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., Sigma (a - b) and Rinit = log(vinit/v0), where Sigma, (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 Sigma (a - b) in the main afterslip area is 0.2 similar to 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.(c) 2022 Elsevier B.V. All rights reserved.

Automated summary

The stress and frictional parameters of a fault system are important for understanding its slip behavior during earthquake cycles. Variations in these parameters are critical for assessing the seismic hazard. Afterslip, which occurs after large earthquakes, can provide information about the fault stress and frictional parameters. In this study, we propose a two-step strategy to determine these parameters using afterslip models. Our analysis of the 2019 Ridgecrest earthquake afterslip process reveals spatial variations in the effective normal stress and suggests differences in pore pressure along the fault rupture. The depth profile of the effective normal stress also indicates changes in permeability.