Volcanism and Crustal Stress Field. Narrowing of Graben Width near Unzen Volcano: Effect of local thinning of lithosphere

The central Kyushu is a north-south spreading region characterized by east-west trending grabens and abundant active volcanoes. Available data indicate that the width of graben is 11-12 km apart from eruption centers of Unzen volcano, whereas it narrows down to about 7 km dose to the eruption centers. Focal mechanisms of earthquakes suggest that north-south tensional axis is not altered by the presence of volcanic conduit. Width of graben in general is pos tively correlated with thickness of lithosphere, and the width of graben is narrowed locally in accordance with the thinning of lithosphere beneath active volcanoes by the presence of magma chamber in middle crust. Evidences supporting this idea include shallow cutoff depth of seismicity and mid-crust low velocity area beneath the active volcano, shallow Curie temperature distribution, and high oxygen isotopic composition of the volcanic rocks.