SCEC Award Number 13020 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Seismicity Patterns, Swarms, and Foreshocks in Southern California
Investigator(s)
Name Organization
Peter Shearer University of California, San Diego
Other Participants Xiaowei Chen, graduate student
SCEC Priorities 2b, 2c, 5d SCEC Groups Seismology, EFP
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
 This SCEC funded research involves continued analysis of earthquakes recorded by the Southern California Seismic Network (SCSN). This has led to greatly improved earthquake locations, focal mechanisms, and estimates of stress drop. We are now using these products to perform integrated studies of seismicity and 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 (i.e., ETAS) triggering models, including details of the foreshock and aftershock behavior for small earthquakes. In particular, we have found that much small earthquake clustering is likely caused by underlying physical drivers, such as fluid flow or slow slip. 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.
Broader Impacts This project helped support female graduate student Xiaowei Chen and incoming student Wei Wang. Our research will help 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 1. Recent analyses of small earthquakes in southern California indicate that many earthquake swarms and several foreshock sequences are caused by an underlying physical driving mechanism, such as fluid flow or slow slip, rather than the earthquake-to-earthquake triggering mechanism that causes aftershock sequences following large earthquakes. See Shearer (2012b) and Chen and Shearer (2013).