. We will study this network more closely in later sections. Surely, though, you can at least appreciate the size and density of it from this map alone.
In addition to the obvious benefit of excellent instrumental coverage, southern California has an advantage for researchers that some parts of the world do not -- every kind of slip can be found here, on at least one of the dozens of significant, active fault zones. The sheer number of active faults in southern California presents researchers with an abundance of opportunities.
The Properties of Faults
To make our study of faults easier, let us first discuss some of the terminology used when describing the basic properties of a fault (see figure).
Below are explanations of the terms found on the diagram above.
Fault
The fault is the actual fracture or zone of fracture in the crust, along which displacement of some sort occurs. Though the fault depicted above appears as a simple and single planar feature, what we call a fault can, in fact, be a complex set of fractures with a very chaotic geometry. If such a set of fractures is large enough, it is often referred to as a fault zone.
The Properties of Faults (continued)
Fault plane
A fault plane is a plane used to represent an actual fault, or a particular segment of a fault. Faults are generally not perfectly flat, smooth planes, so this may not be a true representation of the fault. However, since faults do typically act as planes (even though some, in fact, are so physically complex that trying to draw their structure would be cumbersome and confusing), defining a fault plane is the most convenient way to represent and model a fault.
It is useful to have a reference plane when measuring the characteristics of faults. The standard reference plane is the horizontal. It usually approximates the Earth's surface, but does not vary. Here, the horizontal plane shown is that of sea level, but a horizontal plane of any altitude or depth can be used for reference.
The intersection of a fault plane with the Earth's surface produces what is known as the surface trace of the fault. This intersection is also known as a fault trace, or a fault line, since this is the line drawn to represent a fault on a standard map. The traces of faults are not always obvious at the surface. Some, however, display themselves quite plainly, particularly when the observer knows what to look for.
The trend of a fault trace is the general direction it takes across the Earth's surface. Trend may be used to average out the small, localized bends of a long fault and talk about its overall directionality. This direction is often similar to the strike of a fault (see next page), but the two are fundamentally different, and should not be interchanged.
For a non-vertical fault, this is the part of the Earth's crust above the plane of the fault. Its name originates from mining activities along large, ancient faults which had since been "filled in" with mineral deposits. Miners could hang their lamps from the wall above them, coining the term "hanging wall" for this side of a fault.
The counterpart of the hanging wall, the footwall is the part of the Earth's crust below a fault. As with the hanging wall, the "footwall" was so named by miners, since they would walk on the lower side of a mined-out fault.
Strike
The strike of a fault is the line formed by the intersection of the fault plane with a horizontal plane. The direction of the strike, if noted precisely, should be stated as an angle off of due north, only, since strike is a line (as opposed to the direction of dip, which is a ray). Thus, instead of stating a strike as "50° west of south", it should be "50° east of north". Of course, if precision were not an issue, you could simply say "this is a northeast-striking fault".
Dip
The dip of a fault is given by two measurements: an angle and a direction. You can think of the direction of dip as the direction a marble would roll if placed on a smooth plane exactly parallel to the fault plane. This direction is always perpendicular to the direction of the strike of the fault plane. The angle of the dip is the maximum vertical angle of intersection between the fault plane and a horizontal plane. To state the dip of a fault, both a direction and an angle should be given, though the precision used can vary. For instance, the fault above could be said to dip "at 74° in the direction of 40° east of due south", or you could say it dips "steeply towards the southeast". If a specific directional angle is given, it should always be stated as some angle off of north or south -- in other words, you should say "40° east of south", instead of "50° south of east", even though they represent the same direction. Because the dip of a plane is always perpendicular to its strike, the exact direction of the dip does not need to be given when the strike is precisely defined. The general practice is to simply point out which of the two potential directions is correct, by naming a rough compass direction.Finding Strike and Dip
Measuring the strike and dip of a fault plane at the surface is not always easy, and in the case of a blind fault (explained on the next page), impossible. When faults exist in solid rock, they are well-constrained, because it is far more difficult to break a new fault surface through solid rock than it is to break the pre-existing fault. The material at the surface, however, is rarely solid, bare rock. Typically, soil and loose sediments make up the very top layer of the Earth, and because these materials offer little resistance, there is no need for a fault rupture to break along the exact same surface each time. Measuring the strike and dip of a fault in such material can be misleading. However, if you can find an exposure of the fault in bedrock, you can be pretty sure that the fault you see is indicative of the fault plane's orientation, at least in the immediate area. Geologists rely on these sorts of exposures to measure the strike and dip of a fault along its surface trace
Good exposures of faults at the surface can be few and far between. Confirming the strike and dip of a fault at depth, where it is more likely to be consistent, is a valuable thing. Wells drilled into the ground can provide a source of information about the structures and types of rock beneath the surface, but they are limited in depth. Seismic reflection surveys may also yield clues about structures at depth, but these studies are costly, limited in scope, and do not always provide excellent results.
Another method for finding the strike and dip of a fault beneath the surface involves accurately determining the locations of the hypocenters of naturally occurring earthquakes, and plotting these locations in a cross-sectional or 3-dimensional view. If done correctly with good data, this can allow the "imaging" of fault surfaces deep underground.
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The Properties of Fault Slip
In addition to the properties and characteristics outlined above, a fault also has two other primary traits -- its sense of slip, and its slip rate.
Sense of slip
A fault's sense of slip is defined as the relative motion of the rock on each side of the fault with respect to the other side. Fault slip can be classified in a basic way by its relation to the horizontal. If slip occurs primarily in a vertical sense, it is known as dip slip, since it roughly parallels the dip of the fault. If slip occurs primarily in a horizontal sense, it is known as strike slip, since it roughly parallels the strike of the fault. When slip occurs exactly parallel to the dip, or to the strike, it is known as pure dip, or strike, slip. Fault slip which occurs at a sizable angle with respect to both the dip and the strike of the fault is known as oblique slip, and can be thought of as a combination of both dip slip and strike slip.
More specifically, each of these classes of faults can be further divided and labelled by its directional sense of slip. Dip slip is divided into two main types, or senses: normal andreverse. To distinguish between the two, you must know two pieces of information. First, you need to know how each side of the fault is moving with respect to the other. Second, you need to know in which direction the fault dips. The first two diagrams below illustrate the two basic categories of dip-slip faults. When the hanging wall of a fault is moving down with respect to the footwall, this is called normal faulting. When the footwall is moving down with respect to the hanging wall, this is called reverse faulting. To view these animations in motion, click on each image.
