Julian, B.R. G.R. Foulger and K. Richards-Dinger, Monitoring microearthquake activity and structure changes at the Coso geothermal area, 30th Stanford Workshop on Geothermal Reservoir Engineering, California, 31 January – 2 February, 2005.
Monitoring microearthquake activity and structure changes at the Coso geothermal area
U.S. Geological Survey, 345 Middlefield Rd., MS 977, Menlo Park, CA 94025 United States, [email protected]
Univ. Durham, Science Laboratories, South Rd., Durham, DH1 3LE United Kingdom, [email protected]
Geothermal Program Office, U. S. Navy, China Lake, CA 93555-6001, [email protected]
The Coso geothermal area, at the southern end of Owens Valley in eastern California, has been exploited for electricity generation for more than a decade, during which time a network of three-component digital borehole seismometers operated by the US Navy has monitored seismic activity. The several thousand locatable microearthquakes recorded each year by this network are well suited to using time-dependent crustal structure, hypocenter locations, and moment-tensor focal-mechanisms to monitor changes caused by production and by reservoir-stimulation experiments.
Local-earthquake tomography results reported in 2003 show that between 1996 and 2002 the Vp/Vs ratio decreased in the uppermost 2 km, primarily because of an increase in Vs. This change might be caused by either drying of the rock matrix or a decrease in pore pressure. More detailed tomography, including all years from 1996 to 2003 and having improved horizontal resolution (1 km), now shows that this change occurred primarily in the most recent few years.
Locating microearthquakes within the geothermal field by applying high-resolution relative relocation methods (e.g. Waldhauser & Ellsworth, 2000; Richards-Dinger & Shearer, 2000) to arrival-time differences determined by waveform cross-correlation reduces location errors by an order of magnitude (from 100s of meters to 10s of meters). Examples from reservoir-stimulation experiments in 2003 and 2004, when 14 portable seismometers supplemented the permanent seismic network, clearly delineate small structures upon which microearthquakes occur. High-resolution hypocenters are valuable supplements to complete moment-tensor earthquake mechanisms determined from body-wave amplitude ratios measured using the enlarged seismometer network, and help to constrain the physical processes occurring within the geothermal reservoirs better than either data type alone can do.last update 3rd February, 2006