Abstract
To study interaction of tectonics and earthquakes, we need to address two fundamental questions, Earth's internal structure and the cause of triggered/inhibited seismicity. High-resolution seismic velocity models and analyses of high-precision relocated earthquake have proven to provide useful information from global to regional scale. In areas of active tectonics and/or anthropogenic activities, the stress heterogeneity could largely result from lateral and vertical variation of structure in local scale, which accounts for the non-uniform earthquake distribution. We have mainly conducted two regional studies in Puerto Rico, the plate boundary zone, and the active Coso geothermal field. By inverting for three-dimensional velocity model and relocating earthquakes, we find out the active seismicity in the southwestern island of Puerto Rico and the seismic quiescence in the north could be separated by a high velocity body underneath the Great Southern Puerto Rico Zone dipping to the north. Beneath the Coso geothermal field, our results reveal an anomalous low velocity zone corresponding to the ductile behavior at depths from 6 to 12 km. We suggest that the pervasive melts are not likely to exist in the upper crust and the identified magmatic system could be frozen, felsic, with the inclusion of water. In Coso, by thorough analyses of spatiotemporal distribution of earthquakes, we present a novel observation that the geothermal field is less susceptible to remote triggering than the surrounding areas. Combining with the observations of an abrupt drop of background seismicity with respect to the geothermal operation, the continuous net production rate, the strong subsidence for the reservoir, we imply that the absence of remote triggering could result from loss of pore pressure accompanying the long-term loss of pore fluid. This phenomenon has also be recognized in the Salton Sea geothermal field, which share similar tectonic and anthropogenic settings as the Coso geothermal field. Our results have also shown that the geothermal production process has altered the stress orientation by adding more normal-faulting components. To further understanding how the stress state and failure conditions are affected by the anthropogenic activity, it is necessary to conduct systematic case studies in different areas with presence of both natural and induced earthquake and include other studies such as independent stress measurements, hydraulic response to stress, and mapping structure of faults and fractures.