Regional Seismicity
Regional Seismicity
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Fault rupture is a complex process, difficult to
simulate with basic models. |
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It is possible, with good data, to "view" a fault rupture, despite the
fact that the rupture surface is entirely underground. |
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One of the simplest ways to "picture" or model a fault
is as a plane. To describe the orientation of this plane,
only two measurements -- strike and dip -- are needed. |
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Accurate positioning of earthquake hypocenters can produce a rough
"image" of a fault, allowing sub-surface determination of dip.
Obtaining this much data for a single fault usually requires a
large aftershock sequence. |
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Scarps can be created by non-vertical motion.
(More generally, apparent offset does not always
equate with actual displacement.) |
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Different senses of dip slip can shorten (thicken)
or lengthen (thin) the Earth's crust. |
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Some faults experience appreciable amounts of dip slip
and strike slip simultaneously, and the nomenclature of these faults
reflects this. |
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Since large fault ruptures require the build-up of
a great deal of stress, many years go by between the repeat of
surface rupture on a single fault. The average time between
such ruptures, known as the recurrence interval of that fault,
is a useful measurement for assessing both the slip rate of the fault
and the hazard the fault presents. |
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Any single method for measuring the slip rate of a
fault is subject to substantial errors, both in the accuracy of the
method itself and in interpretation. The properties (including slip
rate) of faults can change over time, or along the length of a fault. |
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The orientation and sense of slip of faults in an
area is largely dependent upon the tectonic forces present in that area. |
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While complex, southern California's network of faults
is largely influenced by a few regional tectonic environments, and
understanding the nature of these makes it possible to make some
simple judgments about the sense of slip of many of the major faults. |
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The distribution of slip -- both in terms of sense and
rate -- along faults in southern California follows definite patterns
and is most strongly influenced by the characteristics of the plate
boundary. |
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Slip can be divided into lower rates among multiple
faults to accomplish the same overall motion between two reference
points as would be provided by a larger slip rate along a single fault
between them. The total slip rate across any arbitrary line can
be thought of as the sum of the slip rates of the individual faults
which cross that line. |
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Earthquakes can occur most anywhere in southern California,
but are typically located in zones associated with major faults. |
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Large earthquakes are generally associated with
major faults, because a large rupture area is required
to produce a large earthquake. |
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Topography -- mountains, hills, and basins, for example -- is
generally the result of movement along faults. Many of these faults may
be currently inactive, but seismicity is typically expected in areas
of steep topography. |
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The maximum depth of seismicity is relatively shallow throughout
southern California, but it does vary, and is related to crustal
thickness. Those areas with deeper hypocenters tend to be associated
with mountains and thrust faulting. Areas of extension
are generally lacking in deep earthquakes. |
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The rate of seismicity varies with time. Though aftershock
sequences strongly influence the rate, they are not the only reason for
the variation. |
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No definite connections have been found between
any potential influencing factor and the occurrence of earthquakes. |
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The shock generated by a large earthquake can
trigger the onset of other earthquakes, even at great distance
from the original hypocenter. |
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By plotting earthquake magnitude against the number of
earthquakes of that magnitude in a given time, a basic characteristic
of the seismicity rate in an area -- the b-value -- can be
determined. |
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The future activity of an aftershock sequence can be
assumed by closely examining the beginning of the sequence. |
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While foreshocks can occasionally be identified in advance of a
mainshock, it is risky to assume this can be done for most earthquakes,
and impossible to apply this to all large earthquakes. |
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The lack of accurate records pre-dating modern seismology
hinders our ability to understand long-term variations, and thus,
possible future trends. |
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Different faults accomodate their slip in different manners,
meaning that there is no definitive relation between the slip rate and
the seismicity rate of a fault. |
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Isoseismal (intensity) maps show irregularities in the
distribution of shaking caused by an earthquake. However, it is still
possible to crudely locate the epicenter of an earthquake using a
well-constructed intensity map. |
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Energy released by an earthquake travels through the
Earth in the form of seismic waves, which cause the shaking we feel.
There are four main kinds of seismic waves. |
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Both the epicenter and origin time of an earthquake can be
located with computations involving the exact arrival times
of seismic waves at three or more seismometers. These arrival
times can be picked by analyzing the waveforms recorded on seismograms. |
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Magnitude is a measure of the energy of an earthquake.
The magnitude scale developed by Charles Richter assigns a numerical
value for the magnitude of an earthquake based upon a comparison of
the maximum amplitude of deflection on a seismogram, as recorded by a
standard Wood-Anderson torsion seismometer, and the distance from that
seismometer to the earthquake's source. That distance can be found by
comparing the arrival times of the first P and S waves to reach the
seismometer. |
