Historical Seismograms - Evidence from the AD 2000 Izu Islands ...

358 Robert A. UhrharnmertNFigure 7. Lower hemisphere stereographic projection of compressional and dilatationalfirst-motions of all events shown in Figure 6. The symbols are the same as in Figure 6.The compression axis is oriented N 17O W and the tension axis is oriented N 73O E.A cross-section of depth versus distance from SHS along the tension axis (T-Tin Figure 7) is shown in Figure 8. The tension axis is oriented N 73' E and itis approximately perpendicular to the trend of the major geological features inthe region. The cross-section indicates that the focal depths of a majority of theearthquakes are relatively shallow (10-20 km). There is an apparent trend towardsshallower focal depths in the N 73' E direction, although the trend is not significantconsidering the increased scatter in the focal depths in that direction. The clusteringof the dilatational first-motions (in Figure 6) is an artifact of choosing thedirection of the cross-section to coincide with the tension axis. Also note that theseismicity stops approximately 25 km to the west of SHS which is consistent withthe observations of Bolt et al. (1968) and Simila (1981) that there are few if anyearthquakes in the Great Valley region. The deeper earthquakes towards N 73' E inthe Cascade Mountains-Modoc Plateau region are similar in depth distribution tothe earthquakes observed in the Sierra Nevada foothills (Wong and Savage, 1983).