If you were very observant when watching the animations above, you may have noticed something interesting about these blocks: though the volume of material in each block remains the same as the faults slip, the area of the upper surface of each block changes! (Imagine looking down at the blocks from overhead to fully appreciate this change.) This is a crucial observation to make in understanding the importance of dip slip faults, and we will take a closer look at this process on the next page.
Dip Slip and Crustal Shortening
Wherever tectonic forces act to squeeze together or pull apart an area of the Earth's crust, you will find dip-slip faults. That's because these faults not only produce obvious vertical offsets, but horizontal offsets as well. They allow a volume of crust to thicken or thin (in depth), in exchange for an expansion or compression of the ground-surface area (area as measured on a map of the region). Thus, where the total ground-surface area of a section of the Earth's crust is under stress to change, dip slip faults are bound to form. The activity below will help you visualize the way these faults accommodate such changes in the crust.
The activity above should have shown you that normal faults thin and extend the Earth's crust, while reverse faults facilitate the crustal shortening and thickening. Although all reverse faults will cause some shortening of the crust, those with a dip of roughly 45 degrees or less are generally indicators of strong compression and a significant crustal shortening. Such faults are thus considered to be a special class of reverse faults, and are called thrust faults. The sense of slip of a thrust fault is still the same as it is for a reverse fault: hanging wall up, footwall down. However, you will note from the animation below that the degree of shortening resulting from a thrust fault is considerably greater than from the more steeply-dipping reverse fault shown on the previous page.
Also shown below, at right, is a diagram of a blind thrust fault. Click on the image to see an animation of the way the fault cuts the lower layer of rock, but merely folds the upper layer, producing not a break, but rather a rounded scarp.
Strike Slip and Oblique Slip
Strike slip is divided into two lateral senses: left-lateral and right-lateral. To understand the difference between these two types of slip, imagine standing on one side of a fault trace, looking across the fault at theother side. Now, imagine the fault breaking, and your view (assuming you can keep your footing in the shaking) shifting due to the movement along the fault. Which way did the things (trees, fences, etc.) on the other side of the fault move? Was it toward your left or your right? If it was toward your left, then the fault experiencedleft-lateral strike slip. If toward the right, that was right-lateral strike slip. The distinction is shown by the two pictures below, as well. Clicking on each image allows you to watch an animation of that sense of slip.
Small half-arrows are often used on maps and diagrams to designate the lateral sense of strike-slip faults. The figure at right shows how this works; essentially, the arrow on each side of the fault points in the direction of that side's relative motion. Similar arrows are often used on cross-sections and cut-away diagrams to indicate the vertical sense of dip-slip faults.
If a fault is oblique in slip -- neither slip component dominates the other -- then the slip of the fault is referred to with a combination of slip terms. The sense of strike slip is used first, followed by the sense of dip slip. For example, if the slip on a fault were such that the hanging wall moved up with respect to the footwall, and the two sides slipped laterally right with respect to each other, you would call this sense of slip "right-lateral reverse", which is sometimes shortened to "right-reverse".
Slip Rate Defined
Slip rate
The slip rate of a fault is the speed with which one side of the fault moves with respect to the other. Since tectonic plates move very slowly, these speeds are measured in units quite different from those we usually associate with measured speeds (for instance, highway traffic moving at kilometers per hour). Slip rates are generally measured in millimeters per year (mm/yr) -- in California, slip rates for faults range from 0 to about 38 mm/yr, though anything over 10 mm/yr is generally considered fast (a slip rate around 1 to 2 mm/yr might be considered average for a major, active fault).
A key fact about slip rates to keep in mind is that they do not represent constant motion along a fault, even though the motions of tectonic plates are constant. Instead, slip rates are averages of the total slip along a fault over a long period of time. Thus, a rate of 2 mm/yr does not mean that two points, one on each side of the fault, which are adjacent to begin with will be exactly two millimeters apart one year later. Instead, it might mean that the two points will remain adjacent for hundreds or thousands of years, then suddenly slide away from each other in a single episode of slip, typically consisting of several centimeters or even meters of offset.
If, as we said, the slip rate of the fault is 2 mm/yr, then the time it takes to move the two points away from each other, divided into the amount of distance between the two points at the end of that time, should yield a rate of roughly 2 mm/yr. This simply follows the formula for finding any average speed:
Figuring out the rate of slip along faults is a key in understanding the relative "importance" of faults in an area, and the hazard those faults present to local residents and developments. Slip rates can also tell us about changes in faults that have happened long ago -- well before recorded history.
There are several ways to determine the slip rate of a fault, and in the following pages, we will investigate how these methods work, and how different methods can produce different results.
Calculating Slip Rates and Recurrence Intervals
One method for finding the slip rate of a fault involves determining the recurrence interval of that fault. A recurrence interval for a fault is the average time between ruptures of a particular size along that fault. Most often, the term is used to denote the repeat time of major events, which are usually defined as those which rupture much or all of the length of the fault, and typically resulting in appreciable surface rupture (if the fault is not a blind fault).
Knowing the recurrence interval between major ruptures along a fault and the average slip of such ruptures allows you to make an estimate of the slip rate of that fault. While the repeat time may vary significantly, an average value can generally be determined if enough geologic evidence exists along the fault trace to date past ruptures. This value can then be used with either a mathematical calculation of the amount of slip a whole-fault rupture would produce, or an estimate of a typical rupture displacement taken from field observations of recent surface ruptures.
These values are then inserted into the equation below to arrive at a slip rate. (Make sure your units are all in agreement!)
For example, if you know from geological studies that the recurrence interval on the (hypothetical) Desert View Fault is 1000 years, and that the average slip in each major rupture (as defined above) is 3.0 meters, then you can estimate a slip rate for the Desert View Fault of 3000 mm divided by 1000 years, or 3.0 mm/yr.
To calculate the slip rate in this manner, you need to know how to arrive at a recurrence interval. For geologists, determining the recurrence interval of a fault can require a lot of field work, and often includes the digging of a trench across the fault's surface trace. In your case, the activity below will let you try out several different approaches to finding recurrence intervals -- without even getting your hands dirty -- including one example using actual data from a real-world fault.
Recurrence Interval| Recurrence intervals sound useful, but how can you determine them? |
While recurrence intervals can be used to calculate slip rates, so too can other methods. Using more than one method on a particular fault allows us to check the accuracy of our findings. Let's study some of these other methods, now, and then compare their results.
A more common and more direct way to estimate the slip rate of a fault is to find a feature, the age of which can be determined, that has been offset by the fault being studied. A stream channel, which may contain plant material that can be radiocarbon-dated, is an excellent example. By dividing the offset distance of the two halves of the channel by the estimated time since the channel was first created (before it was cut by the fault), you can derive a slip rate for the fault.
