Crust and uppermost mantle structure beneath the Kinki district, Japan: Relationship to isolated deep low-frequency earthquakes

Occurrences of deep low-frequency earthquakes are observed in the regions away from volcanoes and the plate boundary in the Kinki district, Japan, and they are referred to as isolated intraplate LFEs. The activation mechanism of these LFEs is not fully understood yet. In this paper, we present the results of travel time tomography around the source areas of the isolated LFEs in the Kinki district. Our velocity model shows two types of velocity features, a low-velocity anomaly beneath the Wakayama earthquake swarm area and a high-velocity anomaly beneath Osaka Bay. Beneath Wakayama area, the low-velocity anomaly extends from the plate boundary to the LFE source area, and the isolated LFEs are located around the boundary of high and low velocity anomaly zones. This result suggests fluid flow from the subducting slab to the earthquake swarm area. In contrast, the uppermost mantle beneath Osaka Bay, where LFEs are also detected, shows a high-velocity anomaly. Our velocity model does not show any signs of fluid pathways from the uppermost mantle to the LFE source area, which were often recognized around volcanic regions in the previous studies. Horizontal fluid influx from surrounding areas or fluid pathways invisible in our model may exist. The high-velocity anomaly zone may represent a cooled melt reservoir formed by past magmatic activity such as Setouchi volcanic activities in the Miocene. Another possible interpretation of the LFE activity beneath Osaka Bay is that ancient magmatic fluids would have remained in the crust, and these fluids could cause the LFEs.