High-resolution electromagnetic imaging of the San Andreas fault in Central California

Journal Article · Fri Jan 01 04:00:00 EST 1999 · Journal of Geophysical Research

DOI:https://doi.org/10.1029/98JB01755· OSTI ID:328586

Unsworth, M ^[1]; Egbert, G ^[2]; Booker, J ^[1]

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1. Geophysics Program, University of Washington, Seattle (United States)
2. College of Oceanic and Atmospheric Sciences, (United States) Oregon State University, Corvallis

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 ({approximately}300{endash}600 m wide) zones of low resistivity extending to depths of 2{endash}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{endash}4 km. In particular, we clearly image resistive basement extending {approximately}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. {copyright} 1999 American Geophysical Union

• OSTI ID: 328586
• Journal Information: Journal of Geophysical Research, Journal Name: Journal of Geophysical Research Journal Issue: B1 Vol. 104; ISSN JGREA2; ISSN 0148-0227
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
CALIFORNIA
EARTH CRUST
EARTHQUAKES
ELECTRIC CONDUCTIVITY
ELECTRIC FIELDS
FLUIDS
GEOLOGIC FAULTS
MAGNETIC FIELDS
MAGNETOTELLURIC SURVEYS
ROCKS
SEISMIC WAVES
SEISMICITY