HR: 12:05h
AN: T22A-07    [Abstracts]
TI: Time-Dependent Tomography and Microearthquake Moment Tensors in the Coso Geothermal Area
AU: * Julian, B R
EM: julian@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd., MS 977, Menlo Park, CA 94025 United States
AU: Foulger, G R
EM: g.r.foulger@durham.ac.uk
AF: Dept. of Earth Sciences, Univ. of Durham, Durham, DH1 3LE United Kingdom
AU: Richards-Dinger, K
EM: Keith.Richards-Dinge@navy.mil
AF: Geothermal Program Office, NAWS, Code 83G00D 1 Administration Circle, China Lake, CA 93555-6001 United States
AU: Monastero, F
EM: frankm@pwd.chinalake.navy.mil
AF: Geothermal Program Office, NAWS, Code 83G00D 1 Administration Circle, China Lake, CA 93555-6001 United States
AB: The Coso geothermal area, California, has produced hot water and steam for electricity generation for more than a decade. During this time, a permanent network of three-component digital borehole seismometers operated by the U.S. Navy has monitored the abundant microearthquake activity in the producing region. A 14-station portable array of three-component digital instruments has supplemented the Navy network since about September 2003 to improve the monitoring near several wells in which fluid injection/hydrofracturing experiments were planned for enhancing geothermal energy production. This combined network records several thousand microearthquakes each year. We used the data for time-dependent seismic tomography to determine changes in seismic wave-speed structure associated with fluid removal. We also determined high-precision locations and microearthquake mechanisms (moment tensors). Inversions of a 4811-earthquake data set comprising 79,822 P- and S-phase travel times reveal low V_p and V_s wave speeds extending to a maximum depth of about 4 km beneath Coso Basin associated with sedimentary fill in the Coso Wash. A negative V_p/V_s anomaly occurs in the northern and eastern part of the geothermal field at 1 km above sea level, and in the northern and southern parts of the field at sea level. Independent graded inversions for each of the years 1996 - 2004 separately show an irregular strengthening of these anomalies. This progressive reduction in V_p/V_sresults predominately from a progressive relative increase of V_s with respect to V_p. Such an increase can result from processes associated with geothermal operations such as a decrease in fluid pressure and the drying of argillaceous minerals such as illite. We will present moment tensors and high-resolution relative earthquake locations for microearthquakes associated with an injection experiment conducted 22nd Feb - 4th March, 2005 in Well 34-9RD2. This well penetrated a deep zone of high permeability, at which time a burst of microearthquakes occurred, before significant injection work was done.
DE: 7205 Continental crust (1219)
DE: 7280 Volcano seismology (8419)
DE: 8135 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8424)
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
DE: 9350 North America
SC: Tectonophysics [T]
MN: Fall Meeting 2005