Reflective body observed by multi-channel reflection survey off Niijima and Kozushima, Japan

Abstract

Seismic reflection characters provide useful information on not only geometry of tectonic structures but also magmatic processes. A multichannel seismic reflection survey has been conducted on November 2000, in order to clarify the subsurface structures around Niijima, Kozushima and Miyakejima of the northern Izu islands, where volcanic activities with earthquake swarm have been repeatedly occurring. As a result, a chain of intermittently continuous reflectors with strong amplitude was observed at 3.5-5 km depth beneath the basement high between Niijima and Kozushima, which is called the Zenisu ridge and an NE-SW oriented echelon ridge across the Northern Izu Arc. The reflective zone has about 10 km length in NW-SE direction whereas it would have extremely thin width in NE-SW direction. Moreover, an isolated reflector is visible below the NW-SE trending basement rise, which is capped by a small seamount chain extending toward SE from Kozushima. Based on the previous study in crustal deformation modeling and seismic characters of the reflections, the reflectors are considered to show the magma intrusions that have been repeatedly occurring there, demonstrating one of the processes of crustal evolution in the study area. Furthermore the seismic attribute analysis shows the remarkable contrast in frequency-dependent attenuation of seismic reflections between the Zenisu ridge and the depression of igneous basement between Miyakejima and the ridge, implying the difference in the extent of destruction of the igneous basement rocks or that in lithology. Finally the AVO and frequency component analyses suggest the existence of volcanic gas at very limited part of the reflectors.