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Published December 1999 | Published
Journal Article Open

Attenuation and Source Properties at the Coso Geothermal Area, California

Abstract

We use a multiple-empirical Green's function method to determine source properties of small (M −0.4 to 1.3) earthquakes and P- and S-wave attenuation at the Coso Geothermal Field, California. Source properties of a previously identified set of clustered events from the Coso geothermal region are first analyzed using an empirical Green's function (EGF) method. Stress-drop values of at least 0.5-1 MPa are inferred for all of the events; in many cases, the corner frequency is outside the usable bandwidth, and the stress drop can only be constrained as being higher than 3 MPa. P- and S-wave stress-drop estimates are identical to the resolution limits of the data. These results are indistinguishable from numerous EGF studies of M 2-5 earthquakes, suggesting a similarity in rupture processes that extends to events that are both tiny and induced, providing further support for Byerlee's Law. Whole-path Q estimates for P and S waves are determined using the multiple-empirical Green's function (MEGF) method of Hough (1997), whereby spectra from clusters of colocated events at a given station are inverted for a single attenuation parameter, κ, with source parameters constrained from EGF analysis. The κ estimates, which we infer to be resolved to within 0.01 sec or better, exhibit almost as much scatter as a function of hypocentral distance as do values from previous single-spectrum studies for which much higher uncertainties in individual κ estimates are expected. The variability in κ estimates determined here therefore suggests real lateral variability in Q structure. Although the ray-path coverage is too sparse to yield a complete three-dimensional attenuation tomographic image, we invert the inferred κ value for three-dimensional structure using a damped least-squares method, and the results do reveal significant lateral variability in Q structure. The inferred attenuation variability corresponds to the heat-flow variations within the geothermal region. A central low-Q region corresponds well with the central high-heat flow region; additional detailed structure is also suggested.

Additional Information

© 1999, by the Seismological Society of America. Manuscript received 15 April 1999. We thank Joan Gomberg, Steve Hartzell, Mike Fehler, and two anonymous reviewers for helpful comments and suggestions; we thank Peter Malin for data and valuable comments, The authors thank the Navy Geothermal Program for funding J. Lees' efforts on this project (Awards N68936-94-R-0139 and N68936-97-C-0001), We further acknowledge CalEnergy Co. Inc. for making available their data, Figure 1 was generated using GMT software (Wessel and Smith, 1991); Figures 9a and 9b were generated using Xmap8 (Lees, 1995).

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August 19, 2023
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