Mathematics – Logic
Scientific paper
Feb 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995georl..22..501e&link_type=abstract
Geophysical Research Letters (ISSN 0094-8276), vol. 22, no. 4, p. 501-504
Mathematics
Logic
10
California, Earthquakes, Geological Faults, Geysers, S Waves, Seismology, Splitting, Anisotropy, Earth Crust, Felsite, Geochemistry, Mathematical Models, Polarization (Waves), Seismic Waves
Scientific paper
Shear-wave splitting from local microearthquakes recorded in The Geysers geothermal field shows that seismic anisotropy is distributed in a complex geographic pattern. At stations within about 2 km of northwest-striking regional faults, the fast polarization direction is parallel to those faults. The geothermal field, lying between two such faults, has both northwest and northeast fast polarization directions, often at the same station. This pattern suggests at least two causes of splitting: (1) extensive dilatancy anisotropy (EDA) and (2) fault-produced fractures or rock fabric. The observed anisotropy may derive from the upper 1.5 km of the crust, averaging 4% there, or it may be heterogeneously distributed throughout the upper 5 km. Fast polarization directions coincide with fracture directions inferred from borehole data for one of the youngest rock types in the region, a felsite pluton of about 1 Ma, and with injectate pathways inferred from microseismicity and geochemistry. Including in reservoir models a permeability anisotropy with a pattern similar to seismic anisotropy may help in optimizing fluid injection and steam recovery.
Evans John R.
Foulger Gillian R.
Julian Bruce R.
Ross Alwyn
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