Assume our hypothetical Desert View Fault cuts a stream channel, as shown in the figure at right. The oldest deposits associated with the channel are dated at 10,000 years old. The stream channel has a pure left-lateral offset of roughly 28 meters. This yields a value of 2.8 mm/yr for the slip rate of the Desert View Fault.
Note that we originally estimated the slip rate as 3.0 mm/yr. Which is correct? Possibly neither, but we now have a good idea that the true slip rate, whatever it may be, lies within the range of 2.8 to 3.0 mm/yr, possibly a little more or a little less. Real studies of slip rates always include uncertainties, denoted by a "plus-or-minus" (+) value after a mean, average, or otherwise preferred value. Here, we might say the slip rate is "2.9 + 0.1 mm/yr". The uncertainty would be greater if our original measurements were not absolutely precise.
GPS Measurements
Yet another way to estimate the relative motion of the two sides of a fault, or to study more generally the deformation across a larger area of faulting, is to make extremely accurate measurements of the positions of at least two points on Earth's surface using the network of 24 satellites which make up the Global Positioning System. This system, commonly referred to as "GPS", uses satellites orbiting high above the Earth in extremely well-known, continually-monitored orbits. The satellites can obtain a precise fix on a particular location on the Earth's surface. Through a combination of state-of-the-art technology and clever mathematical processing, this system allows measurements of position accurate to within a centimeter or less.
This method of obtaining measurements allows us to check the accuracy and completeness of slip rate studies in an area. Although individual faults do not move constantly, the tectonic plates of the Earth's lithosphere do move all the time. This motion manifests itself as strain in the crust. Strain is simply the deformation of a material. If you pull a lump of clay apart, for example, it will stretch somewhat before it snaps into two pieces. A similar phenomenon is at work in the Earth's crust. The "breaks" occur in the form of rupture along faults, creating earthquakes we can feel, but the stretching, "squishing", or warping that occur in elastic materials are present only on a scale well beyond the limit of human perception. People would never notice them without sensitive monitoring instruments like the GPS system.
Thus, an array of instruments across an area spanning an active fault or faults will "see" this deformation as an ongoing process. Holding one station (preferably a fair distance away from any active fault) "fixed" allows an analyst to determine the relative motion of the various stations with regard to that reference point. This, in turn, can provide valuable information about the slip rates of faults in the area. Say, for instance, a particular station is determined to be moving, horizontally, at about 3 mm/yr directly away from a "fixed" station. In between the two stations is only one major fault, running perpendicular to a line connecting the two. In this case, that fault must be a normal fault, and its horizontal component of slip rate should be roughly 3 mm/yr.
In the activity below, you will work through examples of determining slip rates with different methods. When discrepancies appear between the methods, you will decide how best to resolve them. Hypothetical GPS data will be included in three of these examples as a reference guide to show you the value of similar data in real seismological studies.
Different Data, Different Rates?| What methods can we use to find slip rates, and what does it mean when they disagree? |
We've now reviewed most of the properties of active faults, and how they are measured, but we have yet to study how and why they form, and the factors that might guide that formation, giving each fault its own particular characteristics. Let's move on, and investigate these matters.
"How, why and where do faults form?"
Now that we have reviewed plate tectonics and the properties of faults, it's time to put the two together to explain how, why, and where faults form.
Our knowledge of the mechanism of fault formation is still primarily limited to theory and extrapolation. We can create conditions in laboratory experiments to simulate the formation of fractures and faults, but we cannot actually watch faults forming or even lengthening beneath the Earth's surface. Still, the principles of fracturing and fracture propagation are understood well enough to convince us that faulting can be described by applying the basic laws of stress dynamics to the Earth's crust. What insight we do have about fault formation in the real world comes primarily from studying seismicity. New fracturing of the crust produces earthquakes in much the same way that earthquakes are generated by movement along previous fractures. From these movements, we can learn about fault formation indirectly.
Understanding why and where faults form, at least in the context of southern California, requires a simple understanding of plate tectonics. While the models we will be using below are greatly simplified, the general ideas hold true for real-world faulting, as we will show later.
Faults can occur in any orientation and at any angle to horizontal (remember that this angle is known as the dip of the fault), but an active fault's orientation and dip are not necessarily random. These properties are influenced by the regional stress field created by tectonic activity.
This happens because slip along a plane occurs much more easily and efficiently at some angles to the forces being applied than it does at others. For example, if you sit still on a horizontal surface, you will not slip off. However, if someone begins tilting the surface you're sitting on, you will eventually slide off, because the pull of gravity will finally overcome the frictional force keeping you in place.
This is similar to what happens along faults when earthquakes occur. A driving force eventually overcomes a resistant force, resulting in slip. If the surface you sat upon was incredibly sticky -- that is, if the coefficient of friction and the resulting frictional force were large enough -- you might not begin to slip until the surface was tilted almost to vertical (90°). There would be a very narrow range in which the "tectonic stress" (gravity) could be released as slip (you, sliding off the surface). Only a "fault" (the interface between you and the surface) within a certain narrow range of orientations would be efficient at releasing the stress caused by the opposition of the driving and resistant forces, so only such a fault would "form" (experience slip). If your friends were sitting on similar surfaces with lesser tilt angles, they would remain stuck, because their "faults" would not be oriented in a way that encourages slip.
Breaking the Crust
Because tectonic forces are powerful and constant, the rocks subjected to them will tend to break in a way that alleviates the stress they impart to the rocks. This is accomplished by fracturing and slipping in a manner that accommodates the push or pull of the greatest local stress.
For example, let's imagine you are in control of a large, square block of the Earth's crust. If you subject that block to stress by pushing on it from the west edge and the east edge, do you think that a north-south, vertical fracture will be likely to form? No, because such a break would not accomodate movement from the force you're applying to the block.
The activity below provides the opportunity to investigate how tectonic forces apply to faulting. You will have the chance to work through more examples like the one above, but with the help of fault-block figures.
How Tectonic Forces Affect Faults| Determine a fault's slip by knowing the tectonic environment around it -- and vice versa. |
The examples in this activity have hopefully shown you something about the "rules" that govern fault formation and motion. Still, you may wonder, wisely, how these simple rules apply to real-world situations, like that of the plate boundary in southern California. After all, the examples in the activity only used systems with one, two, or three faults -- not the several hundred major faults (not to mention the countless smaller ones) you find in southern California. Why are all those faults needed to accommodate the motion of the plates? After all, if the plate boundary here is a simple transform fault boundary, which plates sliding laterally past each other, wouldn't a single large fault would suffice in moving the two plates "smoothly" past each other? If so, then why are there so many faults here?
