EAR-PF: Properties of the shallow crust around the San Jacinto Fault Zone characterized by joint geodetic and seismic analysis

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Dr. Christopher W. Johnson has been granted an NSF EAR Postdoctoral Fellowship to carry out research and education plans at the University of California San Diego and the University of Southern California. The objective of this project is to develop classifiers for earthquakes, microseisms, and ambient noise using parameters of the waveforms and utilize statistical learning techniques to train a computer program to identify previously undetected microseismic events in upper 1 km of the crust. The properties of the near surface require further exploration to constrain processes that generate earthquakes and characterize the temporal variations in seismic and geodetic records. The expected results from this project will advance the understanding of seismic ground motion, surface deformation, transient deformation signals, and elastic and hydrologic crustal properties for the community at large. This study provides the framework for nascent earthquake detection techniques applicable to regional seismic networks. During the investigation, Dr. Johnson will interact and train undergraduate and graduate students.The dominant fault system along the Pacific-North American plate boundary is the San Andreas Fault (SAF) that bisects dense urban city centers along the California coast. Southeast of Los Angeles, CA the SAF bifurcates into the San Jacinto Fault Zone (SJFZ), where microseismic events occur daily and six M5 earthquakes have ruptured in the past 35 years. Fully characterizing the SAF and SJFZ are of great importance to infrastructure planning and hazard mitigation. A recent dense array deployment of 1,100 seismometers recorded ground motions at the SJFZ for 5 weeks. The data set provides a unique opportunity to develop new techniques that characterize the shallow subsurface from 0-1 km depth and fully describe the fault zone environment from the surface to the brittle-ductile transition zone at ~18 km depth. Preliminary results from the deployment indicate 120 small events in one day of the array data, significantly more than the 15 earthquakes detected by the regional seismic network. Labeling as many features of the data as possible will aid in efficiently processing large volumes of data, thereby reducing the computational expense of microseismic earthquake detection. The detailed classification of continuous data should also produce a qualitative description of patterns in the noise and highlight episodes of coherent energy that may represent previously unexplored failure processes in the shallowest portions of the crust. This is a novel study utilizing data from the Earthscope PBO networks to further advance the understanding of plate boundary fault systems.