Postseismic Process Inversion Using Full Time Series of Surface Deformation: Full Time-Series Inversion (FTI) Theory and Its Application to the 2017 Sarpol-e Zahab Earthquake

Postseismic processes provide important opportunities to probe into and investigate the frictional, viscous, and porous properties of the seismogenic fault and the surrounding Earth media. To accommodate the temporal and spatial resolutions and long-term baseline stability of different deformation data, we develop a full time-series inversion (FTI) technique, which jointly inverts for afterslip patterns using full time series of Global Navigation Satellite System, SAR, and strainmeter data. The FTI linearizes the inversion problem with a prescribed source evolution function to achieve efficient inversion. We conduct synthetic tests to validate the spatial and temporal resolution of the FTI algorithm. FTI outperforms static inversion techniques in terms of inversion stability under high noise level. We apply different parameterization strategies to evaluate its resolution for slip evolution parameters. The tests show that FTI can discriminate spatially separated afterslip with distinct evolution functions. Finally, we apply FTI to investigate the afterslip process following the 2017 M-w 7.3 Sarpol-e Zahab earthquake that occurred along the Iran-Iraq border in northwestern Zagros using Synthetic Aperture Radar Interferometry time series derived from the Sentinel-1 observations 1 year after the mainshock. Similar to the synthetic tests, the algorithm is capable to discriminate afterslip with different evolution functions in the up- and downdip portions of the coseismic rupture zone. By comparing with the stress-driven afterslip model simulated using rate-strengthening frictional law, we demonstrate the stability of FTI in resolving the afterslip process. We emphasize the importance of incorporating early postseismic observations for deciphering afterslip evolution and frictional parameters.