Abstract
We present a method to abtain the in situ seismic velocity in the sourece region of crustal earthquakes. The procedures of the method are as follows.1. Determination of hypocenters of earthquakes in a small area using data at only nearer stations.2. Calculation of a horizental apparent velocity and an emergent direction of seismic waves using travel times of earthquakes at each distant station. When we use the direct waves at a distant station, a horizontal apparent velocity is nearly equal to the true velocity.3. Calculation of the true velocity from horizontal apparent velocity and vertical one which is determined by the focal-depth dependence of travel times at each station.We applied this method to the eastern Yamanashi earthquake in central Japan. The focal depths of these earthquakes are 16-20km, and are somewhat deeper than those of usual crustal earthquakes in Japan. Direct waves were obseved as first-arriving waves at 12 stations with wpicentral distances of 55-156km which we used in this analysis. The horizontal apparent velocity and the true valocity were determined 6.17±0.14 and 6. 09±0.12km/sec, respectively. We can conclude, therefore, that the eastern Yamanashi earthquakes have occirred in the upper crust with the P-wave velocity of 6. 1km/sec.We can also obtain the azimuthal anisotropy in the media, the P-wave velocity is 6.09+0.08 When we assume the azimuthal anisotropy in the velocity is reduce from 0.115 to 0.095km/sec. THe maximum velocity is obtained in the NE-SW direction which is also the orientation of the maximum tension axis of earthquakes in this region.
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