SCEC Award Number 15164 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title A systematic search for tremor using multiple mini seismic arrays near San Jacinto Fault
Investigator(s)
Name Organization
Abhijit Ghosh University of California, Riverside
Other Participants Allie Hutchison, grad student
SCEC Priorities 1c, 5a, 5d SCEC Groups FARM, Transient Detection, Seismology
Report Due Date 03/15/2016 Date Report Submitted 11/10/2016
Project Abstract
 Several short duration tremor events are detected and located in San Jacinto Fault (SJF) near Anza, part of the fault segment characterized by lower seismicity rate relative to either sides. Three mini seismic arrays, network standalone stations, both surface and borehole, are used to detect, cross-check and locate tremor using multiple independent techniques. All techniques locate tremor near the Anza Gap. Array analyses indicate that they occur at the deeper part of the fault. In addition, the locations found in this study are consistent with the locations of the tremor in this area triggered by the 2002 Mw 7.9 Denali earthquake. This study shows that tremor does occur spontaneously under San Jacinto Fault. Based on the preliminary tremor locations, it appears that slow slip at the deeper part of the fault may play a role to accommodate plate motion and resulting stress in the San Jacinto Fault. It has direct implications on the fault dynamics and the seismic hazards in this area. This study has laid the foundation for a more exhaustive and detail study of tremor and transient deformation in the major fault systems in southern California.
Intellectual Merit Studying the full spectrum of fault slip is imperative to understand physics of earthquakes. Different modes of fault slip – fast and slow – typically occur in different parts of fault defining slip dynamics and how fault accommodate stress. So far, slow earthquakes remain elusive in the fault systems of southern California. In this study, we discover ambient tremor in San Jacinto Fault near Anza Gap. Based on the array analyses, they appear to be located at a deeper part of the fault. A comprehensive understanding of slow earthquakes and their spatiotemporal distribution provide new insights into the location of the transition zone at depth and therefore has profound implications on the evolution of state of stress and seismic hazards. In addition, it may help resolving long-standing issues on how SJF accommodate deformation at depth.
Broader Impacts Anza Gap is thought be close to a large damaging earthquakes. Understanding how SJF releases stress has important implications on the seismic hazards in southern California. This project is led by an early career scientist (the PI) and supports a female graduate student. This work will be part of her PhD thesis. In addition, multiple graduate students participated in a field excursion related to this study. Data used in this study and results produced are used in the discussion sessions in many group meetings involving both undergraduate and graduate students.
Exemplary Figure Figure 3: Example of a tremor window detected and located in this study. The map shows the location of tremor, seismic stations and arrays. Bottom left corner shows the waveforms of tremor (shaded time). Colors of the shades indicate different arrays. Bottom right plot shows and compare spectrum of tremor, a small local earthquake and background noise. Three plots in the upper right show beams of the tremor signal. All show focused beam with small slowness suggesting coherent energy from a deep source.
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