Geophysical images of the creeping segment of the San Andreas fault:
implications for the role of crustal fluids in the earthquake process
Tectonophysics, 385 (1-4):
137--158 (Jul 19, 2004)DOI: 10.1016/j.tecto.2004.02.010
Abstract
High-resolution magnetotelluric (MT) studies of the San Andreas fault
(SAF) near Hollister, CA have imaged a zone of high fluid content
flanking the San Andreas fault and extending to midcrustal depths.
This zone, extending northeastward to the Calaveras fault, is imaged
as several focused regions of high conductivity, believed to be the
expression of tectonically bound fluid pockets separated by northeast
dipping, impermeable fault seals. Furthermore, the spatial relationship
between this zone and local seismicity suggests that where present,
fluids inhibit seismicity within the upper crust (0-4 km). The correlation
of coincident seismic and electromagnetic tomography models is used
to sharply delineate geologic and tectonic boundaries. These studies
show that the San Andreas fault plane is vertical below 2 km depth,
bounding the southwest edge of the imaged fault-zone conductor (FZC).
Thus, in the region of study, the San Andreas fault acts both as
a conduit for along-strike fluid flow and a barrier for fluid flow
across the fault. Combined with previous work, these results suggest
that the geologic setting of the San Andreas fault gives rise to
the observed distribution of fluids in and surrounding the fault,
as well as the observed along-strike variation in seismicity.