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

Pattern Analysis of Small Earthquakes and Explosions 355Table 1. Characteristic Pattern ParametersParameter Threshold Probability(percent)R > 3.7 63a: > .77 76f < 3.8 HZ 57F < 5.5 Hz > 99FM C 64ML < 2.0 73T 1800 5 T < 0300 80other individual parameter. Thus the simplest characteristic pattern, for identifyingblasts, is described by the single parameter (F). This parameter probably appliesmore generally, whereas some of the others do not.The dominant frequency (F) of the maximum trace amplitude (AM) is estimatedby the maximum likelihood method using the time differences between five consecutivepeaks and troughs centered on AM. The dominant frequency for a blast isFg = 2.9 f 0.7 Hz and for an earthquake it is FE = 8.9 zt 0.8 Hz. For a thresholdof 5.5 Hz, the probability that an event is a blast, if FI 5.5 Hz, is more than 99per cent. The relatively low-frequency Fb is attributed to the excitation of a fundamentalor higher mode Rayleigh wave (Mooney and Bolt, 1966; Cara and Minster,1981). Theoretically, an explosive surface source will efficiently excite highermodeRayleigh waves at frequencies above 1 Hz in the relatively low velocity surface layers.If F for an event is not available (owing to the trace being too faint to read, say)the next best characteristic pattern is described by a set of three parameters (FM,a, and f). If, simultaneously, FM = C, cy > 0.77 and f < 3.8 HZ for an event, theprobability is 96 per cent that the event is a blast.7. Rate of SeismicityFrom July 1952 through May 1963, 109 earthquakes (1.8 5 ML 5 2.8; 0300 5T c 1800 UTC) were recorded within 50 km of SHS. The cumulative rate of seismicityiswith a corresponding variance oflog N 7 2.80 - 0.87M~, (2)20 log^ = 0.0428 - 0.0196 ML + 0.00798 ML ',where N is the cumulative number of earthquakes with magnitude 2 ML. The coefficientsin (2) and (3) are normalized to earthquakes per year per 10,000 km2. Therate of earthquake occurrence (r = 10I0gN) and its standard error (g,.), calculatedfrom (2) and (3), are given in Table 2. Note that r cannot be reliably extrapolatedabove ML 2 3.0 where r = 1.4 f 0.82 earthquakes per year per 10,000 km2.(3)