Shigekazu Kusumoto

By Yasuto Itoh, Shigekazu Kusumoto, Kazunori Miyamoto and Yoshiyuki Inui

Part of the book: New Frontiers in Tectonic Research

By Shigekazu Kusumoto, Yasuto Itoh, Osamu Takano and Machiko Tamaki

Part of the book: Mechanism of Sedimentary Basin Formation

By Osamu Takano, Yasuto Itoh and Shigekazu Kusumoto

Part of the book: Mechanism of Sedimentary Basin Formation

By Shigekazu Kusumoto

In order to understand the formation history and mechanism of volcanoes and their related structures, including calderas, their subsurface structures play an important role. In recent years, gravity gradiometry survey has been introduced, and new analyses techniques for gravity gradient tensors obtained by the survey have been developed. In this chapter, we first describe the gravity gradient tensor and its characteristics, and the method for obtaining the tensor from the gravity anomaly. Next, we review the semiautomatic interpretation methods for extracting information on subsurface structures, and apply some of the techniques to the volcanic zone of central Kyushu, Japan. The results showed that the horizontal and vertical gravity gradient methods, and the CLP method were useful for extracting outlines of important volcanic and tectonic structures in this region. Using the maximum eigenvector of the gravity gradient tensor, the caldera wall dip of the Aso caldera was successfully estimated to be in the range of 50–70°, and the dip of the Median Tectonic Line which was the largest tectonic line in the southwest Japan was consistent with seismic reflection surveys. In addition, a large circular structure surrounding the Shishimuta caldera with a diameter of 35 km was distinguished in some analyses.

Part of the book: Updates in Volcanology

By Shigekazu Kusumoto, Keiji Takemura and Yasuto Itoh

The Median Tectonic Line (MTL) is the largest tectonic line in southwest Japan, and its eastern portion has moved as a right‐lateral fault with a reverse fault component during the Quaternary. Although a high dip of the MTL has been suggested from geomorphological studies, reflection surveys have indicated a low dip of 30–55°. Thus, the MTL shows contradiction between its fault dip and fault motion. In this study, we attempted to re‐estimate the dip of the MTL by gravity anomaly, gravity gradient tensor, and numerical simulation, restoring topographies caused by lateral faulting. The numerical simulations suggested that a fault dip of 70–75° is a reasonable dip of the MTL. These high‐dip faults are able to move as a lateral fault and have the possibility of reverse fault motion. Deformation patterns caused by faults with these dips are in harmony with the accumulated geological and geomorphological evidence for motions of the MTL. On the other hand, gravity and gravity gradient analyses showed only a material boundary with low dip. This suggested that the MTL does not have conspicuous density contrast at the boundary.

Part of the book: Evolutionary Models of Convergent Margins