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

352 Robert A. Uhrhammereach component. Clock corrections, derived from radio time signals, allowed absolutetime determination to an accuracy of 0.2 seconds. Any unusual characteristicsof the recording were also noted.Only those events with ts-p 5 7 seconds were selected in order to limit thenumber of events to those within a hypocentral distance of approximately 50 km.The amplitude threshold of 0.5 mm was chosen to provide sufficient signal amplitude(above the background noise level) to reliably pick the onset times and first motions.A record amplitude of 0.5 mm is equivalent to a ground motion of 10 nm at afrequency of 1 Hz.The amplitude and time measurements were obtained using a comparator with areticle resolution of 0.1 mm and lengths can be measured accurately to 0.05 mm. Alength resolution of 0.05 mm corresponds to a time resolution of 0.2 seconds on theSHS seismograms (recorded at 15 mm/min). The distance resolution using ts-p isthus approximately 1 km.4. Magnitude CalibrationThe local magnitude (ML) of an event is determined from the maximum traceamplitudes (A) measured (in mm) on the horizontal component SHS seismogramsusing the standard formula (Richter, 1935):ML = logA - logA,, + ~ ML, (1)where CSML is the magnitude adjustment for SHS. ML determined from the maximumtrace amplitudes measured on the standard Wood-Anderson torsion seismogramsrecorded at Mineral (MIN; 85 km southeast of SHS) is adopted as standardto calibrate the magnitude adjustments for SHS. Thirty local events, recorded bythe Benioff seismographs at SHS and also by the Wood-Anderson seismographs atMIN with usable trace amplitudes, were used to determine ~ M= L -0.60 f 0.12 byweighted least-squares.5. Separation of Small Man-made Explosions from Natural EarthquakesA scan of the SHS seismograms immediately showed that a large majority of therecorded events (shown in Figure 4): 1) had compressional first-motion for the onsetof the P-wave; 2) occurred during local daylight hours (PST); 3) were clustered indistance from SHS; and, 4) were approximately the same size. It was clear thatmost of these events were small man-made explosions (equivalent ML 5 2). Theproblem is to find a simple and effective method of culling the small explosions fromthe natural earthquakes.A majority of the events are associated with commercial mining and quarry activityin the west Shasta mining district (Kinkel et al., 1956) shown in Figure 4.The cluster midway between SHS and WDC in Figure 4 matches the location ofthe Iron Mountain Mine (the largest and most active mine in the district) and approximately20 per cent of the events are within this cluster. Also, approximately50 per cent of the events occurred within the mining district which occupies only6 per cent of the total area. In addition, 97 per cent of the events occurring in themining district occurred between 1800 UTC and 0200 UTC during local daylighthours (PST).