Recent earthquake sequences at Coso: Evidence for conjugate faulting and stress loading near a geothermal field

Two recent earthquake sequences near the Coso geothermal field show clear evidence of faulting along conjugate planes. We present results from analyzing an earthquake sequence occurring in 1998 and compare it with a similar sequence that occurred in 1996. The two sequences followed mainshocks that occurred on 27 November 1996 and 6 March 1998. Both mainshocks ruptured approximately colocated regions of the same fault system. Following a comparison with the background seismicity of the Coso region, we have detected evidence of stress loading within the geothermal field that appears to be in response to the 1998 earthquakes. The ML = 5.2 mainshock in the 1998 sequence occurred at 5:47 a.m. UTC and was located approximately 45 km north of the town of Ridgecrest in the Coso range. The mainshock of the 1996 sequence had an ML magnitude of 5.3. There have been no observable surface ruptures associated with either of these sequences. Though the mainshocks for both sequences were located about 900 m apart and have nearly the same local magnitudes, the sequences differ in both their temporal and spatial characteristics. An analysis of the fault-plane solutions of the mainshocks and the aftershock locations suggests that the two sequences ruptured fault planes that are perpendicular to one another. We observe a much faster temporal decay of the 1998 sequence compared to the one in 1996; moreover, while the 1996 sequence was not followed by any sizeable (i.e., ML > 4.0) aftershocks, the 1998 sequence had four such events. From an estimate of the tectonic stressing rate on the fault that produced the 1998 sequence, we infer a repeat cycle of 135 years for an earthquake of comparable magnitude at Coso.