Decoupled co-seismic deformation and stress changes during the 2021 (Mw 6.0, 6.4) Bandar Abbas earthquakes, SE-Syntaxis of Zagros, Iran; New insights into the rupture decoupling process

Abstract

The co-seismic properties of the Mw 6.0 (12:07:03 UTC) and Mw 6.4 (12:08:06 UTC) earthquakes that took place on 14 November 2021, Bandar Abbas Syntaxis, SE-Zagros Simply Folded Belt (SFB), Iran, are thoroughly examined. Understanding the earthquake ruptures and their relationship to the co-seismic deformations, critical to our knowledge about the earthquake source mechanisms, has provided a singular chance to interpret the details of the rupture procedure of these two interrelated earthquakes, to complement previous studies of seismicity. Here, using finite-fault source inversion, we first estimated the co-seismic source models and then the co-seismic displacements during the earthquakes, differentiated into vertical/horizontal components. We inverted the observed teleseismic broadband P-velocity waveforms of the earthquakes to simultaneously estimate the finite-fault rupture process, the slip distribution, the fault geometry and the stress changes. We found that the earthquakes were typical blind thrust-fault types along NW-SE and NE-SW striking fault lengths of ~40-50 km, widths of ~25-30 km, at a depth range of ~3-16 km and ~3-15 km, respectively, with co-seismic surface folding (~7-10 km) to NE controlled by a salt décollement layer at a depth range of ~10-12 km. We also found that the earthquakes consisted of relatively fast rupture sources (VR 3.3 km/s); an initial pure thrust faulting bilateral rupture at a depth of 12 km with a maximum slip of 30 cm and a dip angle of 32o, which was followed by a bilateral rupture with an oblique-slip left-lateral thrust faulting at a depth of 10 km, with a maximum slip of 80 cm and a dip angle of 24o propagated towards the NE. The joint interpretation of estimated Coulomb stress changes imparted by proposed variable slip rupture models, and the salt layer indicated that the stress increased load, triggered the fault planes of both events and influenced along-strike co-seismic strain distribution, providing evidence for the SW-NE trending activation of the stress decoupling between the ruptures, corresponding to the salt décollement. The initial pure thrust motion ruptured and mobilized the salt layer, then triggered and activated the bilateral rupture that generated the co-seismic detachment folds subparallel to the décollement. The weak salt, co-seismically ruptured and rapidly activated, compensated for co-seismic strain through lateral thickness changes from SW to NE and obliquely accommodated the folding in the shallow cover. Thus, basal ductile shear facilitated the change from pure thrust faulting in the basement to oblique thrust faulting in the cover. This finding clarifies differences in rupturing properties and deformation styles of such low-angle thrust faults. Anomalous interference patterns through superimposed fault planes of the Bandar Abbas earthquakes with the salt horizon have illuminated the rupture decoupling process and stress changes of the successive thick-/thin-skinned earthquakes, typical of the Zagros SFB.