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

PATTERN ANALYSIS OF SMALL EARTHQUAKES ANDEXPLOSIONS RECORDED AT SHASTA, CALIFORNIARobert A. UhrhammerSeismographic Station, University of California, BerkeleyBerkeley, California 94720ABSTRACTSeismograms recorded by the three-component short-period seismographsat Shasta Dam, California (SHS; 1951-1963) are analyzed for:1) rate and distribution of seismicity; 2) characteristic patterns whichidentify small explosions; and 3) patterns of the distribution and orientationof earthquake foci. The high quality SHS station preceded theinstallation of a network of stations in the area by 40 years and localevents (within 50 km) were recorded at a rate of approximately one perweek. This study was conducted, in part, to determine how much informationcan be obtained from detailed analysis of records from a singlethree-component station.The local seismicity is 4.2 earthquakes (ML 3 2.5) per year per 10,000square kilometers. The seismicity is not uniformly distributed and therate increases towards the Cascade Mountains to the northeast.Small man-made explosions are identified, with approximately 99 percent reliability, by the characteristic pattern of a relatively low-frequencymaximum trace amplitude. The dominant frequency of the maximumtrace amplitude is 2.9 Hz for explosions and 8.9 Hz for earthquakes.Patterns in the distribution of earthquake foci indicate that: 1) thesource depths are, for the most part, shallow (10-20 km); 2) the dominantmechanism is strike-slip with the tension axis oriented N 73' W; 3) few,if any, earthquakes occur within the Great Valley region; and 4) a fewearthquakes in the Cascade Mountains may originate at a depth of 30 kmor more.1. IntroductionThe University of California Seismographic Stations operated a high-magnificationthree-component short-period Benioff seismograph at Shasta Dam, California(SHS: 40'41.7' N, 122'23.3' W, 312 m elevation) from July 1952 until the stationwas closed in May 1963. SHS recorded many small man-made explosions andearthquakes which occurred in the vicinity of Lake Shasta.The purpose of this paper is three-fold. First, to determine the characteristics ofthe seismicity in a geologically complex region of northern California in the vicinityof Lake Shasta (300 km north of San Francisco) in order to estimate, in part, theprobability and rate of earthquake occurrence. Second, to determine characteristicpatterns on the seismograms that will readily identify explosive sources in the region.Third, to determine the general characteristics of the local seismicity from detailedanalysis of seismograms recorded at a single station. Extraction of a maximumamount of information is necessary in regions that axe without dense seismographicnetworks (Uhrhammer, 1982).Historical Seismogramsand Earthquakes of the World349Copyright CS, 19x8 by Academic Press, Inc.All rights of reproduction in any form rewved.