Fig. 1

Map of the Hakone volcano. a Relief map of the Hakone volcano and its vicinity. Triangles show the locations of seismic stations. The seismic data at the red triangle stations were used to analyze the envelope correlation. Spectrograms recorded at OMZ, N.ASGH, SSN, and TNM stations are shown in Fig. 3. The red star shows the location of Owakudani. The red broken rectangle corresponds to the region showing in b. The right figure denotes the target area for the Japanese islands, indicating the deep low-frequency (DLF) earthquake epicenters (red) determined by the Japan Meteorological Agency (JMA) and active volcanos (blue triangles). The distribution of banded hypocenters in southwest Japan corresponds to non-volcanic low-frequency earthquakes that occur at the boundary of the subducting Philippine Sea Slab (Katsumata and Kamaya 2003). The depth–frequency distribution of DLF earthquakes excepted for the non-volcanic event along to the Philippine Sea Slab is also showing. b Seismic activity beneath the Hakone volcano during the past two decades and c projected on the N–S depth section. The open black circles show the hypocenters of volcano tectonic (VT) earthquakes determined by the Hot Springs Research Institute (HSRI). The red circles represent the hypocenter of DLF earthquakes by JMA. The size of each circle corresponds to the local magnitude of the earthquake