Fault Rerupture during the July 2019 Ridgecrest Earthquake Pair from Joint Slip Inversion of InSAR, Optical Imagery, and GPS

Abstract

ABSTRACT The Ridgecrest earthquake pair ruptured a previously unknown orthogonal fault system in the eastern California shear zone. The stronger of the two, an Mw 7.1 earthquake that occurred on 6 July 2019, was preceded by an Mw 6.4 foreshock that occurred 34 hr earlier. In this study, distinct final slip distributions for the two earthquakes are obtained via joint inversion of Interferometric Synthetic Aperture Radar (InSAR), optical imagery, and Global Positioning System (GPS) measurements. Special attention is paid to the merging of dense (e.g., InSAR and optical imagery) and sparse geodetic (e.g., GPS) datasets. In addition, a new approach is introduced for data and model discretization through intermittent model- and data-space reconditioning that stabilizes the inversion, thus ensuring that small changes in the data space do not cause disproportionate large changes to the model space. Although the coseismic slip of the Mw 6.4 earthquake was complex, involving three distinct asperities distributed among an intersecting orthogonal set of faults, the coseismic slip of the Mw 7.1 earthquake was limited to the main northwest-striking fault. In addition to the Mw 7.1 earthquake, that northwest-striking fault plane also hosted one of the Mw 6.4 asperities. Slip on this coplanar foreshock asperity increased the shear stress at the future site of the Mw 7.1 hypocenter, and triggered a vigorous aftershock activity on the main northwest fault that culminated in its rupture. This, in turn, reactivated the coplanar foreshock asperity. In addition to failing twice within 34 hr, we find that the reruptured asperity slipped about six times more during the Mw 7.1 than during the Mw 6.4 earthquake. This repeated failure is indicative of an incomplete stress drop and premature rupture arrest during the Mw 6.4 foreshock, requiring an efficient frictional strengthening and emphasizing the causal link between highly rate-dependent friction, dynamic frictional restrengthening, and partial stress drop that has been observed in numerical studies of frictional sliding.