The 12 November 2017 M_w 7.3 Ezgeleh‐Sarpolzahab (Iran) Earthquake and Active Tectonics of the Lurestan Arc

Abstract

The 12 November 2017 Mw 7.3 Ezgeleh‐Sarpolzahab earthquake is the largest instrumentally recorded earthquake in the Zagros Simply Folded Belt by a factor of ∼10 in seismic moment. Exploiting local, regional, and teleseismic data and synthetic aperture radar interferometry imagery, we characterize the rupture, its aftershock sequence, background seismicity, and regional tectonics. The mainshock ruptured slowly (∼2 km/s), unilaterally southward, for ∼40 km along an oblique (dextral‐thrust) fault that dips ∼14°E beneath the northwestern Lurestan arc. Slip is confined to basement depths of ∼12–18 km, resolvably beneath the sedimentary cover which is ∼8 km thick in this area. The gentle dip angle and basement location allow for a broad slip area, explaining the large magnitude relative to earthquakes in the main Fars arc of the Zagros, where shallower, steeper faults are limited in rupture extent by weak sedimentary layers. Early aftershocks concentrate around the southern and western edges of the mainshock slip area and therefore cluster in the direction of rupture propagation, implying a contribution from dynamic triggering. A cluster of events ∼100 km to the south near Mandali (Iraq) reactivated the ∼50° dipping Zagros Foredeep Fault. The basement fault responsible for the Ezgeleh‐Sarpolzahab earthquake probably accounts for the ∼1 km elevation contrast between the Lurestan arc and the Kirkuk embayment but is distinct from sections of the Mountain Front Fault that define frontal escarpments elsewhere in the Zagros. It may be related to a seismic interface underlying the central and southern Lurestan arc, and a key concern is whether or not the more extensive regional structure is also seismogenic.

Impulsive Source of the 2017 MW=7.3 Ezgeleh, Iran, Earthquake

“…While the rupture directivity imaged here is visible in published slip models (Chen et al, ; Nissen et al, ), the sharpness of the slip pulse in these studies is blurred by their use of far‐field data and smoothing constraints. The good spatial and temporal resolution of our kinematic slip model enabled by the use of near‐field observations reveals interesting features.…”

“…We use a Bayesian framework to infer a population of coseismic slip models that fit available observations. While currently available studies were either limited to the static final distribution of slip on the fault (Barnhart et al, ; He et al, ; Vajedian et al, ; Wanpeng et al, ; Yang et al, ) or used far‐field teleseismic data (Chen et al, ; Nissen et al, ), we jointly invert interferometric synthetic aperture radar (InSAR) and near‐field strong‐motion data which provide a better resolution (Anderson, ) to propose a kinematic description of the earthquake source. We use a layered velocity model that is routinely used to locate earthquakes by the Iranian Seismological Center, which ensures modeling is performed to the best of our knowledge (supporting information Table S1).…”

Seismotectonics of the Zagros (Iran) From Orogen‐Wide, Calibrated Earthquake Relocations

“…The low level of seismicity within the interior Lurestan arc and adjacent High Zagros supports Berberian's () interpretation that seismic hazard is heavily concentrated along the master blind thrusts. However, more gently dipping faulting responsible for the recent M w 7.3 Ezgeleh‐Sarpolzohab earthquake may underlie interior parts of the Lurestan arc, as discussed by Nissen et al (). Seismicity is much more diffuse in the Dezful embayment where we observe activity concentrated in the interior embayment both on and in between the master thrust faults (Figure ).…”

“…In contrast, seismicity in the Lurestan arc is focused strongly along the frontal escarpments and the mapped locations of the Zagros Foredeep and Mountain Front Faults. This includes the recent M w 7.3 Ezgeleh‐Sarpolzahab (2017) sequence, which ruptured an E dipping (∼15°) dextral‐thrust basement fault in the northern Lurestan arc that is approximately colocated with, albeit highly oblique to Berberian's () Mountain Front Fault (e.g., Barnhart et al, ; Chen et al, ; Nissen et al, ). Several of the other mainshock‐aftershock sequences in the Lurestan arc, such as at Moosiyan (2008, 2012), Qasr‐e Shirin (2013), Murmuri (2014), and Mandali (2018), also appear to have ruptured the Zagros Foredeep or Mountain Front Faults (Copley et al, ; Nippress et al, ; Nissen et al, ).…”

Impulsive source of the 2017, Mw =7.3, Ezgeleh, Iran, earthquake

“…al., 2018; Yang et al, 2018;Vajedian et al, 2018) or used far-field teleseismic data (Chen et al, 2018;Nissen et al, 2019), we jointly invert InSAR and near-field strong-motion data which provide a better resolution (Anderson, 2003) to propose a kinematic description of the earthquake source. We use a layered velocity model that is routinely used to locate earthquakes by the IRSC (Iranian Seismological Center), which ensures modelling is performed to the best of our knowledge ( Supplementary Table T1).…”