Linking seismological observables and dynamic simulations of microseismicity to constrain models and improve observations

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Earthquakes are typically generated by rapid slip, or dynamic rupture, on pre-existing faults inside the Earth, loaded by tectonic plate motion. While large, destructive ruptures occur only rarely, much smaller events, often called microseismicity, occur much more often, with the vast majority of them not felt by people. However, the surface motion due to these much smaller events can be recorded by sensitive instruments called seismometers and analyzed to reveal their location and properties. In fact, microseismicity provides us with a unique window into the stress environment, structure, and physical properties of the Earth's crust and their changes over time. In California, for example, most of microseismicity occurs in the same depth range that is expected to host large, destructive events. However, the ground motions recorded for the microseismicity are currently analyzed based on relatively simple models of earthquake sources from several decades ago. The rapid recent progress in computing, laboratory experiments, and observations has enabled us to create much more realistic numerical models of how these microearthquake ruptures might occur. This study will use new numerical models of microseismicity to advance understanding and improve information about the properties of the crust, gained through the important window of microseismicity studies, while advancing our understanding of fault mechanics and of realistic sources of microseismicity. The resulting insight into earthquake physics will contribute to the physics-based simulation capability for seismic hazard analysis. These results may benefit the society at large.The project will advance understanding of several scientifically and practically important types of microearthquake sources, such as foreshock-like events, repeating earthquakes, ruptures of heterogeneous patches, and fluid-injection-induced events, by producing and analyzing the synthetic far-field seismograms of their dynamic models. The goals are to (i) determine how well the synthetic spectra are fit with the currently used expressions based on the simple models, determine source parameters using the existing seismological approaches, and study how they relate to the actual source properties that can be determine from these models; (ii) look for seismological signatures that would indicate that the sources in question are more complex than the basic seismological assumptions, and potentially distinguish between several characteristic types of sources; and (iii) investigate the possibility of creating alternative approaches to seismological source-parameter estimates for such sources, using models more consistent with the specific seismological signatures identified. The study will examine whether different types of sources can be associated with a typical set of parameters in the currently used spectral fitting expressions, or whether some of them require more advanced approaches such as two-corner-frequency functions. It will also look at how the spectral properties for different types of sources vary over the focal sphere and consider differences between P- and S-wave spectra, which may be different for different types of sources. The research will potentially enable observational scientists to conduct more detailed seismological studies in areas with suitable station coverage, knowledge of the structure, and data quality. The results of the proposed activities will be broadly disseminated. Several graduate and undergraduate students, including from groups that are underrepresented in science, will gain valuable research and teaching experience carrying out the proposed interdisciplinary activities. In addition, a wider group of students will be educated about the issues involved in this research through course projects in two relevant courses taught by the researcher.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.