Daniel R. H. O'Connell

Progressive inversion for hypocenters and P wave and S wave velocity structure: Application to the Geysers, California, Geothermal Field

Seismicity at The Geysers is induced by some aspect of steam production. Accurate earthquake locations, particularly hypocentral depth, are needed to determine the relationship between geothermal energy production and seismicity. Progressive P and S wave velocity-hypocenter inversions were done using data from 39 microearthquakes at The Geysers to estimate microearthquake locations and determine if the geothermal field has a distinctive seismic signature. Comparable final Vp, Vs and Vp/Vs velocity models were obtained from four different starting velocity models. The elevation interval of maximum steam production coincides with minimum observed Vp/ Vs and Vp/ Vs increases below the primary production zone, suggesting that reservoir rock becomes more fluid saturated with increasing depth. A Vp/ Vs peak at the condensation zone-production zone elevation delineates the top of the stream reservoir. Earthquake locations are confined to two depth intervals. A zone of shallow seismicity occurs immediately below the condensation zone and above maximum depths of steam production. A more arealy restricted zone of seismicity is located below maximum production depths and is abruptly terminated at an elevation of −3.7 km.

Source Characterization and Ground-Motion Modeling of the 1892 Vacaville–Winters Earthquake Sequence, California

A sequence of several earthquakes in April 1892 produced significant damage in the towns of Winters, Dixon, Allendale, and Vacaville along the boundary between the southwestern Sacramento Valley and northern Coast Ranges of California. The largest event occurred on 19 April 1892 with a maximum Modified Mercalli intensity of IX and was assigned a moment magnitude ( M ) of 6.5 based on felt area. These earthquakes occurred within a zone of active crustal shortening accommodated by postulated blind thrust faults. Seismotectonic and structural analyses are used to evaluate the depth, geometry, and segmentation of thrust faults that were the probable sources of the 1892 earthquake sequence. Synthetic ground-motion modeling demonstrates that rupture of a 17-km-long segment of the thrust fault system can produce the magnitude and distribution of intensities documented from anecdotal accounts of the 19 April 1892 earthquake, including probable directivity effects east of the range front. Integrated structural and seismotectonic analyses also are used to interpret the role of inferred geometric segment boundaries in arresting the 19 April 1892 earthquake rupture, and the subsequent occurrence of the 21 April 1892 aftershock.

Assessing Ground Shaking

Monitoring and modeling the complex interaction of seismic waves with soils is critical for mitigating earthquake risks.