"Why are there so many faults in southern California?"
The answer to this is related directly to plate tectonics. California is, as was previously mentioned, home to the boundary between the North American Plate and the Pacific Plate. However, the presence of that boundary alone does not explain the complexity of faults in southern California. Studying a fault map of the area around the plate boundary (the San Andreas fault) in central and northern California shows a different picture. While there are multiple active faults in this area, almost all are roughly parallel and moving with the same type of slip: right-lateral strike slip. Since the plate boundary here is a transform fault, and the plates are moving right-laterally with respect to each other, this is not surprising.
However, if you examine a fault map of southern California, the area near the plate boundary is cut by a great number of faults in many different orientations. A large percentage of these faults are not right-lateral strike-slip faults. In fact, every sense of slip, pure and oblique, can be found on at least one significant fault in southern California. Why is this?
"Why are there different fault motions in this area?"
This and the previous question have basically the same answer. In northern and central California, the plates slide reasonably smoothly past each other because the alignment of the plate boundary (the San Andreas fault) is essentially parallel to the relative motion of the plates. In southern California, the plate boundary
is not so simply oriented; there is a bend in the San Andreas fault, often referred to as the "Big Bend". There is even some uncertainty as to the exact nature and location of the plate boundary in the southern half of this bend. Even farther to the south, the plate boundary is no longer represented by the San Andreas fault, or any single transform fault. Instead, it appears as a series of rift zones connected by transform faults, similar to a mid-ocean ridge, located in the Gulf of California.
Ignoring the complexity in the bend and to the south, we can consider the plate boundary in southern California to be a fairly simple right-lateral strike-slip vertical fault with a "kink", as shown at left. The right-lateral motion of the plates causes the two sides of the bend to push against each other, preventing easy sliding, so this particular bend is known as a "restraining" or "convergent" bend. The result of this compression is the uplift of the Transverse Ranges by the action of reverse and thrust faults. If the plate motion were the opposite, and the slip along the San Andreas fault became left-lateral in nature, the existing bend would cause extension in the area near the bend and this bend would be called a "releasing" or "divergent" bend (bottom left). This action would likely cause a basin to form around the bend.
Were the bend oriented the other way (in a north-south direction rather than an east-west orientation) with the existing right-lateral plate motion, as shown at upper right, this bend would become a divergent bend, and a basin would form around the fault, as in the example above. If we took the hypothetical new bend and reversed the plate motion so that things moved left-laterally, this bend would become a convergent bend (bottom right). The situation would then resemble what is currently going on in southern California -- compressional forces causing the uplift of mountains.
Any smaller bend works in the same way, but of course, on a scale to match that of the bend and the fault involved. Convergent bends will always cause compression, as divergent bends will always result in extension. Fault bends can be referred to as "left bends" or "right bends" depending on their configuration. The Big Bend of the San Andreas is a "left bend" -- if you were to walk along the fault, starting on a "straight" section, you would have to veer left when you came to the bend, regardless of the direction of your approach. The hypothetical bend shown in the figures above is a "right bend". By comparing the slip of a fault to a bend along its length, you can quickly tell if it is a convergent or divergent bend. If the sense of strike-slip and the bend have the opposite "handedness" -- that is, left-lateral strike-slip with a right bend, or right-lateral strike-slip with a left bend -- the bend will be convergent. If the handedness is the same, the bend will be divergent.
Fault Steps
Fault steps are features similar, in many ways, to fault bends. Just as fault bends are either left bends or right bends, fault steps are either left steps or right steps. They generally create local compression or extension much like fault bends. However (as shown in the image to the right), in an area with a fault step, the main trace of the fault does not change trend. It merely "steps" over, sideways, and continues along a similar trend. For this reason, fault steps are typically small-scale features, and their effects are limited to the immediate area near the step-over. No fault step even comes close to causing the sort of effects the "Big Bend" of the San Andreas does.
This does not mean that fault steps are not important, nor does it mean they are not the subject of study and debate. Fault steps create landforms which can be used to study past movement along the fault zone, as shown at left, along a hypothetical right-lateral strike-slip fault. While convergent steps create compression and uplift, more geologists find value in divergent steps. These fault steps often create small "pull-apart" basins and sag ponds in which organic material, which can be radiocarbon-dated, piles up. When a major rupture happens, these sediments can be cut and offset, then overlain by more sediment, until the next rupture cuts those. In this manner, a chronological record of major ruptures can be preserved.
Fault steps also affect the progression of major ruptures. Some ruptures progress through fault step-overs; others are stopped. Knowing more about the ease with which fault rupture can "jump over" fault steps would help assess more accurately the risk of major ruptures along certain fault zones. Unfortunately, determining which fault steps allow and which prevent continuous rupture is still a matter of guesswork. With further studies into the nature of fault steps, we may one day have a better understanding of the role they play in rupture progression.
The `Big Bend' and the Plate Boundary
As noted before, the "Big Bend" of the San Andreas fault is responsible for much of the complexity of faulting in southern California. This bend is a convergent (restraining) bend, creating a localized collision of tectonic plates, and a tremendous amount of compressional stress. To release this stress, additional faults have formed over time. A typical response to large-scale compression is crustal shortening. This allows compression to continue by "squeezing" up the rocks in the compressional zone. This is accomplished by thrust faults -- low-angle reverse faults that drive sections of crust over one another to create a thicker pile of crust with a shorter (horizontal) length. The surface traces of such faults are shown in pale yellow on the map view at right. The 1994 Northridge earthquake (magnitude 6.7) occurred on one of these numerous thrust faults.
Not all the compressional force generated by the "Big Bend" of the San Andreas fault goes into thrust faults. The collision boundary is not square with the plate motion, but at an angle, in such a way that some of the material "caught in the middle" has a chance to move laterally out of the way. This is exactly what happens. Large zones of left-lateral faulting, shown here in green, have formed in an effort to relieve some of the stress created by the fault bend. The most obvious example of this left-lateral faulting is the Garlock fault zone, which intersects with the San Andreas near the northern end of the "Big Bend" and continues eastward for several hundred kilometers.
In addition, several right-lateral strike-slip faults south of the Big Bend, and west of the southern San Andreas fault zone, seem to be managing some of the overall slip between the two tectonic plates. These fault zones, shown here in orange, are quite lengthy and roughly parallel the plate boundary.
