The 2019-2020 Khalili (Iran) Earthquake Sequence-Anthropogenic Seismicity in the Zagros Simply Folded Belt?

We investigate the origin of a long-lived earthquake cluster in the Fars arc of the Zagros Simply Folded Belt that is colocated with the major Shanul natural gas field. The cluster emerged in January 2019 and initially comprised small events of M-n similar to 3-4. It culminated on 9 June 2020 with a pair of M-w 5.4 and 5.7 earthquakes, which was followed by >100 aftershocks. We assess the spatiotemporal evolution of the earthquake sequence using multiple event hypocenter relocations, waveform inversions, and Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) measurements and models. We find that the early part of the sequence is spatially distinct from the 9 June 2020 earthquakes and their aftershocks. Moment tensors, centroid depths, and source parameter uncertainties of 15 of the largest (M-n >= 4.0) events show that the sequence is dominated by reverse faulting at shallow depths (mostly <= 4 km) within the sedimentary cover. InSAR modeling shows that the M-w 5.7 mainshock occurred at depths of 2-8 km with a rupture length and maximum slip of similar to 20 km and similar to 0.5 m, respectively. Our results suggest that the 2019-2020 Khalili earthquake sequence was likely influenced by operation of the Shanul field, though elevated natural seismicity in the Zagros makes the association difficult to prove. Understanding how to distinguish man-made from natural seismicity is helpful for hazard and risk assessment, notably in the Zagros, which is both seismically active and rich in oil and gas reserves. Plain Language Summary Earthquakes caused by human activities have been documented in a growing number of regions worldwide, but recognizing these events in areas of naturally elevated seismicity remains challenging. We investigate the origin of a long-lived earthquake cluster in the Fars arc of the Zagros Simply Folded Belt-one of the world's most seismically active mountain belts-that is colocated with a major natural gas field. The seismicity led to public concern and speculation that nearby natural gas extraction was responsible for the seismicity. We assess the spatiotemporal evolution of the earthquake sequence using satellite geodesy and seismology measurements and models. Our results support these being anthropogenic earthquakes, most probably related to operations in the gas field. We suggest that the exploitation of the reservoirs in Iran should be preceded by risk assessment studies and accompanied by the implementation of dedicated, sophisticated monitoring, which would allow seismicity to be detected early and tracked more closely.

Automated summary

A long-lived earthquake cluster in the Fars arc of the Zagros Simply Folded Belt, potentially influenced by the Shanul natural gas field operations, highlights the challenge of distinguishing between anthropogenic and natural seismicity in a region with high natural seismic activity. Understanding this distinction is crucial for hazard and risk assessment in the seismically active and resource-rich Zagros region.