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

Pattern Analysis of Small Earthquakes and Explosions 359U I S TF" I r PI 1-50 -A0 -38 -20 -10 0 10 20 30 40 50+++ ++ ++ 6 +Ot-t$ 0+I I I I++ +0 ++++0$+p ;*+ ++ t+$+t+++++Figure 8. Cross-section of depth versus distance from SHS along the tension axisshown in Figure 7 (N 73' E). The symbols are the same as in Figure 6. The clustering ofdilatational events is an artifact of choosing the direction of the cross-section to coincidewith the tension axis. SHS is located at 0 km.9. ConclusionsThe seismographic records from SHS (1952-1963) are a crucial historical sourceof information for studying seismicity in extreme northern California. SHS precededthe installation of a dense local network in the Lake Shasta area by 40 years.Seismograms from a three-component short-period seismograph with high magnificationcan provide the general characteristics of seismicity in such a moderatelyseismic region. Observatories that operate or have operated such stations shouldensure that the seismograms are preserved (especially if the rate of seismicity isvery low; Uhrhammer, 1983). The rate of events observed within 50 km of SHS isapproximately one per week.The magnitude adjustment for SHS is ~ M= L -0.60 f 0.12 as determined byadopting the ML estimate from the Wood-Anderson seismographs at the MIN station,about 85 km away, as standard. Magnitude calibration is important for scalingthe a-value (2.8) in the Gutenberg-Richter relation given in (3). The rate of seismicity(ML > 2.5) in the vicinity of Lake Shasta averages 4.2f2.0 earthquakes per yearper 10,000 km2. The actual rate decreases towards the Great Valley (southwest)and increases towards the Cascade Mountains-Modoc Plateau (northeast).The search for parameters for a characteristic pattern which will reliably identifya small explosion from the subject population brought to light two particularlyuseful characteristic patterns for identifying small explosions. The first patternrequires only one parameter while the second requires three. Both characteristicpatterns have a high probability (> 95 per cent) of identifying small explosionsusing only the information obtained from analysis of a single vertical-component