The Plate Boundary and the Faults of Southern California
Some of the stress and slip generated by the plate boundary also seems to be shunted off the main plate boundary and onto the Eastern California Shear Zone (ECSZ), shown in yellowish-orange on the map of fault traces at right. This broad area of right-lateral shearing motion branches off from the San Andreas fault zone near Indio, just south of the epicenter of the 1992 Landers earthquake (magnitude 7.3). The ECSZ continues in a north-northwest direction through the Mojave, past Barstow, and on into Owens Valley, crossing the Garlock fault as it goes. Seismicity continues along this trend well into Nevada.
Also contributing to fault activity in this area is the Basin and Range tectonic province to the east of this zone. This region of extensional faulting, shown in light blue, is most pronounced to the north of where the ECSZ crosses the Garlock fault. Unlike the features mentioned above, it may have no direct connection with the Big Bend of the San Andreas fault, though it does most likely originate from the interaction of the North American Plate with the Pacific Plate.
Partitioning Slip"What's so important about the distribution of earthquakes?"
In the first section of this module, we investigated the principles of earthquakes and their occurrence. We studied the properties of faults, the influence of tectonics, and the dynamics of fault rupture. But we stopped short of looking at the occurrence of real-world earthquakes with respect to time and location, and we have yet to investigate whether earthquakes occur in any patterns or are influenced by causal relationships with other phenomena, even other earthquakes. In short, we have covered how earthquakes occur, and a little bit of whythey occur, but we have yet to observe where and when they occur. Those questions are the focus of this section, and our case study will be the regional seismicity of southern California.
"In what parts of southern California are earthquakes most common?"
Given the popular associations between California and earthquakes, this question might actually sound funny to some people. Earthquakes and this region of the United States are so strongly associated for most people that asking where in California earthquakes are most common might sound as absurd as asking which part of the ocean is the wettest. And though it is true that any part of California has a higher risk of strong shaking from an earthquake than the highest such risk you can possibly find in the state of Wisconsin, or Florida, or any one of many other states, it is still useful and interesting to look into variations in both seismic activity and hazard that might exist within this area of active tectonics.
The question at the top of this page assumes that there are variations, with respect to location, in southern California's earthquake activity. Though it is not specifically stated, it also implies that these differences are long-term, and not merely related to the changes brought on by a large aftershock sequence, such as that of the 1992 Landers earthquake. Based on what you have already studied, you may be inclined to agree with these assumptions. But do the records support them, or not?
Do earthquakes happen everywhere in southern California?"
Can an earthquake happen -- that is, can a hypocenter be located -- directly beneath your feet, no matter where you choose to stand in southern California (providing you stand there for a long enough period of time)? And if the answer to that is yes, then is there a smaller chance of it happening in one particular place, compared to another? Such variations in probability can make a big difference when it comes to determining the earthquake risk for a specific area of the state. Let's study the distribution of recorded earthquakes in southern California, to see if we can find and describe any such variations.
Where Do Earthquakes Happen?| Investigate the distribution of earthquakes in southern California. |
The activity above looked at the epicenters of all recorded earthquakes. It's possible that the picture we see might look very different if we limited the focus of our investigation. For example, most people consider only damaging earthquakes to be of any interest, yet in Activity #1, we made no distinction between the magnitudes of the events plotted. If our plot focused only on the larger events, would this make a difference? Is the distribution of the largest earthquakes in southern California somehow different?
"Where do large earthquakes occur in southern California?"
Generally speaking, the larger the magnitude of an earthquake, the greater the chance there is that it will cause damage to structures of injury to people. Naturally, then, the largest earthquakes are of greatest importance to the average person in southern California.
We have noted in previous sections that the rupture area of a fault involved in an earthquake is proportional to the energy released, and thus the magnitude, of that earthquake. This means that large fault surfaces must rupture to produce large earthquakes. Presumably, then, large earthquakes will only occur where large (and thus, probably known and well-mapped) faults exist. Does the earthquake record in southern California support this assumption?
To try and answer these questions, let's look at the activity below.
Where Do Large Earthquakes Occur?| Is the distribution of large earthquakes the same as that of smaller ones? |
Having worked through Activity #2, you may still feel a little unsure that there is really a solid connection between major faults, and large earthquakes, despite your previous learning. This would be understandable; several major earthquakes on the Los Angeles basin map did not seem connected to any major fault. Is there an explanation for them?
"What about the large earthquakes that did not appear connected with any major fault?"
In Activity #2, you saw that large earthquakes are generally associated with major faults, according to the maps you looked at. On the other hand, there may have been enough exceptions to this that you feel unconvinced. Indeed, there were a few flaws associated with this activity which may have been somewhat misleading. How can these "errant" quakes be explained?
The first and more obvious explanation is that not all the active faults capable of producing an earthquake of magnitude 5 were shown on the maps. While most of the largest faults were represented, a few areas may have incorrectly seemed to be lacking active faults.
Also, some of the earthquakes shown occurred deep beneath the surface on thrust or "blind" (buried) thrust faults. These epicenters, as we saw in previous sections, will not plot along the fault trace because of the dip of the fault involved, or in the case of a blind thrust fault, simply because there is no fault trace. These exceptions should have been especially notable in the Los Angeles basin image, where thrust faulting is commonplace. Four of the five highlighted earthquakes on that map occurred along thrust faults, as did many of the smaller earthquakes. Did you note how spread out the aftershocks of the Northridge earthquake were, compared to the more linear distribution of the Long Beach earthquake's aftershocks? These differences are directly related to the faults involved in each earthquake -- a steeply-dipping fault, like that involved in the Long Beach earthquake, looks linear in map view, while a fault with a shallow dip, like the thrust fault responsible for the Northridge earthquake, presents a more open "face" when viewed from above.
If you knew previously that the Los Angeles basin is an area with numerous thrust and blind thrust faults, you may have felt that the pattern of epicenters you saw was not entirely unexpected. Even without that knowledge, however, there were a few clues on the map itself that may have tipped you off to the exceptions above. Did you notice them? Read on to find out what they were.
Topographic Clues
One set of clues that you may have considered using to see through the apparent problems with the activity above is the topography of the Los Angeles basin -- the mountains, hills, and valleys present in the area. Low-angle faults (including some blind faults) can alter the surface, creating plateaus and hills by gradually uplifting a region. When such an uplifted area can be found prominently on one side of a fault, while the other side is low-lying and basically flat, there is a fair probability that the fault has a non-vertical dip, and so epicenters positioned off the fault trace are quite possible. Also, a belt of hills with no associated fault trace is an excellent signal that there may be a blind fault at work beneath those hills. You may wish to go back to the Los Angeles basin map and review the distribution of the earthquakes with these possibilities in mind.
