Geology Reference
In-Depth Information
et al.
(2010) surmise that the shorter boulders -
such as those used in the van der Woerd (2006)
study - have been exhumed since deposition
and, consequently, yield minimum exposure
ages. Because the margins of fans are commonly
indistinct and digitate, rather than linear, and
because channels within the fans may also be
somewhat sinuous, determinations of the mag-
nitude of fan offset typically have inherent
uncertainty. Whereas van der Woerd
et al.
's
(2006) study estimated slip of about 565 m, Behr
et al.
's (2010) reconstructed slip ranged from
680 to 980 m. Ironically, when the differences in
slip are combined with the differences in expo-
sure ages for the two studies, they both happen
to yield similar long-term slip rates - but only
one of them (at most!) is correct in both its esti-
mated age and total slip.
Despite the uncertainies, these slip-rate
estimates have important implications. For
example, even though the San Andreas Fault
has traditionally be viewed as the most
dangerous and seismogenic fault in southern
California, this long-term slip rate is less than
half of the Pacific-North American plate rate.
Recent paleoseismic and geodetic studies
(Bennett
et al.
, 2004; Dorsey, 2002) suggest that
the nearby San Jacinto Fault may be absorbing
much of the plate-boundary slip, and that rates
along these two faults have varied synchronously
and in opposite senses for the past 100 kyr
or more.
Earthquake-related stratigraphy
Some of the clearest indirect paleoseismic
records come from settings where a
stratigraphic signal can be discerned that
appears clearly linked to seismic deformation,
rather than to variability in climate or sediment
supply. For example, the 1980 (
M
=
7.3) El
Asnam earthquake in Algeria caused hanging-
wall uplift and temporarily dammed the Cheliff
River (Meghraoui
et al.
, 1988a,b; Philip and
Meghraoui, 1983), where it traversed the
folded hanging wall through a water gap
(Fig. 6.23A). Eventually, the river eroded
through the dam represented by the upthrown
fault block, and, at that time, the lake drained.
Until that time, lacustrine sediments were
deposited in the earthquake-dammed lake.
This stratigraphic response to a thrusting event
provides a conceptual model for interpreting
the stratigraphy in trenches located on the
footwall at some distance from the main
rupture. Seismic shaking may also induce local
faulting within the lacustrine sediments and
cause sand blows or other features indicative
of faulting, which can be exposed in trenches.
Most importantly, however, lacustrine deposits
will mantle the flooded topography and form
identifiable seismo-stratigraphic markers. With
such a model in mind, it is fairly straight-
forward to interpret the Holocene stratigraphic
record (Fig. 6.23B). At El Asnam, a trench
reveals flood deposits alternating with
paleosols (Meghraoui
et al.
, 1988b). For six
of the lacustrine layers, some associated
indicators of faulting or seismic shaking are
present. Therefore, six stratigraphically defined
earthquakes have been identified as occur-
ring between 6000 yr BP and 1900 yr BP, after
which tilting apparently shifted the position
of the earthquake-dammed lake away from the
trench site. This interpretation is, however,
not without uncertainties. Because the Cheliff
River passes through a water gap, non-
seismically induced landslides could also dam
the river and trigger lacustrine sedimentation.
Within these sediments, a careful search for
features indicative of seismic shaking helps to
rule out non-seismic causes.
Indirect observations of faulting
Vigorous seismic shaking can generate clusters
of landslides, damage the roots of trees, cre-
ate tsunamis, and cause weakly consolidated
sedimentary beds to deform. Each of these
seismically induced consequences produces a
preservable, geological or biological record
that could be interpreted for paleoseismic
purposes. Because non-seismic events, such
as rainfall-driven landslides, high winds, or
abrupt sediment loading, could produce similar
outcomes, however, their interpretation as
paleoseismic recorders commonly becomes
more ambiguous.
