Efforts to understand the build-up and release of stress along active fault zones require detailed studies of individual faults and placement of those results in a physical framework consistent with our knowledge about rock mechanics. One particularly intriguing fault zone behavior that has been observed with increasing frequency in recent years involves the interplay between aseismic slip and seismic swarm activity. Slow fault slip has been observed to trigger earthquake swarms in a variety of tectonic environments including subduction zones, volcanic systems, and continental strike-slip faults such as those in close proximity to population centers in Southern California.
One region that provides a prime environment to study seismic swarms and aseismic creep is the Imperial Valley/Salton Trough in Southern California. Seismic swarms, large (Mw>7) earthquakes, and transient episodes of fault creep are all frequent occurrences along the plate boundary faults within the Salton Trough. The Salton Trough offers a prime opportunity to examine the interplay between these varied types of fault activity, since so many of them are observed within a relatively small geographic area, which also has had very detailed geologic studies and contains one of the world?s densest seismic and geodetic station networks. Detailed studies of aseismic and seismic events offer an opportunity to connect physical properties of the fault-zone such as rock type, depth, and temperature with rock-mechanical properties such as frictional stability, and will allow us to better understand the temporal evolution of seismic hazard associated with individual fault zones.
We propose a combination of an active source seismic study of the 2005 swarm's primary fault zone, and a more detailed analysis of the existing seismic and geodetic data. The core of our effort is a one-week seismic survey using the high-res profiling techniques developed by the USGS-Menlo Park group that will provide constraints on the details of fault-zone geometry and rigidity structure. These results will be used to improve our studies of the 2005 swarm. The combination of these studies will allow us to study the relations between rock-type, rigidity, thermal structure and frictional stability. Because shallow creep and earthquake swarms are general properties of major faults in the Salton Trough, our results will provide inputs to physics based earthquake hazard models in the region.
Within the Imperial Valley/Salton Trough region in Southern California, major faults with the potential for large earthquakes cut through one of the most productive agricultural regions in the country. Extension across the faults has resulted in volcanism and high levels of heat flow that have attracted numerous large geothermal power plants as well, which withdraw and reinject water as part of the process of generating energy. Increasingly, faults within this region have been observed to exhibit anomalous behaviors - instead of the typical "mainshock-aftershock" sequence, there are many earthquake swarms (some potentially linked to human activity) and slow motion along the faults in episodes that last from weeks to years. In some ways this is desirable, since this aseismic motion releases stress along the faults without causing damaging earthquakes. However, aseismic motion can potentially increase stress on adjacent faults that are not sliding. In this project, we combined an active source seismic experiment in the field with additional processing of seismic, GPS and space-based interferometric synthetic aperture radar (InSAR) imagery, to better constrain activity in the subsurface. We focused on the region immediately south of the termination of the San Andreas Fault, where swarms of earthquakes and aseismic slip have been observed every few years. The main outcomes of this project were: Generation of a 3-dimensional velocity model for the subsurface, based on the active source experiment The relocations of seismicity during the swarms and other time periods, using both the standard networks and new stations that were recently made available through the geothermal power companies Reprocessing of the InSAR data at higher resolution, identifying multiple strands of aseismic slip that were not visible during the initial standard processing Identification of a close correlation between the velocity model, seismicity, and the shallow temperature gradient, suggesting that alteration of the subsurface by geothermal fluids is likely controlling the distribution of stress and seismicity Re-assessment of the likely subsurface structures that were active during the aseismic slip/swarm, with implications for the distribution of seismic hazard for this fault system Broader impacts for this project included the support of an early career female PI, as well as the education of multiple graduate students, both through funding of their research activities and through the inclusion of students from three different institutions in the field exercise. Intellectual merit for this activity included the combined use of seismicity, GPS, InSAR and temperature gradient data, assimilated into a new model of the constitutive properties and planes of weakness within the subsurface.
