Geology Reference
In-Depth Information
Observed & Predicted Terrace Elevations
200
200
inner edge elevations
of terraces
projected total uplift due to
repeated Loma Prieta events
zone well matched by
repeated Loma Prieta
deformation
180
180
160
160
zone poorly matched by repeated
Loma Prieta deformation & requiring
displacement on another fault
140
140
120
120
intermediate
terrace
100
100
80
80
oldest terrace
60
60
40
40
youngest terrace
20
20
0
0
40
30
20
10
0
-10
San Andreas-parallel distance (km)
Fig. 4.8
Coseismic shoreline displacement compared with the warped marine terraces.
The variations in coseismic displacement due to the 1989 Loma Prieta
M
=
7.1 earthquake and terrace altitudes
are projected orthogonally on to the trend of the nearby San Andreas Fault. The spatial pattern of coseismic
displacements (gray lines) in the southern half of the transect mimic the height variations of the warped marine
terraces. Such similarity suggests that warping of these terraces could result from multiple earthquakes with Loma
Prieta-like displacements. Modified after Anderson and Menking (1994).
(ii) deformed marine terraces (Anderson and
Menking, 1994). The Loma Prieta earthquake
caused deformation extending from the San
Andreas Fault to the California coast, where
shoreline displacement has been well docu-
mented by detailed surveying. A comparison of
the shoreline coseismic displacement with the
shape of the warped marine terraces clearly
indicates that the long-term terrace deformation
could result from repeated Loma Prieta-like
events. The match between the terraces and the
coseismic strain is very good for the southern
part of the terraces, but is unconvincing farther
north (Fig. 4.8). To explain that northern pat-
tern of deformation, repetitive faulting on
another fault has been invoked (Anderson and
Menking, 1994). Whereas the excellent match
of the warped terraces with the pattern of
coseismic deformation does not prove that
the Loma Prieta rupture was an oft-repeated,
characteristic rupture, it certainly suggests that
this interpretation is reasonable.
Debate persists concerning whether or not
faults exhibiting characteristic earthquakes are
common or rare. Certain aspects of characteris-
tic earthquakes appear incompatible with typi-
cal frequency-magnitude relationships for small
and intermediate earthquakes within a region
(Box 4.1). At present, with few possible excep-
tions, neither the historical nor the paleoseismic
studies provides a sufficiently robust database
to test the characteristic earthquake model.
Nonetheless, in comparison to the unpredictable
hazards posed by random, large earthquakes
on a fault system, the predictive value of
characteristic earthquakes certainly warrants
more focused study to ascertain if, where, and
why such earthquakes occur.
