Аннотация
Although there is increasing evidence that fluids may play a significant
role in the earthquake rupture process, direct observation of fluids
in active fault zones remains difficult. Since the presence of an
electrically conducting fluid, such as saline pore water, strongly
influences the overall conductivity of crustal rocks, electrical
and electromagnetic methods offer great potential for overcoming
this difficulty. Here we present and compare results from high-resolution
magnetotelluric (MT) profiles across two segments of the San Andreas
Fault (SAF) which exhibit very different patterns of seismicity:
Parkfield, which has regular small earthquakes and creep events,
and in the Carrizo Plain, where the fault is seismically quiescent
and apparently locked. In both surveys, electric fields were sampled
continuously, with 100 m long dipoles laid end-to-end across the
fault. From 100 to 0.1 Hz the data from both profiles are consistent
with a two-dimensional (2-D) fault-parallel resistivity model. When
both transverse electric and magnetic (TE and TM) mode data are included
in the interpretation, narrow (\~300-600 m wide) zones of low resistivity
extending to depths of 2-4 km in the core of the fault are required
at both locations. However, at Parkfield the conductance (conductivity
thickness product) of the anomalous region is an order of magnitude
larger than at Carrizo Plain, suggesting much higher concentrations
of fluids for the more seismically active Parkfield segment. We also
image structural differences between the two segments. At Carrizo
Plain, resistive, presumably crystalline, rocks are present on both
sides of the fault at depths below 3-4 km. In particular, we clearly
image resistive basement extending \~10 km or more east of the
SAF, beneath the Elkhorn Hills and Temblor Range. At Parkfield the
situation is quite different with a resistive block of Salinian granite
west of the fault and an electrically conductive, presumably fluid
rich Franciscan complex to the east. It is possible that these structural
differences control the difference in mechanical behavior of the
fault, either directly by affecting fault strength or indirectly
by controlling fluid supply.
Линки и ресурсы
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