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

360 Robert A. Uhrhammerseismogram. The best characteristic pattern is described by a single parameter, thedominant frequency of the maximum trace amplitude (F) recorded on the verticalcomponentseismogram. The characteristic pattern is easily recognizable by eye onthe seismograms because the dominant frequency for a small explosion (2.9zk0.7 Hz)is approximately one-third of that for an earthquake (8.9 f 0.8 Hz).The general characteristics of the seismicity in the vicinity of Lake Shasta aredetermined using only the SHS seismograms. The major characteristics are: 1) theseismicity is predominantly shallow (10-km); 2) the rate of seismicity increasestowards the Cascade Mountains-Modoc Plateau region and decreases to essentiallyzero in the Great Valley; 3) the dominant mechanism is strike-slip with the tensionaxis oriented N 73'E; and, 4) the variance in the depth estimate increases towardsthe Cascade Mountains-Modoc Plateau with a few earthquakes perhaps deeper than30 km. These characteristics are confirmed by the data from the 10 station ShastaDam Microearthquake Network which has been in operation since January 1983(R. LaForge, personal communication, September 1985).ACKNOWLEDGMENTSThe support provided by the Bureau of Reclamation (4PG8124250) to scan andanalyze the Shasta seismograms is acknowledged. I also thank Drs. B. A. Bolt,L. A. Drake, and W. Stauder for enlightening discussions during this work and forcritical comments on the manuscript.REFERENCESBolt, B. A., C. Lomnitz, and T. V. McEvilly (1968). Seismological evidence on the tectonicsof Central and Northern California and the Mendocino Escarpment, Bull. Seism.SOC. Am., 58, 1725-1768.Bolt, B. A. and R. D. Miller (1975). Catalogue of Earthquakes in Northern Californiaand Adjoining Areas, 1 January 1910 - 91 December 1972, Seismographic Stations,University of California, Berkeley, 567 pp.Cara, M. and J. B. Minster (1981). Multi-mode analysis of Rayleigh-type L,, Part 1.Theory and applicability of the method, Bull. Seism. SOC. Am., 71, 973-984.Ewing, W. M., W. S. Jardetzky, and F. Press (1957). Elastic Waves in Layered Media,McGraw-Hill, New York, 380 pp.Gelfand, I. M., S. A. Guberman, V. I. Keilis-Borok, L. Knopoff, F. Press, E. Y. Ranzman,I. M. Rotwain, and A. M. Sadovsky (1976). Pattern Recognition Applied toEarthquake Epicenters in California, Phys. Earth Planet. Interiors, 11, 227-283.Kinkel, A. R., W. E. Hall, and J. P. Albers (1956). Geology and Base-Metal Deposits ofWest Shasta Copper-Zinc District Shasta County, California. U.S. Geol. Sum Prof.Paper 285, 156 pp.Lee, W. H. K. and S. W. Stewart (1981). Principles and Applications of MicroearthquakeNetworks, Adv. in Geophy, Supplement 2, Academic Press, 293 pp.Mooney, H. M. and B. A. Bolt (1966). Dispersive characteristics of the first three Rayleighmodes for a single surface layer, Bull. Seism. SOC. Am., 66, p. 43-67.Richter, C. F. (1935). An instrumental earthquake magnitude scale, Bull. Seism. SOC.Am., 25, 1-32.Simila, G. W. (1981). Seismic Velocity Structure and Associated Tectonics of NorthernCalifornia, Ph.D. Thesis, University of California, Berkeley, 180 pp.Toppozada, R. T., C. R. Real, and D. L. Parke (1981). Preparation of Isoseismal Mapand Summaries of Reported Effects for Pre-1900 California Earthquakes, Calif. Diu.Mines Geol., Open File Rept. 81-11, 182 pp.