With all these warning signs, you might begin to wonder whether the topography of an area can act as an indicator of earthquake risk, even if or especially when the faults in the area are not well-studied.
"Can topographic features be used as a guide in earthquake risk assessment?"
Yes, topographic features can be used as a tool in identifying the risk of active faulting in an area. When scientists and other professionals are making these sorts of assessments, they tend to concentrate most upon landforms near a potentially active fault trace or on the sub-surface structural geology of an area. Even without specialized training, however, it is possible to formulate some simple hypotheses based on a more generalized look at an area.
The activity below gives you a chance to do just that.
Does Topography Signal Earthquake Potential?| Is there a connection between topography and the distribution of earthquakes? |
This activity has shown, as your previous studies should have lead you to assume, that topographic changes and earthquakes are often related. Yet some of the major topographic changes on this map were relatively free of earthquakes, and other areas with a great deal of earthquake activity were essentially free of topography. How can these be explained, and does their existence weaken our assumption?
"Does a change in topography always imply earthquake activity?"
No, a significant change in topography does not necessarily imply that an area will be seismically active. However, with few exceptions, it does mean that the area was once subjected to processes that would have produced earthquakes in some way. When these processes become inactive, the topography they built stays behind, gradually eroding back down (or in the case of a basin, filling up). This explains why some prominent topographic features are lacking in earthquake activity at present.
What explains those areas where major earthquake activity occurs, but no notable topography exists? You may have already guessed the more obvious explanation: pure strike-slip faulting. If no vertical motion of the surrounding rocks is created through either compression or extension (i.e. by pure strike-slip movement), then it is possible that even a large fault will produce very little in the way of topographic changes. This rarely happens for the length of an entire fault, since few fault zones have absolutely no bends or step-overs -- both of which produce noticable topographic features -- or strike across perfectly flat topography.
The more common explanation for high seismicity in an area of little to no topography is the burial or partial burial of faults by sediment. This can be seen in several places in southern California. Basins and valleys, sometimes formed by faulting, may have active strike-slip faults running through them, in the rocks of the valley floor. However, the flow of dirt and debris into these basins is sometimes so large that it completely overwhelms the minor topographic changes created by the fault's motion. Despite the activity that may be going on deep below the valley fill, we see nothing at the surface. A good analogy to this is the topography beneath the ocean, much of which, off the coast of southern California at least, is created by faulting. Only in a few places do these topographic changes stick up above the surface of the water, but that doesn't mean that the ocean bottom everywhere else is flat -- it is simply hidden by the material above it.
"Can we `see' features below the surface at all?"
Our investigation into topographical changes brings up another similar issue often overlooked when casually reviewing earthquake records in map view. Maps, as we have learned, display symbols representing the epicenters of earthquakes. But the true starting point of an earthquake is its hypocenter, a point which can be deep below the surface. With the help of modern seismographic networks, we are capable of determining the depths of hypocenters. Our catalogs therefore contain data in three dimensions, which we have used in specific activities, but not in an overall view of this region.
It is true that we are neglecting a lot of data when we study the earthquake distribution of southern California in only a two-dimensional format, but is that extra data worth viewing? Can the depth of earthquake hypocenters "tell" us anything useful about the nature of this region? Might we be able to see the outlines of fault structures at depth? Are there even any major or systematic variations worth noting?
"Are there variations in the depth of seismicity in southern California?"
Earthquake activity in southern California occurs at shallow depths, relatively speaking. This has to do with both the thickness of the crust, and the type of plate boundary found here. Earthquakes only occur in brittle material. This typically limits their depths to that of normal crustal materials, which may extend as far down as 30 to 40 km below the surface. Collisions between two tectonic plates can push crustal material down deeper, however. In the case of a collision between two continental plates, this can result in brittle materials down to about 70 km below the surface. When at least one oceanic plate is involved in a tectonic collision, however, subduction results, and the oceanic plate which is forced down into the mantle can still produce earthquakes as far down as 700 km below the surface!
The plate boundary in California is not a zone of tectonic collision, but is instead a transform fault. Here the plates move laterally past each other. In such an environment as this, we would expect only shallow earthquakes.
But just how shallow is the seismicity? And do the other tectonic features influence the depth? For instance, the Big Bend of the San Andreas fault does cause compression and crustal thickening, but is this enough to significantly change the maximum depth of seismicity in that area?
To investigate the answers to these questions, look into the activity below.
Distribution in Time?
So far in this section, we have been examining the geographic distribution of earthquakes in southern California. We've looked at how the locations of earthquakes correspond to various features of the landscape, even features deep below the surface. But these are not the only forms of distribution we can use to study seismic activity. We can also investigate the distribution of seismicity with respect to time.
In Section 1, we looked at slip rate and recurrence interval, two concepts related to time, and both important for risk assessment and in
 creating a long-term outlook for the tectonic activity in southern California. But while knowing that a fault may experience a major rupture about once every two millennia is interesting, we would also like to be able to recognize patterns on a more human timescale -- that of days, months, or years. To do this, we need to look at the recent distribution of earthquakes with respect to time.
Indeed, the distribution of earthquakes over time, generally referred to as the seismicity rate, can be a powerful tool for understanding the workings of the Earth's crust. Similar to the way engine noises help an experienced mechanic determine what needs repair, seismicity rates can allow a seismologist to distinguish different types of events, and even in some cases, to anticipate them.
To do this requires a consistent way of finding seismicity rates. Only then can one begin to make comparisons. Or, to continue the engine noise analogy, a mechanic needs to know to what sounds "normal" before he or she can say what sounds unusual. But with seismicity -- much more complicated than any engine -- how can we define what is "normal"?
Defining a Seismicity Rate
Defining a seismicity rate requires several things. First off, you must define a region for which you wish to find a rate. That area can be any size you want it to be. You can even assign boundaries in depth, so that you're actually counting the rate of earthquakes within a particular volume. Whatever you choose, the boundaries should be definite, and fixed.
Naturally, to count earthquakes, you need a way to record and locate earthquakes, or access to a reliable source of data (already recorded for you). Working on this module, you will have access to data recorded by the Southern California Seismic Network, and stored at the SCEC Data Center. There are also many other seismic databases around the world that offer similar information to the public. That's good -- few people could afford to have their own seismic network!
For your rate measurement to be effective, there are two more things you will need to decide: (1) the interval, and (2) the duration. Are you going to count the number of earthquakes per hour, day, month, year, or some other period of time? And for how long will you continue your count?
