HR: 14:40h
AN: V53F-05 INVITED    [Abstracts]
TI: Time-dependent Seismic Tomography of the Coso Geothermal Area, 1996-2006
AU: * Foulger, G R
EM: g.r.foulger@durham.ac.uk
AF: University of Durham, Science Laboratories, South Rd., Durham, UK DH1 3LE, United Kingdom
AU: Julian, B R
EM: julian@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS977, Menlo Park, CA 94025, United States
AU: Monastero, F C
EM: francis.monastero@navy.mil
AF: Geothermal Program Office, US Navy, China Lake, CA 93555, United States
AB: Measurements of temporal changes in Earth structure are commonly determined using local earthquake tomography computer programs that invert multiple seismic-wave arrival time data sets separately and assume that any differences in the structural results arise from real temporal variations. Such an assumption is dangerous because the results of repeated tomography experiments would differ even if the structure did not change, simply because of variation in the seismic ray distribution caused by the natural variation in earthquake locations. Even if the source locations did not change (if only explosion data were used, for example), derived structures would inevitably differ because of observational errors. A much better approach is to invert multiple data sets simultaneously, which makes it possible to determine what changes are truly required by the data. This problem is similar to that of seeking models consistent with initial assumptions, and methods similar to "damped least squares" can solve it. We are developing such a program, dtomo. This program inverts multiple epochs of arrival-time measurements for hypocentral parameters and structural change in the inter-epoch period. We are applying this work to data from the seismically active Coso geothermal area, California. The permanent network operated there by the US Navy, supplemented by temporary stations, provides excellent earthquake arrival-time data. Furthermore, structural change is expected in the area as a result of geothermal exploitation of the resource. We have studied the period 1996 through 2006. Our results show, for a 2-km horizontal grid spacing, an irregular strengthening with time of a negative Vp/Vs anomaly in approximately the upper 2 km of the reservoir. This progressive reduction in Vp/Vs results predominately from a increase of Vs with respect to Vp. Such a change is expected to result from effects of geothermal operations such as decreasing fluid pressure and the drying of argillaceous minerals such as illite.
UR: http://cosomeq.wr.usgs.gov
DE: 7209 Earthquake dynamics (1242)
DE: 7270 Tomography (6982, 8180)
DE: 8135 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8424)
DE: 8180 Tomography (6982, 7270)
DE: 8424 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8135)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting