%0 Journal Article
%@ 0193-5933
%A Viegas, Gisela
%A Hutchings, Lawrence
%A Lawrence Berkeley National Laboratory, Berkeley CA,
%A Lawrence Berkeley National Laboratory, Berkeley CA,
%D 2011
%F epos:1842
%I Geothermal Resources Council
%J Geothermal Resources Council Transactions
%K Geysers, micro-earthquakes, induced seismicity, earthquake source, stress drop, radiated energy, Empirical Green’s function
%P 1773-1780
%T Characterization of Induced Seismicity near an Injection Well at the Northwest Geysers Geothermal Field, California
%U https://episodesplatform.eu/eprints/1842/
%V 35
%X We investigate the temporal and spatial dependence of source parameters of micro-earthquakes (<M3) before and during water injection at the Northwest Geysers between 2005 and 2010. Our objective is to understand the relation among injection, production and source mechanisms of micro-earthquakes. We examine a small area that surrounds an injection well (Prati 9) that extends into the deep zone. We utilize three approaches to determine the source parameters of the micro-earthquakes; the Empirical Green’s Function (EGF) method (Viegas et al., 2010), NetMoment method (Hutchings 2002), and moment tensor inversion (Minson and Dreger, 2008). We first compare the source parameters of 30 earthquakes determined using the three approaches for validation purposes, and then we determine the source parameters for all the earthquakes located within a small volume around the well head before and during injection. We find a good correlation coefficient of 91% between the monthly water injection-rate and the number of induced micro-earthquakes located inside the small volume, with a zero time lag, indicating that the seismic response to water injection is less than a month time. We find the b-value in the Gutenberg−Richter law, which equates the proportion of small earthquakes to large ones, increased from 1.3 to 1.6 with the start of water injection, indicating an increase of the number of small earthquakes relative to larger earthquakes due to reservoir stimulation. Our results indicate that micro-earthquakes at the Northwest Geysers have on average stress drops (mean of 11 MPa) comparable to the ones of natural occurring tectonic earthquakes in the region (around 17 MPa). We notice that the shape of the earthquake cloud is slightly elongated in the SW-NE direction, consistent with the preferential alignment direction of microcracks found in anisotropic studies, indicating that slip is being facilitated in pre-existing cracks. The study of micro-seismicity is a useful tool in reservoir exploration management, as it can   be used to track the release of strain and the injected fluid flow paths, and to characterize the permeability of the reservoir. The source information has implications for understanding the physics of faulting and the principal mechanisms involved in induced seismicity.