Seismic reflector alignment along the volcanic front in northeastern Honshu, Japan

We found seismic reflectors aligned along the east side of the volcanic front in northeastern Honshu, Japan, by analyzing seismograms of intermediate‐depth earthquakes recorded at High Sensitivity Seismograph Network Japan stations. We identified an unusual high‐frequency later phase on the seismograms, which arrives after the direct S waves and continues for >5 s. The characteristics of the waveforms suggest that this later phase is an S wave incidence. We plotted envelope waveforms and identified later phase arrivals on envelope waveforms. To estimate the location of the source of the later phases, we defined an event group that consisted of six to eight events distributed in a region of ∼50 km3. The source of the later phases was located by a combination of a station and an event group using later phase travel times. If we assume that later phases arriving at a station from an event group are generated by a common source, then we can use the observed travel times in a grid search to obtain the source location. The sources of the later phase were found to be aligned along the east side of a volcanic front at a depth of 20–30 km. The averaged ratio of the amplitudes of the later phases to the amplitudes of the incident waves on the later phase source was estimated to be ∼0.6. This result suggests the existence of unusual reflectors spread in the lower crust beneath the fore‐arc region. It may be possible that the later phase sources are steeply inclined bodies filled with fluid.