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We refer to faults that don't reach the surface as blind faults.

Most major faults in southern California reach, and thus intersect with, the surface of the Earth. This intersection of the fault plane with the surface produces a linear feature called a fault trace (often known colloquially as a "fault line"). Some faults -- most commonly, low-angle thrust faults -- do not reach the surface, however. Scientists refer to these as blind faults, and are constantly seeking new way to reveal them, since they can do as much damage as faults that are easily located on the surface; the Whittier Narrows and Northridge earthquakes both occurred on blind thrust faults, and are prime examples of the danger of blind faults. As the block diagram at left shows, a blind thrust fault may reveal a hint of its existence through the creation of a fold belt that may express itself topographically (e.g. as a chain of low hills).

The largest earthquake in southern California during the 20th Century wasn't on the San Andreas fault.

Many people believe that the San Andreas fault is the primary earthquake generator in southern California. In fact, most of the San Andreas fault has been seismically "quiet" in the last century, especially the section between Parkfield and the Cajon Pass. The last major earthquake along this section was, indeed, the largest earthquake in southern California in recorded history, but that was the Fort Tejon earthquake, which happened back in 1857.

The distinction of the largest southern California earthquake of the 20th Century goes to the Kern County earthquake (also known as the Arvin - Tehachapi earthquake) of July 22, 1952, which had a moment magnitude of MW 7.5 (possibly slightly less). It ruptured most of the length of the White Wolf fault zone, located at the southern end of the great San Joaquin Valley. The image at left is a colorized photo of the surface rupture from this earthquake.

(The largest earthquake in all of California during the 20th Century was indeed along the San Andreas fault, but only in central and northern California. This was the Great San Francisco Earthquake of 1906.)

Aftershock zones can be defined in two different ways...

An aftershock is actually just a normal earthquake in every physical detail. Out of context, there is no way to tell the difference between any 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 an "aftershock zone." This is sometimes defined as within one fault-rupture length of the mainshock rupture surface, or alternatively, within an area defined by seismologists based upon early aftershock activity. Second, it must occur within that designated area -- the "aftershock zone" -- before the seismicity rate in that area returns to its "background", meaning pre-mainshock, level. If an earthquake meets these two criteria, seismologists consider it an "aftershock."

The "Big Bend" of California's San Andreas fault

Has it ever occurred to you to wonder, "Why are there so many faults in southern California?" The answer to this is related directly to plate tectonics. California is 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 (like the one at left) 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?

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." This barrier to simple right-lateral motion creates immense stresses within the crust of southern California, and over the millenia, the rocks have responded by deforming or breaking. Where they break, faults are formed in ways that attempt to alleviate the stress. Thus, southern California, home to the Big Bend, is also home to a vast number and variety of faults.

Fault steps, sag ponds and paleoseismology

Fault steps are features similar, in many ways, to fault bends (see the previous fact). 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, 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. Paleoseismology is a field that looks into pre-historic earthquakes, and investigating such sequences of rupture records in sediment is one of that field's key research methods.

Why scientists no longer use the Richter magnitude scale

In the 1930s, when Charles F. Richter first devised his now-famous mathematical scale for rating the amount of energy released by an earthquake, or the "magnitude" (a term suggested by fellow seismologist Harry O. Wood), he based it upon records of earthquakes in southern California made by standard instruments known as Wood-Anderson torsion seismometers. Published in 1935, the original definition of Richter magnitude is:

The magnitude of any shock is taken as the logarithm of the maximum trace amplitude, expressed in microns, with which the standard short-period torsion seismometer would register that shock at an epicentral distance of 100 kilometers.

Because he defined his scale in terms of these torsion seismometers, once these instruments were replaced by more modern equipment, the conversion used to turn seismogram readings into a measure of magnitude was no longer the exact same scale established by Richter in 1935. Today, the algorithm for determining the "local magnitude" (ML) of an earthquake is analogous to the Richter magnitude conversion, but it is not the same. Hence, seismologists no longer truly use the "Richter scale" when calculating the magnitude of earthquakes.

Seismic waves move at different speeds in different rocks

There are two classes of seismic waves: body waves and surface waves. Body waves travel through the interior, or body, of the Earth. Two types of body waves are recognized: P waves and S waves. P waves are the fastest seismic waves, and consequently, the first to arrive at any given location. Because of this fact, they were initially referred to as the primary waves of an earthquake. "Primary" was later shortened simply to "P". Travelling at a speed typically around 60% that of P waves, S waves always arrive at a location after them -- the "S" stands for secondary.

As with sound, the speed at which seismic waves travel depends upon the properties of the matter through which they propagate. In general, the less dense the matter, the slower the waves. Researchers have used observations of seismic waves from known sources to create a velocity model -- a three-dimensional map of the variations in seismic wave velocity throughout the crust beneath southern California. Such a model for P-wave velocity in the Los Angeles basin can be seen at left.

January 1932: the origin of the Richter magnitude scale

Though "the Richter scale" of earthquake magnitude is arguably one of the most famous (or infamous, if you like) means of scientific measurement, few people realize exactly how it was devised, and that it was originally only meant to apply to southern California.

Using data gathered on southern California earthquakes in January 1932, Charles F. Richter made a graph (at left) of the logarithm of maximum amplitude versus epicentral distance from all the seismic recording stations that existed in southern California at that time. From this empirical evidence he drew a parallel best-fit curve (shown in red), arbitrarily positioned so that a "zero-magnitude" earthquake at 100 kilometers would produce a maximum trace amplitude of 0.001 millimeters. He then turned this curve into a table of values that could be used to calculate "magnitudes." All one had to do to find magnitude was to calculate the logarithm (to the base 10) of the maximum amplitude, in millimeters, recorded on a seismogram from a standard (Wood-Anderson) short-period torsion seismometer, and subtract from that the value listed on the table next to the epicentral distance (measured from the location of the instrument that recorded the seismogram).

The idea of earthquake magnitude caught on, and Richter's original scale was adapted for other parts of the world and other types of instruments and measurements,... but it all started with a set of data from southern California earthquakes that struck in January 1932.

The strongest shaking in an earthquake is not necessarily at the epicenter.

The epicenter of an earthquake is the point on the Earth's surface directly above the source -- the hypocenter -- of the earthquake. Although epicenters were originally located by a rough estimate of the zone of strongest shaking in an earthquake, the advent of modern seismology allowed the source of an earthquake to be pinpointed. With this advance, it became clear that the epicenter is not always the site of strongest shaking. The map at left shows the Modified Mercalli intensities recorded during the 1971 San Fernando earthquake. The instrumentally-determined epicenter has been located atop this intensity map. Note how the area of maximum intensity is several miles removed from the epicenter. This disparity can result in a quandary for seismologists attempting to label the earthquake with a name (e.g. "Northridge earthquake"). Some feel that an earthquake should be named for the community that is subjected to the strongest shaking in that quake; others think the location of the epicenter should be the main criterion for selecting a name. When the two are the same, there is no debate, but when they aren't, it is possible to wind up with two different common names for the same event!

More facts about earthquakes, faults, and preparedness can be found in the handbook, Putting Down Roots in Earthquake Country, available online from SCEC.

To order printed copies of this handbook, go to the SCEC Publications Catalog.

Putting Down Roots in Earthquake Country is a product of the Southern California Earthquake Center and the United States Geological Survey, with additional support of organizations listed on the acknowledgements page.

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