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

Pattern Analysis of Small Earthquakes and Explosions35341.211.25*Red Bluff40.C~40.0123.25 121.75Figure 4. Map of study region, including east and west Shasta mining districts (dashedlines) and all events located by SHS. Notice the clusters of events within or near the miningdistricts. These clusters coincide with the locations of mines and quarries.Based on the evidence above, we estimate the rate and distribution of naturalseismicity in the region simply by using only those events with ML 2 1.8 whichoccurred between 0300 UTC and 1800 UTC as shown in Figure 5. The ML thresholdinsures that the seismicity list is uniform for statistical purposes and the timewindow removes almost all of the small explosions. This method eliminates approximately97 per cent of the blasts, but it has the disadvantage of also removingapproximately 38 per cent of the earthquakes from the list. Removal of such a highpercentage of earthquakes from the list may be unacceptable in those cases wherethe seismicity is very low and the sampling time is short, as there may be too fewearthquakes left in the list to reliably estimate the rate and pattern of seismicity.6. Characteristic Patterns for Small Man-made Explosions and NaturalEarthquakesAn independent and more effective method of separation of explosions and earthquakesis to identify characteristics on the seismograms that are associated withsmall explosions. Example seismograms of a natural earthquake and a small explosionaxe shown in Figures 2 and 3, respectively.