Once you have determined all these parameters and have a source of data to work with, you are ready to begin calculating your seismicity rate. The exercise below will take you on a "tour" of seismicity rates in southern California, and then allow you to make your own seismicity rate calculations using a searchable earthquake database.
In the activity above, you saw how the seismicity rate can go up dramatically after a large earthquake, because of the occurrence of numerous aftershocks. You may have heard this term used before, but do you know, specifically, what kind of earthquake qualifies as an aftershock?
Aftershocks Defined
Aftershocks are actually just normal earthquakes in every physical detail. Out of context, there is no way to tell the difference between an arbitrary earthquake and an "aftershock". The only real difference between the two is that an aftershock follows closely in the wake of a larger earthquake, and in roughly the same location as its predecessor. That larger, initial earthquake is usually referred to as the "mainshock".
More specifically, there are two guidelines for labelling an earthquake as an aftershock. First, it must occur within one rupture length of the mainshock rupture surface, or alternatively, within an "aftershock zone" based upon early aftershock activity and defined by seismologists. Second, it must occur within that designated area before the seismicity rate in that area returns to its "background", meaning pre-mainshock, level.
Both of these limitations have somewhat poorly-defined boundaries. The area in which aftershocks from one mainshock occur might overlap the aftershock zone of another mainshock. Other factors might cause an aftershock zone to change over time, or to extend farther than is generally expected.
In addition, defining the normal, "background" seismicity rate for an area can be difficult. Without good records covering decades of activity in an area, it is tough to say for sure what the typical background rate of earthquakes in an area should be, and whether there is any long-term variation in the rate. That makes it difficult to say when an aftershock sequence has ended, and the rate has returned to normal in the (former) aftershock zone.
Mainshocks Defined
As implied before, a mainshock is simply the largest earthquake in a sequence of earthquakes. It is usually significantly greater in size than the second largest earthquake in the same sequence. In terms of magnitude, a numerical measure of the "size" of earthquakes (often abbreviated as a capital M), a mainshock is usually at least one-half of a whole unit of magnitude larger than its largest aftershock (or foreshock, a term we'll get to shortly). For example, if the mainshock of a sequence is of magnitude 6.5, you would generally not expect to see any aftershocks larger than magnitude 6.0 in that same sequence. Most aftershocks will be much smaller -- several units of magnitude less -- than the mainshock. An exception to this rule can be found in earthquake swarms, sequences of earthquakes that do not have a clearly defined mainshock. Swarms are a special case we will look at much later in this section.
It may help to illustrate the relations, as well as the distinctions, among these different classes of earthquakes (foreshocks, mainshocks, and aftershocks) by studying an example of seismicity in southern California. The chart on the left is a list of all earthquakes in southern California, greater than magnitude 3.0, that occurred on April 22, 1992. The time, magnitude, and location (in latitude and longitude) of these events is given on chart. These three measures are all we need to differentiate foreshocks, mainshocks, and aftershocks.
As you can see, one large earthquake on the list far exceeds the others in magnitude. This is the Joshua Tree earthquake, magnitude 6.1. Note its location. Now, look at all of the events that follow it. All these events are smaller in magnitude, but have roughly the same location as the Joshua Tree earthquake. Thus, we can call these events aftershocks of the M 6.1 Joshua Tree mainshock. Notice how the seismicity rate increases dramatically after the Joshua Tree earthquake -- so much so that this list had to be cut short! This is the same sort of jump in seismicity rate you should have seen in the example of March 1998 from Activity #5: Seismicity Rates.
What's a Foreshock?!
Foreshocks are those earthquakes which occur immediately preceeding a mainshock in the exact same area in which the mainshock occurs. Foreshocks may come in groups, or be single events. The time between the last foreshock and the mainshock varies somewhat, but is typically less than a day.
To see an example of this type of event in the real world, let's look again at the chart of earthquakes for April 22, 1992, but this time, let's pay attention to what happened before the Joshua Tree mainshock.
There were three events above M 3.0 that day before the Joshua Tree earthquake. The first happened on the other side of the state, and is unrelated. But as you can see, the M 6.1 mainshock occurred in exactly the same location as, and about two hours after, the two earthquakes preceding it. These earthquakes can therefore be considered foreshocks of the Joshua Tree earthquake.
There is nothing intrinsically characteristic about foreshocks, mainshocks, and aftershocks -- that is, if you looked at the records (seismograms) of them "out of context", with no clues as to which were followed or preceeded by other events, you could never hope to categorize them correctly. They would all look essentially the same; none would have distinguishing features that would let you know its relation to other events. Even magnitude wouldn't let you distinguish with certainty. Foreshocks and aftershocks of very large events can be larger than a moderate mainshock.
Thus, foreshocks cannot be positively identified as foreshocks until after the mainshock has occurred. Even then, the "mainshock" will sometimes prove to be a large foreshock of an even bigger earthquake, which will then assume the position of the mainshock in that earthquake sequence. An important thing to keep in mind is that while all events above a certain magnitude will have aftershocks, foreshocks do not always occur before large earthquakes. This will be important when we discuss the possibilities of earthquake prediction later in this section. For now, let's return to our investigation of seismicity rates and those things that can affect them.
Seismicity Rates Applied
Now that you have some experience in determining seismicity rates, and know a little of the basic jargon, the next obvious step is to learn how to apply this skill in such a way that you can begin to draw conclusions about the nature of ongoing seismicity in an area. But be careful! As with the task of determining seismicity rates, there are factors here you must also keep in mind, or risk producing ridiculous results and poor conclusions.
In Activity 5, when you compared the rate of southern California seismicity from one month to that of another, that was a simple application of
 seismicity rates. It addressed the question, "Does the seismicity rate vary from month to month?" But what if you wanted to ask the question, "Is the present monthly seismicity rate similar to the rate from 50 years ago?" Could you simply check the earthquake catalog from 50 years past and compare it to today's? No. Any conclusion you made from such a comparison would be questionable, because of a significant difference in your two sets of data. In this example, that difference would be the size of the network recording the data. Fifty years ago, the number of seismic recording instruments in southern California was but a small fraction of those that make up today's seismic network, and so only large or favorably located earthquakes would be recorded by enough stations for seismologists to accurately determine their origins.
There are plenty of other ways to reach poor conclusions when working with data. It is important to be mindful of the limitations of the data set with which you're working. Is it too small, or biased in some way? If you're comparing two different sets, were they collected in essentially identical ways? In our example above, the two sets were not collected identically, and that could have led to an erroneous conclusion: that the seismicity rate in 1948 was seven times lower than in 1998! Of course, this comparison was also performed with a data set that was much too small to produce meaningful results, given the generality of the conclusion reached.
