Source Characteristics of Small Earthquakes at the Northwest Geysers Geothermal Field, California

We investigate the source parameters of small earthquakes (<=M4) at the Northwest Geysers, CA, near an injection well, before and during water injection. Our objective is to understand the relation among injection, production and source mechanisms of earthquakes. To determine the earthquakes source parameters, such as fault radius, stress drop, seismic moment and radiated energy, we use two techniques: the Empirical Green's Function (EGF) method; and the NetMoment (NM) method. The EGF method is very efficient at extracting source information of micro-earthquakes in a heavily attenuating media such as The Geysers, as it empirically corrects for attenuation and site effects. However, the number of earthquakes for which it can be applied is limited by the availability of an EGF earthquake which acts as the media transfer function. The NM method is a good method to apply when several stations record the same event. It uses a simultaneous inversion for attenuation and source properties for a single event and all stations. NM runs automatically and works well with large datasets such as the one at The Geysers, with more than 19,000 small earthquakes reported in 2006. However the method potentially carries large uncertainties in the attenuation correction, as there is a trade-off between attenuation and source corner frequency. In this study, we use the source parameter estimates obtained with the EGF approach of a small number of earthquakes, to validate and further constrain the source parameters of a larger dataset obtained with the NM method. Our EGF results indicate that earthquakes at the Northwest Geysers have radii comparable to or smaller than, and stress drops (median of 31.4 MPa) comparable to or higher than those of deeper, larger (>M5.5), natural occurring tectonic earthquakes (where stress drop ranges from 17 to 55 MPa). We do not observe a systematic change of earthquake source properties with time (before and during injection), but the number of earthquakes analyzed before injection with the EGF method is very limited and not statistically significant. We expect to improve the statistical significance once we add results using the NM method. The source information of the Geysers earthquakes can be used to estimated the total area of the fractured surface of the reservoir, in which fluid will flow, has implications for the understanding of the physics of faulting and can help in the assessment of the seismic potential in areas of ongoing geothermal exploration.