Mapping of a magma reservoir beneath Nikko-Shirane volcano in northern Kanto, Japan, from travel time and seismogram shape anomalies

Since ray paths of direct P and S waves are largely distorted in a magma body, a seismic shadow zone is created. However, scattered or diffracted waves with very small amplitudes can arrive in the shadow zone because their ray paths are different from those of the direct waves. They become the first arrival in the shadow zone. More than 100 seismologists from many institutions in Japan joined together to perform an intensified seismic study in the Nikko-Ashio area, northern Kanto, Japan. It was suggested that there is a magma body beneath Mount Nikko-Shirane, an active volcano, that causes a seismic shadow zone, because P waves recorded by the joint observation are anomalously attenuated in cases when they propagate through a zone beneath the eastern part of the volcano. We developed a new method to estimate the shape of the anomalous attenuation zone by proposing the use of a new parameter, the energy ratio between directP or S waves and their coda waves, to discriminate whether or not each observation station is located inside or outside the shadow zone. We estimated the shape of the anomalous attenuation zone by dividing the study area into blocks with dimensions of 2 km × 2 km × 1 km and using the energy ratio data of P waves for local events. The obtained result shows the existence of an anomalous attenuation zone in the area east of Nikko-Shirane volcano with a diameter of about 10 km at depths greater than 3 km. It was also found from plots of deep event seismograms recorded by the same observation that travel times are delayed by about 0.7 s for ray paths crossing the anomalous attenuation zone obtained by the energy ratio data. This value requires the P wave velocity in the anomalous zone to be less than that of the surrounding crustal material by at least about 30%, suggesting the existence of a huge magma reservoir in this area.