SCEC Award Number 12017 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Collaborative proposal from UCSD and Caltech: SCEC Community Data Products of Relocated Seismicity, Improved Focal Mechanisms, and Waveform Spectra for Resolving Fine-Scale Fault Structures and State of Stress in Southern California
Investigator(s)
Name Organization
Peter Shearer University of California, San Diego Egill Hauksson California Institute of Technology
Other Participants Xiaowei Chen (UCSD graduate student)
Postdoc (Caltech, to be named)
SCEC Priorities 2a, 2d, 2f SCEC Groups Seismology, USR, Geology
Report Due Date 03/15/2013 Date Report Submitted N/A
Project Abstract
 This research involves a collaboration between Caltech and UCSD to perform automatic processing of the Southern California Seismic Network (SCSN) waveform archive. Our SCEC work has focused on improving earthquake locations using waveform cross-correlation, and on computing spectra for use in studies of earthquake source properties and attenuation. In 2011, we released the latest version of our relocated catalog (the HYS catalog) containing high-precision locations of over 500,000 events from 1981 to June 2011. The complex spatial distribution of the events reflects the different processes that contribute to the generation of both background and triggered seismicity. The Pacific North-America plate tectonic deformation is the main process that causes major earthquakes and their aftershock sequences. Secondary processes such as geothermal exploitation, extensional gravitation collapse, and crustal delamination also cause ongoing seismicity. The overall pattern of seismicity reveals fault structures on many scales, as well as mainshock-aftershock sequences such as 1992 Landers, 1994 Northridge, 1999 Hector Mine, and 2010 El Mayor-Cucapah. Other regions such as the southern Sierra and Coso regions, the San Jacinto fault, and the Salton Trough also have high ongoing levels of seismicity. The latest significant earthquake sequence that we have analyzed is the 2012 Brawley swarm. It started near the town of Brawley at 4h30m (GMT) on August 26 with three events of M < 2.0 occurring within 5 minutes. The seismic activity picked up again at 11h43m (GMT), and continued at a steady rate. The three largest earthquakes (M5.3, M4.9, and M5.4) in the sequence occurred over a period of 90 minutes, starting at 19h31m. The largest (M5.4) earthquake was widely felt across southernmost California, northern Baja California and western Arizona.
Intellectual Merit Our research relates to many key SCEC objectives, including characterizing seismicity on a variety of spatial and temporal scales. Our precise earthquake location catalogs are important because they provide direct constraints on physical properties at depth. These constraints help address key questions in earthquake physics and crustal deformation, including the relationship between microseismicity and major faults, the size and scale lengths of variations in absolute stress orientation and in earthquake stress drops, and the driving mechanism for aftershocks and swarm activity.
Broader Impacts This project helped support female graduate student Xiaowei Chen. Outreach activities consisted of providing the relocated catalog to SCEC scientists and others doing research on seismicity in southern California. The relocated catalog is available at the Southern California Earthquake Data Center (SCEDC). Our research helps to better characterize faults and earthquake clustering, which has broad implications for earthquake forecasting and predictability. Advances in these areas would have clear societal benefits.
Exemplary Figure Figure 2. The seismicity from 1981 to 2012 August is shown as black open circles, and the events of the 2012 August Brawley swarm are shown as red circles. Lower hemisphere focal mechanisms of the M > 5 events are shown. IRC - Imler Road creepmeter; WLA – Wildlife Liquefaction Array. The east and west boundaries of the Brawley seismic zone as defined by Johnson and Hill (1982) are shown with red dash-lines. Late Quaternary fault traces (magenta) from Jennings and Bryant (2010).