Seismic attenuation tomography of the source zone of the 2016 Kumamoto earthquake (M 7.3)

We study the three-dimensional seismic attenuation (QP and QS) structure in the source area of the 2016 Kumamoto earthquake (M 7.3) by using 18,296 t(P)* and 29,668 t(S)* data from 742 local earthquakes recorded by a dense seismic network consisting of 112 stations deployed in Kyushu Island. Our results show that significant low-Q (high-attenuation) anomalies exist in the crust and mantle wedge beneath the volcanic front and back-arc area, which reflect hot and wet zones caused by convective circulation in the mantle wedge and dehydration of the young Philippine Sea (PHS) slab. The 2016 Kumamoto earthquake occurred in a high-Q and high-velocity (high-V) zone in the upper crust, which is surrounded and underlain by low-Q and low-V anomalies in the lower crust and upper mantle. These results suggest that the 2016 Kumamoto earthquake took place in a brittle seismogenic layer in the upper crust, but its rupture nucleation was affected by fluids and arc magma ascending from the mantle wedge. In addition, an obvious low-Q zone is revealed in the fore-arc mantle wedge, which reflects serpentinization of the fore-arc mantle due to abundant fluids from the PHS slab dehydration.