INTERPRETATION OF SEISMIC AND VOLCANIC ACTIVITIES IN THE IZU BLOCK IN RELATION TO COLLISION TECTONICS

Distinctive seismic and volcanic activities in the Izu block are interpreted as mechanical processes responding to the subduction and collision of the Philippine Sea plate. Systematic propagation of seismicity and interconnection between some seismic and volcanic events confirm that the activities are of a mechanical origin. It is inferred that the western part of the Izu block deforms so as to adjust the difference of plate motion between subducting and colliding segments along the Suruga trough, while the eastern part involves a seismo-volcanic activity like the earthquake swarms east off the Izu Peninsula described by Hill's model. In this seismo-volcanic activity, ascending magma intrudes into parallel fissures and induces seismic slips on the connected faults. Such system of fissures and faults is able to yield a NW-SE contraction in response to the collision of the Izu Peninsula and a NE-SW extension that gives a downgoing component of the plate motion along the Sagami trough. The remarkable crustal inflation observed in recent activities may reflect the volume expansion of ascending magma associated with the vesiculation of dissolved volatiles. The monogenetic volcanoes in the eastern Izu block have been progressively constructed by the seismo-volcanic processes repeated in the late Quaternary.