Figure 13.11 shows the best-fitting coseismic slip and
afterslip distributions. The cumulative afterslip after 9 months
is shown. The afterslip is largely localized within two
patches. One patch is in the upper 5 km, just above the larger
coseismic slip patch. The other patch of afterslip occurs above
the hypocenter.
The fit to the GPS time-series is shown in Figure 13.12.
The model reproduces the rapid postseismic velocities during the
first 0.1-0.2 years and the less rapid velocities during the later
time periods. There is a tendency for the model to under-fit the
displacements at later times indicating that the model relaxes and
returns to the pre-earthquake rate too quickly. This may indicate
that another (un-modeled) deformation mechanism, such as deep
distributed flow, may dominate the signal at later times.

Figure 13.11:
A.
Best-fitting coseismic slip distribution and, B. corresponding
9-month cumulative rate-state afterslip.

Figure 13.12:
Fit to the
postseismic GPS time series. Only a subset of data is shown
because of space limitations. Data shown with 95% confidence
error bars. Solid line is model. Vertical axis is displacement in
mm.

Table 1 shows the approximate 95% confidence intervals on the
frictional parameters. The values for A and B assume a uniform
effective normal stress on the fault of 50 MPa. The parameters
vary linearly with depth, so we report the values at the center of
the top and bottom rows of patches. A-B is on the order of
to , which is about an order of magnitude lower
than experimental values for granite at conditions well above or
below the transition from potentially unstable (negative A-B) to
nominally stable (positive A-B) friction. It is also an order of
magnitude lower than an estimate from an afterslip inversion of
the Tokachi-oki, Japan, earthquake (Miyazaki et al., 2004).
However, the estimated A-B values fall within a wide range of
experimental values reported for Serpentinite, which crops out
along the San Andreas fault zone and is frequently mentioned as an
important factor allowing for velocity-strengthening behavior of
some faults (Reinen et al., 1994). The low values of A-B
might be indicative of a transition zone from velocity-weakening
(negative A-B) to velocity-strengthening (positive A-B). Estimates
of are of the order 0.01-0.1 m, in reasonably good agreement
with the seismic estimate near Parkfield (Fletcher and
Spudich, 1998). The scaling relationship of Marone and
Kilgore (1993) infers a fault gouge thickness of 1-10 meters.

Table 13.1:
Table 1. 95% Confidence intervals on frictional
parameters