Poor Conclusions Make Great Myths
Many people draw their own conclusions about earthquakes based upon a very few occasions that stand out in their memory. These oversimplified observations are akin to the comment that "the phone always rings when you're in the shower". This isn't really true, but since people only tend to note the events that fit a certain pattern and forget about the mundane ones, it has a "ring" of truth. To avoid creating or supporting similar myths about seismicity, it's important that we not only consider all the appropriate data (in the example above, all showers and phone calls, not just those that coincide), but that we have enough data to reach an accurate conclusion. Otherwise, chance or bias can influence that end result.
One currently popular myth about the timing of earthquakes in southern California is that big earthquakes always happen in the morning. This is primarily due to the fact that most people only remember the last few large earthquakes that have occurred, perhaps just from the past decade. Farther back in time than that, their knowledge of when earthquakes happened is less complete. Since two of the most recent damaging earthquakes, Landers and Northridge, struck in the morning hours, many people now have the impression that big earthquakes will always happen in the morning. But were those same people alive in 1941, it's possible they'd think big earthquakes happen only in the evening, since the two most destructive earthquake prior to that time were the Long Beach and Imperial Valley earthquakes, both of which struck in the evening hours.
What other problems might you find while making these kinds of comparisons? The activity below should get you thinking about the pitfalls you may run into while attempting to draw conclusions from seismological data about what factors, if any, can influence the distribution of earthquakes in time.
It's difficult for people to admit that the timing of earthquakes is something complex and potentially beyond our ability to anticipate. That's why myths develop so easily -- we want them to be true, because we want to feel like we have some control or understanding of earthquakes, to make them less terrifying. The truth is that we are only beginning to understand what influences the occurrence of earthquakes. And despite the fact that we are making gains in our knowledge, we still have a long way to go.
The Proper Application of Observations
Many assertions have been made of a correlation between the timing of earthquakes and some natural force or cycle. A lot of people talk about "earthquake weather", as though faults several kilometers underground can "sense" changes in humidity and temperature at the surface. Even scientists have been trying for decades, with no success, to connect the timing of earthquakes to tidal forces (we will suggest, later, why these efforts fail, though tidal forces may play a role in affecting seismicity). The fact remains, however, that the only natural process ever convincingly shown to affect the timing of earthquakes is... another earthquake!
 "Do any man-made influences affect seismicity?"
The influence that one earthquake can have upon the timing of others is most glaringly obvious in the outbursts of aftershocks that follow large earthquakes. But why do aftershocks occur?
It is generally thought that aftershocks are a response to sudden changes in the stress on rocks in the Earth's crust. Remember that earthquakes occur when rocks, or at least the pre-existing fractures within the rocks, are stressed to the point of breaking. Though this stress builds up gradually by means of tectonic plate motion, the amount of stress in the crust can be altered suddenly and locally when a fault ruptures, because this shifts large masses of rock into new positions. While the primary result is a release of energy, and a decrease in stress, some parts of the crust near the fault rupture experience an increase in stress. That seems to induce other, typically much smaller, earthquakes to occur.
The figure at right shows a map view of stress change from the 1979 Homestead Valley earthquake sequence. The black line at center represents the trace of the fault that ruptured. White dots represent aftershock epicenters. Note that most fall in the areas where the stress increased (yellow to red), and few happened where it decreased (blue to purple).
"Can earthquakes trigger other earthquakes -- those outside of their `aftershock zones'?"
Evidence suggests that large earthquakes can also trigger earthquakes outside of their aftershock zones, where the stress change resulting from the ground rupture should be minimal. The exact reasons for this, and the mechanisms behind such "triggering", are debatable. While it's assumed that aftershocks (which are a type of triggered event) are induced by the radical changes in stress on the rocks near a fault rupture, such changes should not extend as far from the rupture as "triggered earthquakes" have been observed. So why do triggered events occur?
Many scientists feel it has something to do with the ground motion of a large earthquake, and how this shaking affects stressed and faulted rocks. Think of it as bumping an unstable object, inducing it to fall over, even if the tumble is not immediate. The relationship seen in examples of large earthquakes with triggered events is not as simple as one might expect in a standard "chain reaction" -- when one event immediately and directly leads to another, and another, and so on. As with aftershocks, the time between a triggered earthquake and the event that causes it can be hours, days, weeks, or even months or years.
This is one explanation for the lack of an obvious correlation between tidal forces (which could act as a trigger in a way similar to the force of earthquake shaking) and the onset of seismicity. If the time between a triggered event and its cause can vary greatly, then we would probably see no obvious correlation between tides and earthquake activity, which is indeed the case. This does not mean tidal effects do not influence earthquakes at all, but rather that any influence they have will be unpredictable, and therefore useless to consider in any practical application involving earthquake forecasting.
To see how seismologists arrived at the conclusion that triggered earthquakes can occur outside of aftershock zones, work through the activity below, which focuses on the extended effects of the 1992 Landers earthquake in southern California.
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Seismicity Rates
Is Earthquake Timing Influenced?
the city of Haicheng, and began a massive evacuation effort. The earthquake, measuring magnitude 7.3, followed about nine hours later, just as the evening chill was setting in -- which had residents contemplating a return to the warmth of their homes. Thankfully, most remained outside until the earthquake occurred, and thus while 90% of the city's buildings were severely damaged or destroyed, casualties were few.
wintertime. Had the predicted earthquake come a few hours later, the effort might have been a failure in spite of its relative accuracy. Imagine trying to suspend all the functions of a city like Los Angeles for nine hours, awaiting an event many people doubt will happen! Then imagine the uproar that would result should the predicted event fail to occur. Predictions need to be accurate and dependable, and that is simply beyond seismology's present ability to accomplish.
happen again, for certain, but the time between earthquakes like that is generally more than the 100 years we sampled. On other faults, the repeat time can even be in the thousands of years.
Slip Rates vs. Seismicity Rates
to enlarge it, if you need to) that the amount of historic seismicity along certain stretches of the infamous San Andreas fault zone (in red) is minimal, even non-existent. Conversely, you can see that while many of the faults with low slip rates display very little activity, some areas of the map are densely covered with blue and purple pixels that each represent the epicenter of an earthquake. Though this view is somewhat biased in places, due to large aftershock sequences on faults with low slip rates (e.g. Landers), it is nonetheless fair to say that there isn't a strict correlation between the two rates.
these two mature fault zones coincide with an area of complexity -- where the fault zone looks less like a single planar feature and is more of an intricate set of fault steps, bends, and parallel strands. Conversely, the quietest places tend to be those with a simple geometry.