An active seismic survey with unprecedented density of seismometers found a high velocity zone to the west of the summit of Asama Volcano, Japan. The high velocity zone coincides with an area of magma-filled crack (dike) intrusion associated with the 2004 eruptions inferred from precise earthquake relocations and ground deformation modeling. It also coincides with an area of high resistivity surrounded by low resistivity, indicating that the solidification of magma due to repeating intrusions is responsible for the high velocity. This thus endorses the magma pathway previously speculated by seismic and geodetic observations. These findings demonstrate that dense seismic exploration combined with geophysical monitoring is an effective way to understand the dynamics of volcanic eruptions.
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