Understanding variability in the size and location of large earthquakes along subduction margins is crucial for evaluating seismic and tsunami hazards. We present a coseismic slip model for the 2021 M 8.2 Chignik earthquake and investigate the relationship of this earthquake to previous major events in the Alaska Peninsula region and to interseismic coupling. Stress changes from the 2020 M 7.8 Simeonof event triggered the Chignik event, and together, the earthquakes partially filled an unruptured section along a 3000-km subduction margin that has experienced a series of ruptures along almost its entire length over the past century. Variations in coupling and structural characteristics may make the region more prone to nucleating M 7 to M 8 events rather than larger M > 8.5 earthquakes. Stress changes and rupture areas suggest that the two recent earthquakes may be part of an 80-year-long rupture cascade and may have advanced seismic hazard in the region.

The Ölfus seismic belt lies at the western end of the ~E‐W sinistral transform shear zone in South Iceland, called the South Iceland Seismic Zone (SISZ), where most seismicity and surface faulting show ~N‐S dextral slip. Unlike the rest of SISZ, seismicity in west Ölfus is predominantly along the ~ENE‐WSW direction. Throughout recorded history, Ölfus has shown an interactive behavior with the Hengill volcanic system that lies northwest of the zone. For instance, the 13 November 1998 Mw 5.1 earthquake in the Hjalli area (west Ölfus) and its ~ENE trending aftershock sequence were likely triggered by the 4 June 1998 Mw 5.4 Hengill earthquake sequence. These events point to an interplay between conjugate ~N‐S and ~ENE‐WSW faults in the region. Relative relocations of earthquakes in Hjalli‐Ölfus from July 1991 to December 1999 (Icelandic Meteorological Office, 2017) are chiefly limited to 4‐ to 8‐km depth along the ~ENE direction with a few distributed on smaller ~N‐S faults. The foreshocks of the November 1998 earthquake occurred on a ~N‐S fault until a day prior to the mainshock when they shifted to the ~ENE direction. The subsequent aftershocks are also mainly restricted to the ~ENE direction. We find that the Mw 5.1 (Global Centroid Moment Tensor moment = 5.43 × 1016 N‐m) Hjalli‐Ölfus earthquake ruptured a near‐vertical ~ENE fault area of 24–40 km2 with left‐lateral average slip of 5–8 cm. Multiple relocations of the mainshock using various constraints indicate that the event likely occurred close to the junction of the conjugate ~ENE‐WSW and ~N‐S faults.

No abstract

No abstract