Underground Structure in and around Toga Bay, Oga Peninsula, NE Japan, Inferred from Gravity Survey

Abstract

Recent research by Kano et al. (2002) confirmed the existence of a maar-type crater in and around Toga Bay. They named it Toga volcano. In this paper we reveal features of the above crater by analyzing its gravity anomaly. For this purpose we combined gravity data from two sources; AU (Akita University) and SK (Sekiyushigen-Kaihatu Co.). AU data was acquired mainly in 1981 and additionally in 1987. SK data was acquired for oil exploration in 1957. For this publication all data were reprocessed into a unified system. An isolated negative anomaly which was clearly detected on the local anomaly map was interpreted as evidence of low density pumiceous sediment filling the crater. Peak value of negative anomaly is -5.7 mGal. Basement is mainly volcanic rocks. A two-layer structure was assumed in the inverse analysis, which is based on iterative accommodation. On the assumption of 0.6 g/cm³ in density difference, the crater was analyzed as about 380 m in maximum depth and about 1.8-2.2 km in diameter. The mass deficiency was estimated as 2.9×10⁸ ton by Gauss's theorem. Possible variations of the structure are discussed in relation to allowable modifications of the density difference. Some related comments are given on the effects of measurement error and ill-balanced distribution of gravity stations.