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Absolute and relative focal depth determination of moderate-sized earthquakes: An example from the 2010 El Mayor-Cucapah earthquake sequence

Chunquan Yu, Egill Hauksson, Zhongwen Zhan, Elizabeth S. Cochran, & Donald V. Helmberger

Published August 14, 2018, SCEC Contribution #8520, 2018 SCEC Annual Meeting Poster #056

Earthquake focal depth is a key parameter for understanding the geometry of fault zones, the rheology of the lithosphere and the tectonics of the continents. It is also crucial for seismic hazard assessment. Routine determination of earthquake focal depth relies on travel times of major body-wave phases. It works well in the presence of a dense nearby seismic network, but the uncertainty greatly increases as the distance to the nearby station increases. Precise focal depth can be determined, however, if regional depth phases are identified along with their reference phases. The differential travel time between the depth phase and its reference phase, which is most sensitive to the focal depth, can be determined with regional waveform modeling. We demonstrate our procedure using data from the 2010 Mw 7.2 El Mayor-Cucapah earthquake sequence (M>4.0). The focal depths for this earthquake sequence were poorly determined in the Southern California catalog as they are located near the edge of, or outside, the Southern California Seismic Network. We determine the absolute depth of ~30% of all M>4.0 earthquakes (122) in this sequence using regional waveform modeling. For other events with weak or complicated depth phases, we further develop a relative relocation method using Pn differential travel times in addition to Pg and Sg differential travel times. The incorporation of Pn differential travel times greatly reduces the uncertainty of relative depth estimation. In total, we are able to determine the focal depth of about ~75% of the 122 M>4.0 events. Our results show tight depth distribution of the 2010 El Mayor-Cucapah earthquake sequence, with the majority located between 3-10 km. The depth distribution of these earthquakes is similar to focal depths determined using a temporary seismic network. The relatively shallow depth range is consistent with the expectation from high surface heat flows.

Yu, C., Hauksson, E., Zhan, Z., Cochran, E. S., & Helmberger, D. V. (2018, 08). Absolute and relative focal depth determination of moderate-sized earthquakes: An example from the 2010 El Mayor-Cucapah earthquake sequence. Poster Presentation at 2018 SCEC Annual Meeting.

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