Decoupled co-seismic deformation and stress changes during the 2021 (Mw 6.0, 6.4) North Bandar Abbas doublet earthquakes in Fin region, SE-syntaxis of Zagros, Iran: new insights into the tectonic deformation 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 nite-fault source inversion, we rst 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 nite-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 (V R 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 32 o , 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 24 o 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 in uenced 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 nding clari es 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-/thinskinned earthquakes, typical of the Zagros SFB. al., 2011) and centroid depths of high-quality...

Uncovering a Seismogenic Fault in Southern Iran through Co-Seismic Deformation of the Mw 6.1 Doublet Earthquake of 14 November 2021

“…Golshadi et al [16] endeavored to distinguish between a thrust and a back thrust in the Khurgu (Namak) and Handun anticlines. Toker et al [17] presented the following results: the first fault plane was characterized by parameter values of 114°, 32°, and 91° for the strike, dip, and rake, with a SW dip, while the second fault plane had parameter values of 49°, 24°, and 27°, with a SE dip. They stated that the co-seismic displacement was confined to a depth range of 3 to 16 km.…”

“…The investigations by Golshadi et al [16] and Toker et al [17] revealed significant discrepancies in fault locations a ributable to differing interpretations and strike orientations of the causative faults. From our perspective, the identified faults, particularly the back thrust, are only approximations due to insufficiently reliable data.…”

“…Previous studies, such as those conducted by Golshadi et al [16], Toker et al [17], and Rezapour and Jamalreyhani [41], predominantly relied on local seismic networks.…”

“…Toker et al [17] used teleseismically recorded broadband P-velocity waveforms in their analysis of finite-fault source inversion using data from the IRSC and identified two faults as the cause of the events.…”

“…Golshadi et al [16] and Toker et al [ 17] a ributed the events to two different faults. However, these faults are approximately 50 km apart and have different orientations.…”