Mapping Offshore Faults with Marine EM
Abstract
The California continental borderland, part of the tectonic boundary between the Pacific and North American plates, is characterized by northwest-trending, dextral strike-slip faults that create pull-apart basins and pressure ridges. One such basin, the 10~km wide San Diego Trough (SDT), has been used to test marine EM equipment designed for hydrocarbon exploration, because of its location, water depth (1100~m), and relatively flat seafloor. The SDT is bounded to the east and west by the Coronado Bank fault and the Thirtymile Bank fault, while the SDT fault runs approximately down the center. Inversion of marine magnetotelluric (MT) data across the SDT provides a good map of basement structure, with 1-2~km of ~1~Ωm sediment underlain by >1000~Ωm igneous basement. Independent estimates of sediment resistivity and thickness are provided by controlled source EM data that are fit by 2300~m of 1.9~Ωm sediments over resistive basement. The MT conductivity image suggests a dipping Thirtymile Bank fault, although the western part of the section is dominated by what appears to be a resistive, tilted, slump block. There is no conductivity signature associated with the SDT fault. However, our preliminary inversions feature a 10~km deep, 2~km wide, 10~Ω conductivity anomaly associated with the Coronado Bank fault, whose appearance is remarkably similar to land MT images of the San Andreas fault. This suggests that marine EM may be a useful technique for assessing seismic hazard associated with offshore faults.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2005
- Bibcode:
- 2005AGUFMGP34A..05L
- Keywords:
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- 0925 Magnetic and electrical methods (5109);
- 3006 Marine electromagnetics;
- 3045 Seafloor morphology;
- geology;
- and geophysics;
- 3094 Instruments and techniques