Hi All. This in part has been taken from Jonathan M. Lees page who is an Associate Professor of Geology and Geophysics. Take Care…Don in creepy town

Two major features with low Poisson's ratio are identified at geothermal production depth (1--3 km) around stations S2-S6 and S1-S3-S4. The two low \sigma features are separated by a northwest-southeast trending high \sigma belt with variable width of 1 ~ 3 km. A high \Psi body is found around S2 and S6, and extends down in depth. A circular, low \Psi belt corresponding to the high \sigma belt is located around S2-S6 and is linked to a previously reported structure in V_s tomography. This low \Psi (highly porous) belt is probably a horizontal conduit/reservoir of geothermal fluid. A vertical, low \sigma and high \Psi channel beneath triangle S1-S3-S4 corresponds to a high attenuation, dome-like feature. We propose an upwell-and-spread magma intrusion model for the last major magmatism in the Coso region. The magmatic upwelling is centered in the S1-S3-S4 area. The model predicts potential geothermal resources to the south and west of triangle S1-S3-S4 based on local faulting patterns.

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, which occurred on November 27, 1996 and March 6, 1998. Both mainshocks ruptured approximately co-located 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 M_L = 5.2 mainshock in the 1998 sequence occurred at 5:47 am UTC, and was located approximately 45 km north of the town of Ridgecrest in the Coso range. The mainshock of the 1996 sequence had a M_L 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, which 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., M_L > 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.

• Jonathan M. Lees Page