Crustal Structure Beneath the Swarm Earthquake Zone in the West of Miyake-jima Island and the Related Tectonics of the Northern Part of the Zenisu Ridge

Abstract

In response to the eruption of Miyake-jima volcano starting in June 27, 2000 and the associated swarm earthquakes in the west of lvliyake-jima island, the comprehensive surveys and ocean bottom earthquake observation were conducted by the Japanese Hydrographic Department. Some of the survey results and the derived sub-bottom structure were presented with consideration of teetonics as to the northern part of the Zenisu ridge inferred from geophysical data of topography, magnetic and gravity anomalies covering the northern part of the Izu-shoto region The narrow multi-beam soundings in the west of the Miyake-jima island show the small volcanic cones in the southeast offing of the Kozu sluma island The seismic records proved the existence of many normal faults segmenting the sedimentary layers, which may imply the compressive stress field in NW-SE direction. It is also confirmed that the swarm earthquake region coincides well with the northeastern edge of magnetic basement high. Magnetic basement highs implying the dyke intru sion were also recognized between the Nii-jima and Kozu-shima islands. These features may suggest upwelling of magma through NW SE trending deep fractures caused by regional stress field. The general distribution of magnetic and gravity anomalies covering the northern part of the Izu shoto region, show the good correspondence with the tectonic lines mferred from seismic activity The northern part of the Zenisu ridge has been segmented into three part, Omurodashi ,Ni i jima, and Kozu-shima blocks, and in between there exist magnetic basement highs implying the dyke mtrusion thiough the deep fractures in the crust. The deflection of the major axis of Nii-jima islands is ascribed to the rotational movement caused by the huge dyke intrusions occurred in the east of the island.