Site Specific Probabilistic Seismic Hazard and Risk Analysis for Surrounding Communities of The Geysers Geothermal Development Area

Abstract:

We conduct a probabilistic seismic hazard and risk analysis from induced and tectonic earthquakes for a 50 km radius area centered on The Geysers, California and for the next ten years. We calculate hazard with both a conventional and physics-based approach. We estimate site specific hazard. We convert hazard to risk of nuisance and damage to structures per year and map the risk. For the conventional PSHA we assume the past ten years is indicative of hazard for the next ten years from M<4.5 earthquakes. Larger earthquakes are added from evaluation of b-values. For the physics-based appraoch, we utilize computations (Heidbach and Altmann, 2013) to calculate pressure as a function of position throughout The Geysers for the next 10 years. We then use another geo-mechanical modeling code (Bachmann et al., 2012) to calculate the number of earthquakes that will occur. We identify amplification factors for specific sites within each geologic unit from recordings of noise. Then, we interpolate within each geologic unit in finely gridded points. All grid points within a unit are weighted by distance from each data collection point. The entire process is repeated for all of the other types of geologic units until the entire area is gridded and assigned a hazard value for every grid points.

We found that nuisance and damage risks calculated by both conventional and physics-based approaches provided almost identical results. This is very surprising since they were calculated by completely independent means. The conventional approach used the actual catalog of the past ten years of earthquakes to estimate the hazard for the next ten year. While the physics-based approach used geotechnical modeling to calculate the catalog for the next ten years. Similarly, for the conventional PSHA, we utilized attenuation relations from past earthquakes recorded at the Geysers to translate the ground motion from the source to the site. While for the physics-based approach we calculated ground motion from simulation of actual earthquake rupture. Finally, the source of the earthquakes was the actual source for the conventional PSHA. While, we assumed random fractures for the physics-based approach. From all this, we consider the calculation of the conventional approach, based on actual data, to validate the physics-based approach used.