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

350 Robert A. UhrhammerLake Shasta and vicinity are near the juxtaposition of the southeastern part ofthe Klamath Mountains with the Cascade Mountains and the Great Valley. Thisregion exhibits a relatively low level of seismicity and no earthquakes with Mr, largerthan approximately 5 are known to have occurred in historical times (Townley andAllen, 1939; Bolt and Miller, 1975; Toppozada et al., 1981; Toppozada and Park,1982). Historical seismicity (ML 2 3.0) from Bolt and Miller (1975) is shown inFigure 1.Seismographic records from the Shasta station (1952-1963) were examined, and568 events were identified. The local magnitude (ML), estimated from the SHSrecords, is calibrated using the standard Wood-Anderson seismograms recorded atthe Mineral station (MIN; 85 km to the southeast in Figure 1). Approximately 80per cent of the events recorded are small explosions (man-made blasts) associatedwith mining and quarry activity in the Shasta mining districts.The following regional seismicity properties were inferred from the three-componentshort-period SHS seismograms: epicentral locations; the distribution of focaldepths; a composite focal mechanism; and a depth cross-section for the local earthquakes.The inferred spatial characteristics of the earthquakes are subsequentlycompared the few other seismicity studies yet published for this region.2. InstrumentationThe instrumentation at SHS consisted of three 50 kg moving coil Benioff seismometerscoupled to galvanometers and recording on 35 mm film. The seismometerand galvanometer free periods were Ts = 1.0 sec and Tc = 2.0 sec, respectively,and the magnification was 50k (at 7’s). Four 35 mm by 935 mm film strips wererecorded each day, one for each of the three components and one for time. Examples41.50%341.5Figure 1. Map of historical seismicity (ML 2 3.0; 1928-1982). The 50 km radius circlecentered on SHS defines the area studied in this paper.