The Linked Complexity of Coseismic and Postseismic Faulting Revealed by Seismo‐Geodetic Dynamic Inversion of the 2004 Parkfield Earthquake

Abstract

Several regularly recurring moderate-size earthquakes motivated dense instrumentation of theParkfield section of the San Andreas fault (SAF), providing an invaluable near-fault observatory. We present aseismo-geodetic dynamic inversion of the 2004 Parkfield earthquake, which illuminates the interlinkedcomplexity of faulting across time scales. Using fast-velocity-weakening rate-and-state friction, we jointlymodel coseismic dynamic rupture and the 90-day evolution of postseismic slip in a 3D domain. We utilize aparallel tempering Markov chain Monte Carlo approach to solve this non-linear high-dimensional inverseproblem, constraining spatially varying prestress and fault friction parameters by 30 strong motion and 12 GPSstations. From visiting >2 million models, we discern complex coseismic rupture dynamics that transition froma strongly radiating pulse-like phase to a mildly radiating crack-like phase. Both coseismic phases are separatedby a shallow strength barrier that nearly arrests rupture and leads to a gap in the afterslip, reflecting the geologicheterogeneity along this segment of the SAF. Coseismic rupture termination involves distinct arrest mechanismsthat imprint on afterslip kinematics. A backward propagating afterslip front may drive delayed aftershockactivity above the hypocenter. Trade-off analysis of the 10,500 best-fitting models uncovers local correlationsbetween prestress levels and the reference friction coefficient, alongside an anticorrelation between prestressand rate-state parameters b a. We find that a complex, fault-local interplay of dynamic parametersdetermines the nucleation, propagation, and arrest of both, co- and postseismic faulting. This study demonstratesthe potential of inverse physics-based modeling to reveal novel insights and detailed characterizations of well-recorded earthquakes.