SCEC Award Number 12018 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Seismicity Patterns, Swarms, and Foreshocks in Southern California
Name Organization
Peter Shearer University of California, San Diego
Other Participants Xiaowei Chen (UCSD graduate student)
SCEC Priorities 2b, 2c, 2e SCEC Groups Seismology, EFP, WGCEP
Report Due Date 03/15/2013 Date Report Submitted N/A
Project Abstract
We study seismicity patterns in southern California to address a number of issues related to seismic hazard. We have recently focused on studying earthquake triggering models and their relationship to swarms and foreshock sequences. We have identified several aspects of the space/time clustering of seismicity that cannot be explained with standard earthquake-to-earthquake triggering models, including differences in precursory seismicity behavior between large and small earthquakes and details of the foreshock and aftershock behavior for small earthquakes. We have developed a method to quantify seismicity migration in event clusters and find that most swarms exhibit migration whereas most aftershock sequences do not. Swarm migration velocities suggest both slow slip and fluid diffusion mechanisms are involved. We find that extended foreshock sequences more closely resemble swarms than earthquake-to-earthquake triggering cascades, in which there is no fundamental difference between foreshocks, mainshocks, and aftershocks. Our results support previous work that has suggested that major California foreshock sequences are not caused by static stress triggering and may be driven by aseismic processes. Ongoing results of this work include a more detailed understanding of earthquake source properties and seismicity patterns. This knowledge contributes to quantitative assessments of earthquake potential and seismic hazard in southern California.
Intellectual Merit Our research relates to many key SCEC objectives, including characterizing seismicity clustering and its implications for earthquake prediction. Our main contribution has been to systematically and objectively examine large amounts of earthquake data, to test whether existing models are adequate to explain the observations. This knowledge contributes to quantitative assessments of earthquake potential and seismic hazard in southern California.
Broader Impacts This project helped support female graduate student Xiaowei Chen. Our research helps to quantify earthquake clustering and triggering, which has broad implications for earthquake forecasting and predictability. Advances in these areas would have clear societal benefits.
Exemplary Figure Figure 3. (left) Estimated event stress drops versus time for foreshocks and aftershocks of the Landers, Hector Mine, and El Major earthquakes. Note the lower median stress drops for the foreshock sequences. (right) Stacked source spectra for the foreshocks (red) versus aftershocks (black), normalized for moment differences, confirming the lower frequency content of the foreshocks.