Coseismic and early post-seismic deformations due to the 2019 earthquake sequence in Ridgecrest, California

Abstract

SUMMARYThe 2019 Ridgecrest earthquake sequence in the Mojave Desert that ruptured a complex orthogonal conjugate fault system offers a rare opportunity to probe the mechanics of the intraplate lithosphere of the central eastern California shear zone (ECSZ). We used space geodetic data to investigate the coseismic and post-seismic deformations attributable to this earthquake sequence. A triangular dislocation inversion scheme was used to derive the coseismic and post-seismic slip distributions on five non-planar faults in a homogeneous elastic crust. A wide range of viscoelastic relaxation models with varying upper mantle viscosities were also tested to constrain the rheological structure. The inferred coseismic slip exhibited a pronounced (approximately 60 per cent) shallow slip deficit (SSD), only a small proportion of which was recovered by early aseismic afterslip. Inversions of GPS and InSAR data suggest that the near-field post-seismic transient was dominated by afterslip at depths of 3–8 km. Viscosity modelling yielded a robust lower bound of approximately 8.0 × 1017 Pa s on the viscosity of the upper mantle. Both the GPS observations and the modelled viscosity at each individual station showed a lateral heterogeneous relaxation process on both northeast and southwest ECSZ. The effective viscosity of the mantle asthenosphere to the northeast was inferred to be approximately two times lower than that to the southwest, which is consistent with the pattern expected for regional heat flow. A comparison among the static coseismically induced Coulomb stress changes, the cumulative distribution of aftershocks and the afterslip distribution suggests that aftershocks and shallow afterslip could be responses to coseismically induced stresses, but the aftershock and afterslip distribution are poorly correlated. We argue that the pronounced SSD and lack of shallow afterslip during the Ridgecrest earthquake sequence are indicative of an immature fault. Furthermore, we suggest similar rheological structures for the southern and central ECSZ and infer a relatively low effective viscosity for the mantle to the northeast because of its high heat flow. We also propose that the afterslip may illuminate the rate-strengthening regions that mostly slip aseismically, but the aftershocks may illuminate fluid-saturated areas near ruptures.