Supershear earthquakes, propagating faster than the Earth's shear wave velocity, can generate strong ground motion at distances far from the ruptured fault. Despite the hazards associated with these earthquakes, the exact fault properties that govern their occurrence are not well constrained. Although early studies associated supershear ruptures with simple fault geometries, recent dynamic rupture models have revealed a supershear transition mechanism over complex fault geometries such as fault stepovers.Here we present the first observation of a supershear transition on a fault stepover system during the 2017 M w 7.7 Komandorsky Islands earthquake. Using a high-resolution back-projection technique, we find that the earthquake's rupture velocity accelerates from 2.1 to 5.0 km/s between two offset faults, demonstrating the viability of a new supershear transition mechanism occurring in nature. Given the fault complexity of the Earth's transform plate boundaries, this result may improve our understanding of supershear rupture processes and their associated hazards.Plain Language Summary Earthquakes traveling faster than the shear wave velocity of the earth are occasionally observed in large strike-slip events. These so-called supershear earthquakes generate a potentially destructive shock wave analogous to the sonic boom of a supersonic aircraft. Although many supershear earthquakes are coincident with straight, continuous fault sections, we find that the 2017 Komandorsky Islands earthquake reached supershear speeds following a jump in rupture across two fault segments. The results of this study confirm that supershear ruptures can occur on complex fault systems, allowing the seismic hazard on similar fault systems to be better evaluated.
An episode of unrest began at Kīlauea in April 2018 that produced both significant volcanic output and high rates of seismicity, including a Mw 6.9 earthquake on 4 May 2018. In this study, we image the rupture process of this earthquake using a genetic algorithm‐based back‐projection technique. The dominant feature of the earthquake is a slowly propagating western rupture, which shares similar characteristics with the region's largest recorded event in 1975 (Mw 7.7). The location of this western segment suggests that small asperities on this section of the décollement that frequently fail as slow slip events may achieve seismic slip rates when rupture is initiated on adjacent sections of the fault. Given the interaction between volcanic and seismic activity in this region, imaging the rupture properties of these events can improve our understanding of future geologic hazards in this region.
Summary Subduction zones are associated with significant seismic hazards around the world and determining the future locations of large earthquakes within these systems is a perpetual challenge of the Earth sciences. This study presents back-projection results from the 2021 Mw 7.1 Fukushima earthquake which show that the rupture area of this event filled a previously identified coseismic gap within the rupture area of the 2011 Mw 9.1 Tohoku-oki earthquake. These results, combined with observations of a similar coseismic gap from the 2010 Mw 8.8 Maule, Chile earthquake that was subsequently filled by a Mw 7.1 aftershock, demonstrate that future assessments of seismic hazards following giant earthquakes should include the identification of coseismic gaps left within mainshock rupture areas.
Deep‐focus earthquakes provide insight into how subducting slabs deform over a range of spatial and temporal scales as they descend into the mantle. This study uses a 4D source imaging approach to determine centroid locations of the 2015 Mw 7.9 Bonin Islands deep‐focus earthquake and its aftershock sequence. Imaged sources of the mainshock show a complex rupture, but one that is compatible with a sub‐horizontal rupture plane. Previously undetected early aftershocks are imaged down to depths of approximately 750 km and represent the first reported earthquakes that initiate in the lower mantle. These events and a previously reported group of shallower distal aftershocks occur at the lower and upper boundaries of an imaged slab segment that deforms as it penetrates into the lower mantle. We hypothesize that mainshock failure allowed gravitational settling of the slab segment to occur which produced the distal aftershock sequences.
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