A two-step inversion for fault frictional properties using a temporally varying afterslip model and its application to the 2019 Ridgecrest earthquake

The stress status and frictional parameters of a fault system control its slip behaviors over earthquake cycles. These parameters generally exhibit large spatial variations in a real fault system, and resolving such variations is of critical importance for realistic assessments of the seismic hazard. Slowly evolving afterslip reflects the slip response of a fault system to the coseismic stress loading, which is commonly observed following large earthquakes, thus providing information for constraining the fault stress and frictional parameters. In this study, we demonstrate that two independent and determinable frictional properties (i.e., σ(a−b) and Rinit=log⁡(vinit/v0), where σ, (a−b), vinit and v0 are the effective normal stress, velocity-dependence frictional parameters, initial slip velocity, and reference velocity, respectively) can be obtained from an afterslip evolution process. We propose a two-step strategy that uses the temporally segmented afterslip models to invert for the independent parameters. After validating the performance of our inversion procedure by synthetic tests, we apply it to the 2019 Ridgecrest earthquake afterslip process. Our results show that σ(a−b) in the main afterslip area is 0.2∼0.5 MPa. The spatial distribution of this parameter suggests a significant difference of pore pressure at the two ends of the coseismic rupture. The depth profile of the effective normal stress in the afterslip concentration area reveals that the pore fluid pressure is hydrostatic above 5 km depth and increases to lithostatic from 5 to 20 km, indicating a gradual permeability decrease in this depth range. Our analysis also shows that the afterslip models which assume a uniform slip evolution function cannot be directly used for the estimation of the frictional properties.