3D Basement Structure of the Fukui Plain, Central Japan, Inferred from Gravity Anomalies

Abstract

The Fukui Earthquake of M 7.1 was occurred in 26 June, 1948. The epicenter is located at the central part of an alluvial plain (the Fukui plain). In order to understand locations both of the epicenter of the earthquake and active faults surrounding the plain in terms of crustal structure and geology, the basement structure beneath the plain was estimated by using gravity data of about 1, 000 points. Applying 2D-Talwani's method iteratively, we calculated 3D density structure beneath the plain up to a depth of 4km. Three-layers model is assumed in this study: i. e., from top to bottom, each layer corresponds to the Quaternary sediments (density 2.1g/cm³), the Neogene sedimentary layers (2.4) and the lowest Neogene layer and the lower strata (2.67), respectively. An elongated depression of about 3km depth with N-S orientation is found in the central part of the Fukui plain. Epicenter of the Fukui earthquake is located on the deepest part of the depression. The results also imply that active faults in the Fukui plain are situated along boundaries where the basement structure steeply changes.