Geology Reference
In-Depth Information
A
Variable-Slip Model
Observations
variable displacement per event
at a point
constant slip rate along length
variable earthquake size
distance along fault
B
Uniform-Slip Model
constant displacement per event
at a point
constat slip rate along length
constant size large earthquakes:
more frequent moderate earthquakes
Fig. 4.7
Models for slip
accumulation along a
regional suite of faults.
Models for (A) variable
slip, (B) uniform slip, and
(C) characteristic
earthquakes. Vertical
arrows depict coseismic
slip in individual
earthquakes. Modified
after Schwartz and
Coppersmith (1984).
distance along fault
C
Characteristic Earthquake Model
constant displacement per event
at a point
variable slip rate along length
constant size large earthquakes:
infrequent moderate earthquakes
distance along fault
How could one test whether a fault displays
characteristic earthquakes? In the vicinity of
the “Parkfield asperity” on the San Andreas
Fault, for example, five moderate thrust-fault
earthquakes (
M
∼
6) with rather similar rupture
patterns have occurred during the 20th century
(Bakun and McEvilly, 1984). Such repetition is
suggestive of characteristic earthquake dis-
placement patterns. Usually, however, the his-
torical record is both too brief and too
incomplete to permit reconstruction of the dis-
placement patterns during several ruptures of
an entire fault or a segment of it. Consequently,
we often have to interpret the geological record
of deformation and faulting to assess past rup-
ture patterns. At least two different approaches
can be used for such an assessment. In one, the
variability of displacement along a fault is com-
piled for several past earthquakes using
measured displacements of strata, structures,
and geomorphic features. We describe this
approach in a subsequent chapter on paleoseis-
micity. In the second approach, the deformation
observed to have resulted from a single, recent
earthquake is compared with nearby deformed
geological markers, such as the surface of a fold
or deformed marine or fluvial terraces, that rep-
resent the cumulative deformation due to multi-
ple earthquakes. If the spatial pattern of
deformation shown by the long-lived structure
could be created through repeated duplication
of the deformation due to the individual fault-
ing event (Stein
et al.
, 1988), then it is reasona-
ble to suggest that the fault may have a
characteristic behavior. An example (Fig. 4.8) of
such a comparison can be made using (i) the
strain resulting from the 1989
M
=
7.1 Loma
Prieta earthquake south of San Francisco, and
