Geology Reference
In-Depth Information
Box 6.3
Lessons from the San Andreas Fault.
In the 35 years since the pioneering paleoseismic studies of Kerry Sieh on the San Andreas
Fault as a doctoral student in the 1970s, this fault has provided a treasure trove of insights on
how large strike-slip faults behave. Dozens of trenches have been excavated along the length
of the fault to determine when earthquakes occurred during the past millennium and to try to
deduce the patterns of past ruptures. Despite these many studies, the imprecision of radiocarbon
dating introduces sufficient temporal ambiguity that correlation of individual rupture events
from one site to another remains somewhat uncertain. Nonetheless, a consensus has gradually
emerged about the lateral extent of many of the earthquake ruptures of the past millennium.
Surprisingly, despite numerous paleoseismic records from those many trenches, until recently
relatively little was known either about how much slip occurred at a given site in each rupture
or about overall slip rates at a site. (The lack of information on slip per rupture is a typical
consequence of trenching perpendicular to a strike-slip fault and not having complementary
trenches parallel (Sieh, 1984) to it for salami-slicing (see Fig. 6.3) in order to reconstruct the
slip record.)
Dating of offset geomorphic features is now providing a more detailed record of slip
variations (e.g., Weldon and Sieh, 1985; Liu
et al.
, 2004; van der Woerd
et al.
, 2006). Several
of the longer paleoseismic records (see Fig. 6.8) on the San Andreas Fault contained as many
as 10 events (Sieh
et al.
, 1989) and allowed geologists to calculate recurrence intervals and
to consider whether San Andreas earthquakes occurred in clusters or not. But time series of
10 or fewer earthquakes (most trenches had fewer) are too short to make a statistically
reliable assessment of whether slip rates on the San Andreas Fault are steady, whether the
fault behaves in a time-predictable or a slip-predictable manner, or whether a predictable
pattern for a sequence of San Andreas earthquakes exists at any given site.
Wrightwood Location
Sierra
Nevada
paleoseismic
trenches
0
50
km
100
Mojave
Desert
1857
rupture
Wright-
wood
LA
Pacific
Ocean
A
A. Location of Wrightwood, CA, studies.
All these uncertainties are beginning to be addressed through studies by Ray Weldon
and his associates along the San Andreas Fault near Wrightwood (see figure A) at the east-
ern end of the San Gabriel Mountains (Weldon
et al.
, 2004). Over an 18-year period, they
have excavated more than 40 trenches and radiocarbon-dated over 50 peat layers in order
