Characteristics of P-coda and Crustal Inhomogeneity beneath Nikko Area Estimated from Seismic Array Observations

Recent studies made by several researchers show that most parts of P-coda are composed of wavelets incident from the direction of the source region for a frequency range lower than a few Helz. To study characteristics of P-coda wave in a region where strength of inhomogeneity is high, we carried out two seismic array observations in and around a volcanic region of northeastern Japan. One seismic array (Tone array), which is composed of 40 stations, was set up in a volcanic region. The other (GZD array) with 38 stations was set up in a region about 20km away from the former. Seismic data were recorded by personal computers at a sampling frequency of 200Hz. These data were band-pass filters at central frequencies of 2, 8 and 16Hz to be analyzed by using semblance coefficients. Analysis for the GZD array shows that the maximum semblance directions in the P-coda parts almost coincide with the hypocenter direction. On the other hands, the maximum semblance directions for 8 and 16Hz in the P-coda parts are much scattered for the Tone array. These results show that inhomogeneity in the volcanic region is relatively stronger than surrounding region at scale lengths shorter than a few hundreds meters. The mean free path beneath the Tone array, which is located in the volcanic region, estimated to be shorter than 10km based on a single scattering model.