Coupled mechanism of the postseismic deformation of blind thrust fault : Case study of the 2017 Iran/Iraq M_W 7. 3 earthquake

The phase of postseismic deformation constitutes a pivotal component within the seismic cycle, and a comprehensive exploration of its underlying mechanistic origins serves to elevate our comprehension of not only the intricate trajectory of seismic evolution but also the inherent characteristics of faults and the evaluative discernment of seismic risk. The abundant InSAR observational data from the 2017 M-W 7. 3 earthquake in Iran presents a substantial resource, affording an entry point for the investigation of the postseismic deformation mechanisms along blind thrust faults within the piedmont fold belt of the Zagros orogenic belt. A prerequisite for investigating post-earthquake afterslip or viscoelastic relaxation is the development of a detailed seismic source model and precise deformation observations. In this study, we combine multi-perspective InSAR data with far-field waveform information to invert the slip distribution of the 2017 M-W 7. 3 earthquake in Iran, and obtain the pseudo-3D surface displacement in the source area. Utilizing InSAR time series analysis, we extract post-earthquake deformation characteristics within the epicentral area over a year and a half following the main shock. The results reveal similarities between postseismic deformation and coseismic displacement fields. Along the LOS profile, the maximum LOS deformation rate reaches around 8 cm center dot a(-1), suggesting significant postseismic stress adjustment within the source area. Constraining the purely kinematic afterslip model with deformation data obtained within six months post-earthquake reveals an energy release equivalent to that of a moment magnitude MW 6. 7 seismic event during the same period. Further investigation into the respective contributions of stress -driven afterslip and viscoelastic relaxation within the lower crust to the ensuing postseismic deformation is undertaken. Through simulation employing a layered viscoelastic model, it is demonstrated that a coupled model encompassing both afterslip and viscoelastic relaxation effects yields a more comprehensive explication of the observed postseismic deformation characteristics. In this coupled model, postseismic afterslip is primarily concentrated in the shallow upper region above the co-seismic rupture area, exerting a predominant influence on surface postseismic deformation within a year and a half following the main shock. It is conjectured that the minimum viscosity coefficient of the lithospheric layer beneath the source region approximates 1/10(19) Pa center dot s. The postseismic deformation also exhibits a certain degree of facilitative impact on the growth of the Zagros fold belt. It is noteworthy that the investigation of longer-period postseismic deformation mechanisms necessitates sustained, high-precision observations and a consideration of lateral heterogeneity within the lithospheric rheological structure.