On August 12, 2021 a > 220 s lasting complex earthquake with Mw > 8.2 hit the central and southern South Sandwich trench. Due to its remote location and short interevent times, reported earthquake parameters varied significantly between different international agencies. We studied the complex rupture by combining different seismic source characterization techniques sensitive to different frequency ranges based on teleseismic broadband recordings from 0.001–2 Hz, including point and finite fault inversions and the back-projection of high-frequency signals. We also determined moment tensor solutions for 88 aftershocks. The rupture initiated simultaneously with a Mw 7.6 thrust earthquake in the deep part of the seismogenic zone in the central subduction interface and a shallow megathrust rupture which propagated unilaterally to the south with a very slow rupture velocity of 1.2 km/s and varying strike following the curvature of the trench. The slow rupture covered nearly two thirds of the entire subduction zone length, and with Mw 8.2 released the bulk of the total moment of the earthquake. Tsunami modelling indicates the inferred shallow rupture can explain the tsunami records. The southern segment of the shallow rupture overlaps with another activation of the deeper part of the megathrust equivalent to a Mw 7.6. The aftershock distribution confirms the extent and curvature of the rupture. Some mechanisms are consistent with the mainshocks, but many indicate also activation of secondary faults. Rupture velocities and radiated frequencies varied strongly between different stages of the rupture, which might explain the variability of published source parameters.

We investigate the origin of a long-lived earthquake cluster in the Fars arc of the Zagros Simply Folded Belt that is co-located with the major Shanul natural gas field near the small settlement of Khalili. The cluster emerged in January 2019 and initially comprised small events of w 5.4 and 5.7 earthquakes, which were followed by > 100 aftershocks. We assess the spatio-temporal 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 June 9, 2020 earthquakes and their aftershocks. Moment tensors, centroid depths, and source parameter uncertainties of fifteen of the largest (Mn ≥ 4.0) events show that the sequence is dominated by reverse faulting at shallow depths (mostly ≤ 4 km) within the sedimentary cover. InSAR modelling shows that the Mw 5.7 mainshock occurred at depths of 2–8 km, with a rupture length and maximum slip of ~20 km and ~0.5 m, respectively. Our results strongly suggest that the 2019-2020 Khalili earthquake sequence was influenced by the operation of the Shanul field, making these the first known examples of gas extraction anthropogenic earthquakes in Zagros. Understanding the genesis of such events to distinguish man-made seismicity from natural earthquakes is helpful for hazard and risk assessment, notably in Iran which is both seismically-active and rich in oil and gas reserves.

The month-to-year-long deformation of the Earth’s crust where active subduction zones terminate is poorly explored. Here we report on a multidisciplinary dataset that captures the synergy of slow-slip events, earthquake swarms and fault-interactions during the ~5 years leading up to the 2018 M 6.9 Zakynthos Earthquake at the western termination of the Hellenic Subduction System (HSS). It appears that this long-lasting preparatory phase initiated due to a slow-slip event that lasted ~4 months and released strain equivalent to a ~M 6.3 earthquake. We propose that the slow-slip event, which is the first to be reported in the HSS, tectonically destabilised the upper 20-40 km of the crust, producing alternating phases of seismic and aseismic deformation, including intense microseismicity (M<4) on neighbouring faults, earthquake swarms in the epicentral area of the M 6.9 earthquake ~1.5 years before the main event, another episode of slow-slip immediately preceding the mainshock and, eventually, the large (M6.9) Zakynthos Earthquake. Tectonic instability in the area is evidenced by a prolonged (~4 years) period of overall suppressed b-values (<1) and strong earthquake interactions on discrete strike-slip, thrust and normal faults. We propose that composite faulting patterns accompanied by alternating (seismic/aseismic) deformation styles may characterise multi-fault subduction-termination zones and may operate over a range of timescales (from individual earthquakes to millions of years).