Strong inhomogeneity beneath quaternary volcanoes revealed from the peak delay analysis of S-wave seismograms of microearthquakes in northeastern Japan

High-frequency S-wave envelopes of microearthquakes well reflect the medium inhomogeneity of the Earth. Defining the peak delay time as the time lag from the direct S-wave onset to the maximum amplitude arrival of its envelope, we use this quantity to evaluate the strength of multiple forward scattering and diffraction due to random inhomogeneities along the seismic ray path. Analysing peak delay times of many microearthquakes occurred along the subducting Pacific Plate for 2-4, 4-8, 8-16 and 16-32 Hz frequency bands, we find a clear path dependence of the peak delay time in relation to the distribution of Quaternary volcanoes in northeastern Japan. Peak delay times of less than 2 s are usually observed at most of the stations in the study area, but large peak delay times of more than 5 s are observed in the backarc side stations for the case that S wave propagates beneath Quaternary volcanoes. The large peak delay times are inferred to be generated at a depth of 20-60 km beneath Quaternary volcanoes by considering ray paths under a 1-D velocity structure. These strongly inhomogeneous regions are located at low-velocity and high Vp/Vs regions revealed from tomography, which suggests the inhomogeneity may be related to dykes and melts of ascending magma.