Geology Reference
In-Depth Information
6 Paleoseismology:
ruptures and slip rates
Geological maps of either Quaternary deposits
or older bedrock often indicate the presence of
faults in a given area. Through examination
of  displaced land features or of lithologic
contacts in outcrops, we can tell that these faults
have been active at some time in the past.
But,  how  active have they been? Have they
ruptured  in  large earthquakes that broke long
sections of the fault, or are they associated with
smaller displacements? Have the faults moved
aseismically and continuously or in discrete
events that generated earthquakes? When did
the last rupture and the ones previous to that
occur? Is there regularity to the timing of events?
Is there a spatial pattern to the rupture that is
repeated from event to event? Can the fault be
divided into independent segments that rupture
in characteristic events? Does an earthquake on
one fault increase or decrease the likelihood of
rupture on other nearby faults, and how are
those changes dependent on the orientation of
the fault? Do arrays of nearby faults exhibit
systematic rupture patterns (Fig. 6.1)? Can we
estimate the likelihood that a fault will produce
an earthquake of a given magnitude within a
specified period of time? What has the past slip
rate on a fault been and how does that compare
with geodetically determined rates? What can be
done to estimate rupture histories when a fault
is not exposed?
Answers to questions such as these constitute
the focus of paleoseismological studies. Such
investigations typically use stratigraphic, structural,
geomorphic, and biological evidence to
reconstruct the sequence of displacements on a
fault. When combined with dating of displaced
features or of other indicators of faulting, the
timing of past ruptures can also be determined.
When multiple offsets can be dated on a single
fault, it becomes possible to determine recur-
rence intervals and longer-term rates of
displacement and to define the variation in the
displacement that occurred during each of
several earthquakes along the same fault.
Why do we want to know this? If we are
interested in understanding how the brittle
upper crust responds to imposed stresses,
we  need to know how and when it ruptures
during earthquakes. In order to develop a basis
for predicting the location and magnitude of
future earthquakes, we need many details of
past earthquakes. If we want to determine
whether or not a fault exhibits “characteristic” or
repeatable behavior, we have to know its rup-
ture history and the variability of displacement
along it in the past. We recognize that, in areas
of active deformation, the interplay between
tectonic movements and surface processes
controls the geomorphology of these deforming
landscapes. In order to quantify such interactions,
we need to know both the magnitude and three-
dimensional geometry of faulting or folding
events, as well as the timing of these events.
A remarkably diverse array of approaches -
ranging from determining the growth record of
trees that grew along a fault trace to interpreting
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