A distinction technique between volcanic and tectonic depression structures based on the restoration modeling of gravity anomaly: a case study of the Hohi volcanic zone, central Kyushu, Japan

In this study, we propose a numerical modeling technique which restores the gravity anomaly of tectonic origin and identifies the gravity low of caldera origin. The identification is performed just by comparing the restored gravity anomalies with the observed gravity anomalies, thus we do not need detailed geophysical and geological information around the buried caldera. The technique has been successfully applied to distinguish the gravity low originated in the buried Shishimuta caldera from other gravity lows in the Hohi volcanic zone, central Kyushu in Japan.     

A method for determination of volcanic glass concentrations in sedimentary sequences and its application to quaternary studies

A method for determination of volcanic glass concentration in sedimentary sequences is presented. This method consists of two steps: (1) systematic extraction of volcanic glass particles from samples taken at close intervals in a sedimentary section, and the determination of their concentration; (2) precise measurement of the refractive indices of the separated glass particles. A recently developed measuring system (RIMS 86) based on the thermal immersion method permits quick and accurate measurement of the refractive index of a large number of grains within samples. Based on these kinds of data, we typically recognize five types of concentration of volcanic glass shards in sedimentary sequences. The method proposed here enables the simultaneous determination of the refractive index of glass shards and the proportion of glass shards in a sample, which makes it possible to recognize certain ash-fall-horizons, even those undetectable through visual observation of a sedimentary column. © 1994 John Wiley & Sons, Inc.