We estimate the 3-D P wave attenuation (Q p −1 ) structure of central Japan to investigate arc magmatism. We first determine the corner frequencies of the earthquakes and then conduct a joint inversion to determine attenuation terms. The 3-D Q p −1 structure is finally obtained via a tomographic inversion. High-attenuation areas are located in the uppermost mantle along the volcanic front, which are interpreted as zones of partial melting at 30-to 100-km depth due to the supply of slab-derived fluids. The subducted Philippine Sea plate acts as a barrier to the upward migration of melt, resulting in the volcanic gap. The distribution of volcanoes is controlled primarily by the geometry of the subducted Philippine Sea plate. The attenuation structures beneath Mount Fuji and Mount Hakone show contrasting melt transport pathways, which are separated at ≤50-km depth, suggesting that the independent melt supply system results in the distinct activities of these volcanoes.Plain Language Summary It is important to investigate the magmatic system beneath active volcanoes, especially for disaster prevention. However, the details of deeper magmatic systems are poorly understood in central Japan. We estimate the three-dimensional seismic attenuation structure of central Japan to reveal the magmatic system beneath active volcanoes. Our findings suggest that the distribution of volcanoes is controlled primarily by the geometry of the subducted Philippine Sea plate and that the differences in volcanic activity at Mount Fuji and Mount Hakone reflect their respective melt transport pathways.

Investigating heterogeneous structures beneath active volcanoes is important for better understanding of volcanic activity and improved mitigation of volcanic risk. In central Japan, Hakone volcano has recently shown shallow earthquake swarms and deep low-frequency earthquakes (DLFEs), which are probably related to geothermal or deep magmatic activity. In order to image the feeding system beneath this volcano, we estimate 3-D P wave attenuation structure using waveform data recorded at permanent and densely distributed temporary seismograph stations. We first determine corner frequencies of the earthquakes and then perform a joint inversion to obtain attenuation terms (t*) and site responses. Values of t* are finally inverted to estimate the attenuation structure to a depth of 50 km. High-attenuation zones at depths ≤5 km suggest that fracture zones are permeated with hydrothermal fluids. A high-strain rate zone revealed by geodetic observations is spatially correlated with a high-attenuation volume at depths of 5-10 km, suggesting that the anelastic deformation is dominant in the high-attenuation zones. A subvertical volume of moderate attenuation that is imaged at depths of 10-20 km beneath Hakone volcano is connected to the top of a zone of partial melting beneath Mt. Fuji at depths ≥30 km through a subhorizontal channel at a depth of~25 km. DLFEs in the lower crust and fluid-related activity such as earthquake swarms in the upper crust occur in this volume. The zone is thus interpreted as a fluid-bearing pathway, which allows the rise of magmatic fluids to Hakone volcano.