Abstract
P-wave travel time and amplitude anomalies from 59 teleseismic earthquakes are analyzed for 17 stations of a seismic network in Hokkaido region, Japan. Relative travel time and log amplitude variations within the network are parameterized by an azimuth-independent term (average), and first and second azimuthal terms. For the travel time, the azimuth-independent terms, which represent shallow near-station velocity anomalies, show an obvious geographical variation and a significant correlation with gravity (Bouguer) anomalies. The azimuth-independent terms vary from - 0.73 to 0.58 s. The slow directions of the first azimuthal terms for stations surrounding the Hidaka Mountains point consistently toward an area near the axis of the mountains, suggesting the existence of a low-velocity zone in the uppermost mantle beneath the Hidaka Mountains. For the amplitude, the azimuth-independent terms have large positive values (amplification) at stations where the geological setting is young, i.e., Quaternary. The first azimuthal terms for stations surrounding the Hidaka Mountains show some indication of large amplitudes for the incidence azimuth in the direction of the mountains. If the azimuth-independent terms are subtracted from the observed values, the travel time residuals and log amplitudes show some correlation, suggesting the focusing and defocusing of rays by the heterogeneity in the uppermost mantle beneath the network.
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