Tomographic inversion of P-wave velocity and Q structures beneath the Kirishima volcanic complex, southern Japan, based on finite difference calculations of complex traveltimes

We estimate the P-wave velocity and attenuation structures beneath the Kiri shima volcanic complex, southern Japan, by inverting the complex traveltime s (arrival times and pulse widths) of waveform data obtained during an acti ve seismic experiment conducted in 1994. In this experiment, six 200-250 kg shots were recorded at 163 temporary seismic stations deployed on the volc anic complex. We use first-arrival times for the shots, which were hand-mea sured interactively. The waveform. data are Fourier transformed into the fr equency domain and analysed using a new method based on autoregressive mode lling of complex decaying oscillations in the frequency domain to determine pulse widths for the first-arrival phases. A non-linear inversion method i s used to invert 893 first-arrival times and 325 pulse widths to estimate t he velocity and attenuation structures of the volcanic complex. Wavefronts for the inversion are calculated with a finite difference method based on t he Eikonal equation, which is well suited to estimating the complex travelt imes for the structures of the Kirishima volcano complex, where large struc tural heterogeneities are expected. The attenuation structure is derived us ing ray paths derived from the velocity structure. We obtain 3-D velocity a nd attenuation structures down to 1.5 and 0.5 km below sea level, respectiv ely. High-velocity pipe-like structures with correspondingly low attenuatio n are found under the summit craters. These pipe-like structures are interp reted as remnant conduits of solidified magma. No evidence of a shallow mag ma chamber is visible in the tomographic images.