Modeling of co- and early postseismic deformation due to the 2019 Ridgecrest earthquake sequence

Kang Wang, & Roland Bürgmann

Published August 15, 2019, SCEC Contribution #9887, 2019 SCEC Annual Meeting Poster #234

On July 4th, 2019, a Mw 6.4 earthquake struck the Searles Valley near the town of Ridgecrest in southern California. The Mw 6.4 earthquake was followed by a Mw 7.1 event about 30 hours later. Both the Mw 6.4 foreshock and the Mw 7.1 mainshock occurred on previously unidentified faults, although there has been intense background seismicity in this area, including along the complex Little Lake fault zone and Airport Lake fault zone to the west of the 2019 rupture sequence. Reports from field surveys and satellite imagery, including InSAR and optical images, indicate a complex network of faults activated by the 2019 Ridgecrest earthquakes. Here we use Sentinel-1 InSAR and GPS data to study the co- and early postseismic deformation due to this earthquake sequence. Since first postseismic SAR images from both ascending and descending tracks were taken after the Mw 7.1 earthquake, we invert for the combined coseismic slip due to both the Mw 6.4 and the Mw 7.1 earthquakes. The preferred slip model is characterized by a combination of left-lateral slip on a southwestward (SW) trending fault, which likely primarily ruptured during the Mw 6.4 foreshock, and right-lateral slip on a series of northwestward (NW) trending segments, which probably includes contributions from both events. The slip mostly concentrates in a depth range above 10 km, with a maximum slip of ~5 m. Assuming that the Mw 6.4 foreshock primarily ruptured the SW-trending fault, we find that the Mw 6.4 rupture imparted positive shear stress change and unclamping on the NW-trending faults near the Mw 7.1 hypocenter. Relocated aftershocks reveal that south of the Mw 7.1 epicenter, where the rupture trace bifurcates, almost all aftershocks a few days after the mainshock occurred along a structure 3-4 km east of the main rupture trace. This is consistent with our geodetic inversion results that favor an east dipping fault segment along the main rupture trace that connects to the eastern branch at depth. We also observe clear near-field postseismic deformation one month after the mainshock. The early postseismic InSAR observations are characterized by subtle linear range change features across many of the fault traces, and significant deformation near junctions and stepovers of the fault segments. The postseismic deformation is indicative of both modest shallow afterslip and poroelastic rebound.

Citation
Wang, K., & Bürgmann, R. (2019, 08). Modeling of co- and early postseismic deformation due to the 2019 Ridgecrest earthquake sequence. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Ridgecrest Earthquakes