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

354Robert A. Uhrhammer41.2.41.25*Red Bluff40.0140.0123.25 121.75Figure 5. Plot of all located SHS events with ML 2 1.8 which occurred between 0300UTC and 1800 UTC. These events are predominantly natural earthquakes.We approach identification as a problem in pattern recognition (Gelfand et al.,1976). A set of parameters and their corresponding thresholds, which describecharacteristic patterns, needs to be identified in order to distinguish between twoclasses of events, namely, explosions and earthquakes. One difficulty is in selectingan easily estimated set of parameters that consistently distinguish small explosionsfrom natural earthquakes with a high degree of reliability. We have examinedthe following parameters (measured on the vertical component SHS seismograms):1) amplitude ratio of maximum trace to first cycle of P-wave (R); 2) rate of decayof P-wave coda (a); 3) dominant frequency of P-wave (f); 4) dominant frequency ofmaximum trace amplitude (F); 5) first-motion of P-wave (FM); 6) local magnitudeof event (AIL); and 7) origin time of event (T). The primary goal is to ascertaincharacteristic patterns which reliably identify the explosions (also called blasts) sothey can be culled from the list of events recorded at SHS in order to estimate therate and pattern of seismicity in the region.A summary of the probabilities for the seven parameters, discussed above, isgiven in Table 1. The dominant frequency of the maximum trace amplitude (F)has a much higher probability for identifying blasts (> 99 per cent) than does any