Akihiko Yamamoto

Variations of gravity anomaly roughness in Chugoku district, Japan: Relationship with distributions of topographic lineaments

Tectonic movement may disturb lateral continuities of crustal structures at weak zones. These lateral discontinuities of density structure cause undulations of the gravity anomaly field whose complexity can be an indicator of past crustal instability. On the other hand, topographic lineaments are formed along weak crustal zones. We would expect therefore that gravity anomaly roughness bears some relation to the distribution of surface lineaments. We develop a statistical method by employing the Bouguer anomaly standard deviation as an index of gravity anomaly roughness in terms of which we interpret the spatial distribution of topographic lineaments in Chugoku district, Japan. Locations of the lineaments, which could be diagnostic of the heterogeneity of the previously damaged crust, tend to coincide with those of high roughness areas of gravity anomaly fields.

A Gravity Database of Southwest Japan: Application to Bouguer Gravity Imaging in Kyushu District, Southwest Japan

In order to investigate subsurface density structures with the highest precision, two major gravity databases and some unpublished data files, which cover Southwest Japan, are recompiled. This paper presents some examples of the detailed gravity anomaly maps in Kyushu District, southwest Japan. These maps indicate various fine subsurface structures, which may arose with tectonic movements and/or volcanic activities of this region. With an aid of geological and topographical information, we discussed relationships between characteristic patterns of the detailed gravity anomalies and subsurface structures in this region. The most conspicuous feature is an existence of a steep horizontal gradient zone of gravity anomalies running through the Aso Caldera in the ENE-WSW direction. Although the location of the westward extension of the Median Tectonic Line (MTL) in this region is still debatable, this zone seems to be the westernmost part of MTL.