Yutaka Nagaoka

Temporal change of phase velocity beneath Mt. Asama, Japan, inferred from coda wave interferometry

[1] We estimated the temporal changes of phase velocity of Rayleigh waves extracted from cross correlations of S-coda waves recorded at 12 stations around Mt. Asama, Japan. First, we extracted a Rayleigh wave by taking cross correlations of S-coda waves for 315 regional earthquakes between October 2005 and February 2009. The dispersion curve of the Rayleigh wave was measured and compared with the one extracted from 18 days of ambient seismic noise. We found that both dispersion curves are consistent with each other, demonstrating the dominance of the fundamental Rayleigh waves. We then divided the entire time period into sub-periods, each of which consists of 80 earthquakes, to measure the temporal changes at frequencies between 0.3 and 0.6 Hz. The result shows the reduction of phase velocity by 1.5 % and the subsequent recovery before the eruption of Mt. Asama in 2008.

Magma pathway and its structural controls of Asama Volcano, Japan

Abstract Asama Volcano, Japan, is one of the most active volcanoes in the Japanese islands. Recent development of geophysical monitoring in Asama Volcano allows us to infer the magma pathway and its structural controls beneath the volcano. Combining geodetic data and precise earthquake locations during recent eruptions suggests that the magma intrudes several kilometres to the west of the summit to a depth of about 1 km below sea level as a nearly east–west-trending dyke. The vertically intruded magma then moves horizontally by several kilometres to beneath the summit before it ascends vertically to make the surface. Combining the P-wave velocity and the resistivity structure shows that the intrusions are under structural controls. Frozen and fractureless magma associated with volcanic activity until 24 000 years ago impedes the ascent of rising magma on its way to the surface. The S-wave velocity structure inferred from ambient noise tomography reveals a low-velocity body beneath the modelled dyke. From independent information, we have inferred that this low-velocity body is likely to be a magma chamber.