Restoration of the depression structure at the eastern part of central Kyushu, Japan by means of dislocation modeling

Abstract

We have attempted to restore a subsurface structure in the eastern part of central Kyushu, Japan, by combining fault motions which were modeled as dislocation planes embedded in an elastic isotropic half space. The simulated crustal deformation pattern was compared with the subsurface structures estimated from gravity anomalies and/or seismic prospecting. The modeling procedure successfully restored all tectonic basins in the area without any need for motive forces for uplift or subsidence. The results also suggest that two major tectonic events have occurred in this region. Those are (1) the formation of half-graben caused by north–south extension, and (2) the formation of the pull-apart basin caused by east–west compression.

Introduction

Subsurface structure shows the present contemporary static basement form, and is also considered to indicate the accumulated result of crustal deformation in the past. Accordingly, the restoration of a subsurface structure provides us important information on the tectonic history of the region concerned.

There are a number of alternative approaches to basin structure restoration. Katzman et al. (1995), for instance, employed the Boundary Element Method for the study of tectonics in the Dead Sea. However, in our paper, a dislocation model was used to restore the subsurface structures by calculating the extent of static vertical displacement caused by fault motion, which is obtained as a superposition of analytical solutions for dislocation planes embedded in an elastic isotropic half space (Rodgers, 1980). If the subsurface structures are actually formed by fault motions, they should be restored by combining dislocation plane motions.

As Chinnery and Petrak (1968), Katzman et al. (1995)or others have pointed out, the dislocation model has the disadvantage of a so called `edge effect' which causes singularities at the edges of the fault plane. This effect is due to the unrealistic assumption of a constant discontinuity over the fault plane, and causes indeterminate displacement and infinite stress at the edges. In spite of this disadvantage, we employed the dislocation model because it is a simple calculation which represents the essential characteristics of regional tectonic movements and does not require the assumption of a dynamic frictional constant over the fault plane.

The eastern part of central Kyushu is a key area for understanding the tectonic history of Southwest Japan because the region is located at an intersection of tectonic lines and is near an inflection point of the subducting Philippine Sea plate margin (Fig. 1). This region is also characterized by gravity lows which are caused by the tectonic and volcanic depression structures. Although many studies have discussed the formation of the region, a definitive model of the mechanical formation has not been established so far. We therefore propose the following dislocation model as one important aspect of our understanding of the tectonic history of the region.