Long-lived afterslip of the 2013 Mw 6.1 Minab earthquake detected by Persistent Scatterer Interferometry along the Irer fault (western Makran-Zagros transition zone, Iran)

Abstract

SUMMARY The ratio of seismogenic to aseismic deformation along active faults is needed to estimate their seismogenic potential and hazards. Seismologic and geodetic methods routinely capture coseismic displacements, but data acquisition requirements to fully document post-seismic deformation are not well known. Our study documents afterslip between about 18 months and 4 years after a mid-size earthquake and, based on remote structural mapping, we document fault rupture segments not previously associated with that earthquake. Persistent scatterer interferometric analysis of Sentinel-1A aperture radar data acquired between October 2014 and December 2018 reveals prolonged post-seismic deformation following the 11 May 2013 Mw 6.1 Minab earthquake and its aftershocks. The surface deformation data yield a sharp contrast across both the main seismogenic fault (here named the Irer fault) and its northeastern splay, and it is compatible with left-lateral motion along both faults. The PSI data helped us to identify and map the splay fault in the satellite imagery. We could then measure the geological offset along both faults, finding maximum displacements of about 1 km (main fault) and 350 m (splay). Our modelling of the observed post-seismic surface deformation pattern shows that post-seismic deformation was accommodated by left-lateral afterslip, not viscoelastic relaxation. This result is consistent with previous propositions that Mw 6 earthquakes do not measurably excite deeply seated viscoelastic relaxation mechanisms. Our afterslip modelling yields a slip pattern from the surface to a depth of 6 km to maximum 16 km, in agreement with the depth of the coseismic slip-distribution, and a maximum displacement of ∼7 cm along the fault, but located ∼8 km to the east of the coseismic slip maximum. Moment release during the observed afterslip in our study is Mw 5.7, or 12% of the coseismic moment released by main shock and aftershocks together. Combined with previously published results for the early post-seismic period (first 2 months), we estimate the aseismic moment to be at least ∼37% of the total, implying a high ratio of aseismic to seismic moment release for the Irer fault. Our results show that observation time windows well beyond 5 years are needed to record afterslip following mid-sized earthquakes. Thus, progress in understanding the transition from post-seismic to interseismic fault behaviour critically depends on the availability of data provided by satellite missions such as Copernicus Sentinel-1A. Similarly, robust comparison of the post-seismic rates with long-term geological rates requires palaeoseismic study and dating of related morphotectonic features.