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Time-dependent earthquake tomography in Southern California

Jing Hu, Hongrui Qiu, Pieter-Ewald Share, Jiawei Qian, Haijiang Zhang, & Yehuda Ben-Zion

Published August 14, 2019, SCEC Contribution #9558, 2019 SCEC Annual Meeting Poster #042

Temporal changes of seismic velocities have been determined through analyses of earthquake and ambient noise data in Southern California and elsewhere, but there is limited understanding of the spatial distribution and amplitudes of these changes. In this study, we apply a time-dependent earthquake tomography technique to systematically monitor 3-D velocity perturbations in the upper crust of Southern California. The employed time-dependent earthquake tomography utilizes differential times between pairs of nearby earthquakes that occurred at different times. We use P first arrival picks of earthquakes from 2000 to 2016 recorded at 365 stations in Southern California. The 17-year time span is separated into 99 intervals, each containing ~1000 events, and velocity changes are monitored between successive time periods. We first use all the P first arrivals and the CVM-H15.1 as an initial model to derive a common 3-D Vp model and refined earthquake locations, using double-difference inversions on both pairs of nearby sources and pairs of stations. Second, we utilize the common 3-D Vp model as an initial model to separately invert a 3-D Vp model for each time period using the associated P data. Finally, we resolve velocity changes between two adjacent time periods by employing the inverted 3-D Vp model and relocated earthquakes of the earlier time period as the initial velocity model and event locations. We divide the study area into five regions and compute the average velocity changes in each region. We find that Vp increases and then significantly decreases at a depth interval of 10-13 km in the aftershock zone of the 2010 Mw 7.2 El Mayor-Cucapah earthquake during five days following the mainshock. Numerous other velocity changes are observed in various regions during the 17 yr duration of this study, but none appears to be significant. Updated results will be presented in the meeting.

Key Words
Velocity changes, time-dependent tomography, Southern California

Hu, J., Qiu, H., Share, P., Qian, J., Zhang, H., & Ben-Zion, Y. (2019, 08). Time-dependent earthquake tomography in Southern California. Poster Presentation at 2019 SCEC Annual Meeting.

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