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
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
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