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.