5:15 PM - 7:15 PM
[SCG61-P03] Crustal heterogeneity beneath the Median Tectonic Line estimated by the seismic velocity structure
Keywords:Median Tectonic Line, active fault, seismic velocity structure
The Median Tectonic Line (MTL) is a major geological discontinuity that separates southwestern Japan into two regions, the Inner and Outer zones. From the western Kii Peninsula to the Shikoku region, the MTL active fault system extends along and near the material boundary MTL. The seismic velocity structure along the MTL active fault system has been investigated using travel time tomography. Previous studies indicated that the deeper part of the MTL active fault system shows a low-velocity anomaly. Although the material boundary MTL extends to the Chubu (or Kanto) region, crustal structures of the deeper part of the material boundary MTL, particularly the Chubu region, have not been thoroughly investigated using seismic velocity analysis. To understand crustal heterogeneity along the material boundary MTL better, we conducted travel time tomography to estimate the three-dimensional velocity structure beneath southwestern Japan.
Our velocity model shows a linear low-velocity anomaly along the material boundary MTL from the west of Lake Hamana to the Kyusyu region. This low-velocity anomaly is recognized at depths of 10-20 km. While a low-velocity anomaly along the material boundary MTL (MTL active fault system) from the western part of the Kii Peninsula to the Shikoku region has been pointed out so far, the low-velocity anomaly zone in the east of the Kii Peninsula has not received much attention. Although this linear low-velocity anomaly zone extends to the Kyusyu region, it does not necessarily follow that the low-velocity anomaly in the Kyusyu is related to the MTL, because the central Kyusyu is located in the volcanic area. It is possible that fluids derived from the Philippine Sea slab, old remanent fluids, or fracture zones would exist along the material boundary MTL.
In contrast, our velocity model does not show any significant velocity anomaly east of Lake Hamana. The material boundary MTL bends significantly in this region. This bending structure is related to the collision of the Izu-Bonin Arc. We consider that the greatly disturbed crustal structures beneath the material boundary MTL caused by the collision of the Izu-Bonin Arc would be related to the absence of a low-velocity structure.
Our velocity model shows a linear low-velocity anomaly along the material boundary MTL from the west of Lake Hamana to the Kyusyu region. This low-velocity anomaly is recognized at depths of 10-20 km. While a low-velocity anomaly along the material boundary MTL (MTL active fault system) from the western part of the Kii Peninsula to the Shikoku region has been pointed out so far, the low-velocity anomaly zone in the east of the Kii Peninsula has not received much attention. Although this linear low-velocity anomaly zone extends to the Kyusyu region, it does not necessarily follow that the low-velocity anomaly in the Kyusyu is related to the MTL, because the central Kyusyu is located in the volcanic area. It is possible that fluids derived from the Philippine Sea slab, old remanent fluids, or fracture zones would exist along the material boundary MTL.
In contrast, our velocity model does not show any significant velocity anomaly east of Lake Hamana. The material boundary MTL bends significantly in this region. This bending structure is related to the collision of the Izu-Bonin Arc. We consider that the greatly disturbed crustal structures beneath the material boundary MTL caused by the collision of the Izu-Bonin Arc would be related to the absence of a low-velocity structure.