5:15 PM - 7:15 PM
[SSS08-P05] Crustal structure beneath the Kinki region in southwestern Japan: Relationship to the activation of deep low-frequency earthquakes away from volcanoes and the plate boundary
Keywords:deep low frequency earthquakes , seismic velocity structure, travel time tomography
Deep Low-Frequency Earthquakes (DLFEs) typically occur around the Moho discontinuity beneath volcanoes or on the plate boundary such as the Philippine Sea plate in the Japanese Island. However, in the Kinki region of southwestern Japan, isolated DLFEs have also been detected away from volcanoes and the plate boundary. It is hypothesized that these isolated DLFEs may activate in a unique situation unlike volcanic areas. Yet, the crustal and upper mantle structure around these isolated DLFEs remains poorly understood. In this study, we conducted travel time tomography to estimate the three-dimensional seismic velocity structure in southwestern Japan, especially around the Kinki region and identified crustal heterogeneity associated with the isolated DLFEs activity in the Kinki region.
In the western part of the Kii Peninsula (Wakayama region) isolated DLFEs have occurred beneath a nonvolcanic earthquake swarm area. Our velocity model shows a low-velocity anomaly zone beneath this earthquake swarm area, consistent with previous tomographic studies. Furthermore, isolated DLFEs are found to be located at the edge of the low-velocity anomaly zone. This result suggests fluids flow to the earthquake swarm area and ascend from the upper part of the subducting Philippine Sea slab.
In contrast, the uppermost mantle (40 - 50 km depth) beneath Osaka Bay, where isolated DLFEs are also detected, shows a high-velocity anomaly. This is unusual because the uppermost mantle beneath the volcanic DLFEs area typically shows a low-velocity anomaly. Although it is expected that fluids would flow toward the isolated DLFEs area beneath Osaka Bay, our velocity model does not show any signs of fluid paths from the uppermost mantle to the isolated DLFEs area as estimated around the volcanic region. Fluid influx from surrounding areas or fluid paths invisible in our velocity model may exist. It was assumed that extensive magmatic activity that formed the Setouchi volcanic rocks occurred in 12 - 15 Ma. There is a possibility that old magmatic fluids would have remained in the crust, and these fluids could cause DLFEs beneath Osaka Bay. The high-velocity anomaly zone may represent the cooled magma chamber formed by the past magmatic activity. We cannot determine which hypothesis is optimal based on our velocity model because we cannot estimate absolute velocity value accurately. However, the isolated DLFEs zone beneath Osaka Bay would at least differ from the volcanic and Wakayama areas where DLFEs occur. These unique crustal structures under Osaka Bay may be associated with the occurrence of DLFEs away from volcanoes and the plate boundary.
In the western part of the Kii Peninsula (Wakayama region) isolated DLFEs have occurred beneath a nonvolcanic earthquake swarm area. Our velocity model shows a low-velocity anomaly zone beneath this earthquake swarm area, consistent with previous tomographic studies. Furthermore, isolated DLFEs are found to be located at the edge of the low-velocity anomaly zone. This result suggests fluids flow to the earthquake swarm area and ascend from the upper part of the subducting Philippine Sea slab.
In contrast, the uppermost mantle (40 - 50 km depth) beneath Osaka Bay, where isolated DLFEs are also detected, shows a high-velocity anomaly. This is unusual because the uppermost mantle beneath the volcanic DLFEs area typically shows a low-velocity anomaly. Although it is expected that fluids would flow toward the isolated DLFEs area beneath Osaka Bay, our velocity model does not show any signs of fluid paths from the uppermost mantle to the isolated DLFEs area as estimated around the volcanic region. Fluid influx from surrounding areas or fluid paths invisible in our velocity model may exist. It was assumed that extensive magmatic activity that formed the Setouchi volcanic rocks occurred in 12 - 15 Ma. There is a possibility that old magmatic fluids would have remained in the crust, and these fluids could cause DLFEs beneath Osaka Bay. The high-velocity anomaly zone may represent the cooled magma chamber formed by the past magmatic activity. We cannot determine which hypothesis is optimal based on our velocity model because we cannot estimate absolute velocity value accurately. However, the isolated DLFEs zone beneath Osaka Bay would at least differ from the volcanic and Wakayama areas where DLFEs occur. These unique crustal structures under Osaka Bay may be associated with the occurrence of DLFEs away from volcanoes and the plate boundary.