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 the Parkfield section of the San Andreas fault (SAF), providing an invaluable near‐fault observatory. We present a seismo‐geodetic dynamic inversion of the 2004 Parkfield earthquake, which illuminates the interlinked complexity of faulting across time scales. Using fast‐velocity‐weakening rate‐and‐state friction, we jointly model coseismic dynamic rupture and the 90‐day evolution of postseismic slip in a 3D domain. We utilize a parallel tempering Markov chain Monte Carlo approach to solve this non‐linear high‐dimensional inverse problem, constraining spatially varying prestress and fault friction parameters by 30 strong motion and 12 GPS stations. From visiting 2 million models, we discern complex coseismic rupture dynamics that transition from a strongly radiating pulse‐like phase to a mildly radiating crack‐like phase. Both coseismic phases are separated by a shallow strength barrier that nearly arrests rupture and leads to a gap in the afterslip, reflecting the geologic heterogeneity along this segment of the SAF. Coseismic rupture termination involves distinct arrest mechanisms that imprint on afterslip kinematics. A backward propagating afterslip front may drive delayed aftershock activity above the hypocenter. Trade‐off analysis of the 10,500 best‐fitting models uncovers local correlations between prestress levels and the reference friction coefficient, alongside an anticorrelation between prestress and rate‐state parameters . We find that a complex, fault‐local interplay of dynamic parameters determines the nucleation, propagation, and arrest of both, co‐ and postseismic faulting. This study demonstrates the potential of inverse physics‐based modeling to reveal novel insights and detailed characterizations of well‐recorded earthquakes.