Structural Analysis of Calderas by Semiautomatic Interpretation of the Gravity Gradient Tensor: A Case Study in Central Kyushu, Japan

Abstract

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.

Eigenvector of gravity gradient tensor for estimating fault dips considering fault type

“…1b), applied Beiki's technique (Beiki and Pedersen 2010;Beiki 2013) to the estimation of fault dips. This method provided results wherein the fault dip estimated by the gravity gradient tensor harmonized with the dip observed from seismic surveys (Kusumoto 2015(Kusumoto , 2016a. In addition, the dip of an earthquake source fault of the Kumamoto Earthquake that occurred in April 2016 estimated from the gravity gradient tensor also corresponded with the dip of the fault model (normal fault of 60°), thus explaining the crustal movement observed by GNSS (Global Navigation Satellite System) (Kusumoto 2016b).…”

“…5a was considered a high-density body that intruded into the low-density layer, and the dip of the normal fault was given by the dip of the maximum eigenvector. I believe Kusumoto (2015Kusumoto ( , 2016aKusumoto ( , 2016b) was able to obtain results that coincided with seismic surveys since he estimated the fault dip in normal fault regions by the maximum eigenvector of the tensor.…”

“…In addition, the dip of an earthquake source fault of the Kumamoto Earthquake that occurred in April 2016 estimated from the gravity gradient tensor also corresponded with the dip of the fault model (normal fault of 60°), thus explaining the crustal movement observed by GNSS (Global Navigation Satellite System) (Kusumoto 2016b). The range for which this method is applicable is wide from low dip to high dip (e.g., Beiki 2013;Kusumoto 2015Kusumoto , 2016aKusumoto , 2016b, although it has some numerical instability to the vertical fault (e.g., Kusumoto 2015).…”