Heterogeneous b-value Distributions Measured Over an Extensive Region from the Northern Okinawa Trough to Southern Kyushu Island, Japan

Spatiotemporal b-value maps are presented for an extensive region from the northern Okinawa trough to off southern Kyushu Island, Japan. This region shows high seismic activity associated with a tectonic setting characterized by seafloor spreading caused by the opening of a back-arc basin in the Ryukyu arc–trench system and the presence of a left-lateral shear zone on southern Kyushu Island. The obtained spatial and temporal distributions of b-value for the analyzed period are highly heterogeneous, reflecting the influence of the tectonic features and processes of the region. A comparison of the results with other geophysical observations suggests deep fluids associated with mantle upwelling below the seismogenic layer are the dominant control on the observed b-value variations. In addition, the hypocentral area of the largest earthquake (M7.1), which occurred in 2015, corresponds approximately to a region with low b-values (b = 0.5–0.7). Another region with low b-values (b = 0.5–0.7) occurs in the west of the analyzed region off southern Kyushu Island, where the occurrence of large-magnitude events is not clearly recognized. During the analysis period, there were few moderate to large earthquakes in the analyzed region. Furthermore, some active faults that have the potential to generate large earthquakes of ~ M7 are located in those parts of the analyzed region with low b-values. These findings suggest a high likelihood of nucleation of large earthquakes in the focal region.