Earthquake‐Related Structures Beneath the Southernmost Portion of the Ryukyu Arc and Forearc

Moderate to large earthquakes repeatedly occur beneath the southernmost Ryukyu arc and forearc, but the seismogenic structures were poorly studied. To better understand the southernmost Ryukyu seismogenic structures, we have deployed ocean bottom seismometers to record the aftershocks of a large earthquake in 2015. As a result, several groups of aftershocks are associated with the seismogenic structures. A deep group coincides with the north‐dipping seismogenic zone that terminates near the tip of the mantle wedge beneath the Ryukyu Arc. A shallow group dips southward from the southern slope of the Ryukyu Arc to the southern end of the aftershocks; this group shows south‐dipping normal faulting character and accompanies the lower crust exhumation of Ryukyu Arc above the seismogenic zone. A plate interface step‐down beneath the seismogenic zone is inferred. A crustal underplating and exhumation around the seismogenic zone have taken place synchronously during the earthquake event. Plain Language Summary Large earthquakes frequently occur at the plate boundary where two plates converge. Those earthquakes usually appear at a segment of the downgoing plate surface, called a seismogenic zone. Seismogenic zones are extensively studied in order to understand the possible earthquake cycles, crustal stress variation, mechanisms of earthquake nucleation, rupture propagation, and so on. Historically, the southernmost Ryukyu seismogenic zone has generated numerous earthquakes but its seismogenic structures are still unclear. By studying aftershocks of a large earthquake in 2015, we show the general characters of the southernmost Ryukyu seismogenic zone and indicate a synchronous occurrence of the uplift of the south Ryukyu Arc and accumulation of some subducted material beneath the Ryukyu Arc. Key Points A better resolution of the tectonic events and plate interface geometry beneath the southernmost Ryukyu arc and forearc Tectonic exhumation occurs in the frontal portion of the southernmost Ryukyu seismogenic zone Tectonic underplating occurs beneath the southernmost Ryukyu seismogenic zone