SCEC Award Number 19207 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Understanding the triggering process of the foreshock sequence of the 2010 M7.2 El-Mayor-Cucapah earthquake
Investigator(s)
Name Organization
Xiaowei Chen University of Oklahoma
Other Participants Colin Pennington (graduate student)
SCEC Priorities 1d, 4a, 5a SCEC Groups Seismology, FARM, EFP
Report Due Date 04/30/2020 Date Report Submitted 11/03/2020
Project Abstract
 Understanding the triggering mechanism of foreshocks and their relationship to large earthquakes is very important for earthquake forecasting and hazard mitigation. Two models have been proposed to foreshocks: “pre-slip” where foreshocks are triggered by aseismic processes in the mainshock nucleation zone; “cascading failure” where foreshocks are triggered by series of cascading failure. Continuing debates are ongoing for the triggering mechanism of foreshocks. In this study, we reanalyze the foreshocks for the 2010 M7.2 El-Mayor-Cucapah earthquake using new data, improved spectral analysis method and new locations. The results from the new method show good agreement with results from spectral ratio analysis. The results show increased level of stress interaction within 0.5 km of the mainshock starting about 9 days before the mainshock, which accelerated during the last day. This suggests cascading failure during foreshocks, and between foreshocks and the mainshock. The results also show some isolated foreshocks with no stress interactions, and some foreshocks occurring within rupture zones of previous earthquakes, which suggests possible aseismic stressing process. Collectively, these observations support an alternative foreshock model: “swarmy foreshock”, where the foreshock themselves are independent of the mainshock nucleation process, but the foreshocks and the possibly associate aseismic process contribute to the stress loading at the mainshock hypocenter.
Intellectual Merit The research contributes to new understanding of the triggering process during the El-Mayor-Cucapah earthquake mainshock. The results suggest the importance of improved data recording and analysis method in understanding triggering processes. In practice, there is no clear boundary between the two end-members of foreshock triggering processes. Both aseismic slip and cascading failure can occur during foreshock sequences. The results support the alternative “swarmy foreshock” model, and highlight the importance to understand earthquake swarms in seismic hazard evaluation. In addition, comparison of source parameter estimates suggests the improved stacking method can obtain the same results for the same events from spectral ratio analysis.
Broader Impacts The project results are beneficial for learning earthquake hazards, risks and earthquake physics. The project contributed to the training and education of students and postdocs at OU. Graduate student Jiewen Zhang adapted and refined an improved stacking algorithm, and postdoc Qimin Wu developed and refined a new time-domain deconvolution method.
Exemplary Figure Figure 3. Left: Along-depth view of stress changes. Right: Map view of stress changes. The black “+” indicates the M7.2 mainshock in both figures. Red color is stress increase, and blue is stress decrease. All the following figures use the same colorbar (+/-0.1 MPa).
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