Random inhomogeneities in the northern Izu-Bonin arc estimated by tomographic inversion of peak delay times ofSwave seismograms

Abstract

[1] Random inhomogeneity in the lithosphere scatters seismic waves at high frequencies (>1 Hz). The accumulated scattering effect of S waves can be measured from the peak delay time, defined as the time lag from the S wave onset to the maximal amplitude arrival. This study estimated the spatial distribution of the power spectral density function (PSDF) of random velocity inhomogeneities in the northern Izu-Bonin arc by inversion analysis of the peak delay times assuming von Karman-type spectrum. Peak delay times were measured at plural frequency bands (4–8, 8–16, and 16–32 Hz) by using waveform data recorded by ocean bottom seismographs. The inversion result at 0–30 km depth shows strong inhomogeneities beneath the Quaternary volcanoes Aoga-shima, Sumisu-jima, and Tori-shima. At 30–70 km depth, strongly inhomogeneous regions were imaged beneath the Quaternary volcanoes and weak inhomogeneities were imaged on the fore-arc side. The PSDF beneath the Quaternary volcanoes was estimated as P(m) ~ 0.029 m−3.7 km3 at 0.5 < m < 50 km−1, where m is the wave number (km−1). The PSDF on the fore-arc side is P(m) ~ 0.0023 m−4.4 km3. The spectral roll-off is significantly different between these two regions, indicating that differences in random inhomogeneities become pronounced at smaller spatial scales. Petrological studies of the Izu-Bonin Mariana arc have suggested that magma and restite transport occurs across the Moho. We speculate that small magma inclusions or restite may account for the strong inhomogeneities beneath the Quaternary volcanoes and that the stochastic characteristics of their spatial distribution have changed the spectral roll-off there.