3He/4He ratio anomalies associated with the 1984 Western Nagano Earthquake: Possibly induced by a diapiric magma

Abstract

A magnitude 6.8 earthquake occurred near Ontake Volcano in the western Nagano Prefecture of central Honshu, Japan, on September 14, 1984. After the event, 3He/4He ratios of bubbling gases in hot springs located less than 10 km from the newly formed fault were found to have increased significantly, by up to 15%. Although the number of data is small, considering that 3He can only be of magmatic origin, we propose a model that the earthquake was triggered by an upward migration of 3He‐containing fluids associated with magma intrusion beneath the source region. The proposed model is supported by the uplifting observed in the region since 1967, the continuation of seismic swarm activities in the area since 1976, an unprecedented eruption of Ontake Volcano in October 1978, and the existence of a small melted body in the lower crust beneath the present fault region.