SCEC Award Number 11158 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Relocating the Northernmost Aftershocks Following the 4 April 2010 Mw 7.2 El Mayor-Cucapah Earthquake
Investigator(s)
Name Organization
Elizabeth Cochran University of California, Riverside
Other Participants Kayla Kroll (UCR graduate student)
SCEC Priorities A4, A10, A7 SCEC Groups Seismology
Report Due Date 02/29/2012 Date Report Submitted N/A
Project Abstract
 Following the 4 April 2010 Mw 7.2 El Mayor-Cucapah earthquake, eight temporary seismometers were installed in the Yuha Desert region north of the Mexican border. During the deployment period, between 6 April and 14 June 2010, over 4,300 aftershocks ranging in magnitude from M 0.1 to M 4.85, within a 20 km by 14 km study area are reported in the Southern California Earthquake Data Center catalog. We compute the double difference hypocenter relocations using hypoDD with both manually picked P and S phase arrivals and waveform cross-correlations. We initially locate the events with Hypoinverse using manual phase picks and the Southern California Earthquake Center Community Velocity Model, Version 4 (SCEC CVM-S4), Imperial Valley model. We improve the locations by jointly inverting for a new velocity model, station corrections and locations with the VELEST algorithm. After relocation, the mean horizontal and vertical relative relocation errors were reduced to 23 m and 82 m, respectively. Relocated seismicity is highly correlated with faults observed to have surface slip as mapped by Rymer et al., (2011) (i.e. Laguna Salada-East, Yuha Well Fault, Yuha Fault, Vista de Anza Fault, and several normal faults). We identify spatio-temporal aftershock patterns, with some faults active for short durations during the observation period. We propose that the dense cluster of aftershocks in the Yuha Desert region is the result of increased stress at the end of the mainshock rupture in region of complex, crosscutting, immature faults.
Intellectual Merit Aftershocks of the 2010 El Mayor – Cucapah earthquake cluster in two areas, one southwest of the epicenter near the northern tip of the Gulf of California and the other approximately 75 km northwest of the epicenter in the Yuha Desert region (west of Calexico, CA), that separates the Laguna Salada fault in the south and the Elsinore and San Jacinto faults in the north (Figure 1). The northern cluster of aftershocks provides a unique opportunity to conduct a forensic investigation of the fine scale fault structure between the termination of the San Andreas fault in the Salton Trough region and the fault systems of the US-Mexico border zone where plate motion is being accommodate in a complex manner. Additionally, we are able to explore the uncertain relationship between the Laguna Salada and Elsinore faults. Identification of a link between these faults is particularly important for seismic hazard, as rupture on both segments could produce earthquakes > M7 in an urban area.
Broader Impacts The project involved both graduate and undergraduate students in installing seismometers following the El Mayor Cucapah earthquake. The research reported here forms the Masters thesis of Kayla Kroll an underrepresented (female) student in the Earth Sciences.
Exemplary Figure Figure 3: Relocated seismicity (red dots) overlain on faults previously mapped (green lines) by Isaac (1986) and faults that experienced surface offset or creep during the EMC (black lines) as mapped by Rymer et al. (2011).