Geology Reference
In-Depth Information
10
Earthquakes
Abstract
This chapter introduces earthquake seismology, in particular friction
models for earthquake genesis, source mechanisms, moment tensors, and
the Gutenberg-Richter law. The objective is to acquire confidence with the
focal mechanisms representation (beach balls) and the recurrence time of
earthquakes.
along the fault plane. A direct consequence of
the model is that the relative velocity vectors
will attain the magnitude determined by global
plate kinematics only at some distance from plate
boundaries (Fig.
10.1
). During this
interseismic
phase
, which may be several thousands of years
long after large earthquakes, the upper crust de-
forms elastically and accumulates strain energy
and stress. When the accumulated stress exceeds
some threshold, a “rupture” along the stick region
determines sudden sliding and release of strain
energy (Fig.
10.1
). The (almost) instantaneous
displacement during this
coseismic phase
can
reach tens of meters and cause devastating earth-
quakes. The rupture and the subsequent evolution
of the stress and strain fields leading to the next
earthquake is known as the
seismic cycle
.A
complete seismic cycle typically also includes a
postseismic phase
, which generally spans a few
months after an earthquake but may continue
for some years. In this instance, a fault that
has experienced a significant rupture continues
to accommodate some slip after the earthquake.
Today observations of surface deformation using
geodetic techniques are used to monitor the slow
10.1
Reid's Model
Earthquakes represent the most evident effect
of short-term plate tectonics, because of their
dramatic and destructive impact on humanity. Al-
though they are mostly concentrated along plate
boundaries, strong earthquakes associated with
intra-plate deformation or other phenomena are
also possible and in some cases may represent
an even greater hazard for human life. An earth-
quake can be viewed as an instantaneous release
of accumulated strain energy (hence stress) in
a small region along an active fault. The idea
that an earthquake results from a sudden release
of accumulated stress in the crust was first pro-
posed by Reid (
1910
), an American engineer who
studied the ground displacement field across the
San Andreas Fault after the 1906 earthquake in
San Francisco. His analysis led to the
elastic
rebound theory
for the origin of earthquakes.
In this model, slip along a fault plane may be
locked for centuries, whereas the field of rel-
ative velocities is nonzero and increases pro-
gressively as we move away from a stick zone
