The first few days of the 2019 Ridgecrest earthquake sequence

Benjamin Idini, Mark Simons, Minyan Zhong, Oliver Stephenson, Zachary E. Ross, Eric J. Fielding, Sang-Ho Yun, Egill Hauksson, Chris Milliner, Angelyn W. Moore, & Zhen Liu

Published August 15, 2019, SCEC Contribution #9884, 2019 SCEC Annual Meeting Poster #276

A nearly 20-year-long hiatus in significant seismic activity in southern California ended on July 4, 2019 with a sequence of earthquakes near the city of Ridgecrest, CA. This sequence included a M6.4 foreshock followed by a M7.1 mainshock nearly 34 hours later. Here, we focus on the use of available GNSS, satellite imaging and seismic data. We use SAR data from the ALOS-2 and Sentinel-1 satellites to produce interferograms from ascending and descending viewing geometries. We also derive range and azimuth offsets from speckle tracking. From the geodetic imaging data combined with a seismicity catalog derived from template matching, we derive a complex pattern of surface faulting. Many of the characteristics of the ruptures are reminiscent of those seen previously for the 1992 Landers (CA) and 1999 Hector Mine (CA) earthquakes which occurred further south in the Eastern California Shear Zone. Unfortunately, the satellite radar data we have spans the time period of the two largest events. Thus, we initially infer a model for subsurface slip that combines the effects of both events. Our approach relies on a Bayesian unregularized inversion method that is solved with using an MCMC-based algorithm. This approach attempts to explicitly include the impact of both errors in our observations as well as errors in our forward model. The assumed fault geometry is simplified for the purposes of the slip inversion. We attempt to explicitly separate the effects of the foreshock and mainshock, through the inclusion of observations made using cross-correlation of optical satellite imagery from Planet Labs that includes scenes taken in the period between the two primary events. We also use the GNSS data that resolve the events separately. Preliminary models for the combined events indicate a minimum of 6 m of slip at depth, with slip dominated by 3 asperities, one corresponding to the SW-NE trending M6.4 event, one on the NW-SE trending fault near the hypocenter of the M7.1, and a 3rd one on the southern extent of the same structure approaching but not crossing the Garlock Fault. We also discuss the distribution of slip as a function of depth and the relationship between the inferred distribution of subsurface slip and seismicity.

Citation
Idini, B., Simons, M., Zhong, M., Stephenson, O., Ross, Z. E., Fielding, E. J., Yun, S., Hauksson, E., Milliner, C., Moore, A. W., & Liu, Z. (2019, 08). The first few days of the 2019 Ridgecrest earthquake sequence. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Ridgecrest Earthquakes