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

His to ric a1 Se as rn og ra ms and In t e rp re tat ao n of Strong Earth q u a Ice s 59on absorbing and disseminating characteristics of medium. Incoherent radiationoriginates in the unevenness of rupture propagation and friction at a rupture and,possibly, at various secondary ruptures, according to Kostrov (1967).Because short-period waves are easily scattered by small-scale inhomogeneities ofthe Earth, short-period source radiation is essentially complicated with dispersedwaves. Therefore, it is worthwhile to undertake bandpass filtering of oscillations aswell as statistical screening of dispersed waves in order to specify, in pure form, theeffects related to the source. An important conclusion from this investigation is thatthe source radiates practically nothing in the time intervals when the envelope ofthe strong earthquake P-wave attenuates in a manner similar to the coda of P-wavesfor weak earthquakes. This important conclusion enables us to determine easily theinstant of stopping of the rupture propagation using the envelope of the P-waveof a strong earthquake, which includes the moment of source impulse termination.Figure 1 shows the envelope of P-waves and P-codas in the Aleutian earthquake.The envelope is plotted on the basis of the local maximum amplitudes on recordsof the TCHISS instruments with the one hertz frequency. It is seen that for weakearthquakes (M = 5.0) maximum amplitudes are reached in one to two secondswith their subsequent monotonous attenuation corresponding to dispersed waves -P-coda. At the same time for stronger earthquakes (M = 7.2) oscillations buildup for nearly 12 seconds, after which the source practically comes to a still sincethe envelope goes similar to the P-coda of a weak signal. For the earthquake witha magnitude M = 8.2 the record is more complicated - several large impulses ofincoherent radiation may be found there. Similarity to the envelope of P-codas of aweak earthquake is not observed until 180 sec after the P-wave arrival. This valuemay be assumed a typical duration of incoherent radiation for great earthquakes.Thus, the analysis of short-period records of historical seismograms of the strongestearthquakes help to obtain new information on the source mechanism, the depth,and the length of the radiation pulse, which are essential in particular for specifyingthe tsunami-generating capacity of earthquakes employing a relatively simplemethod.A-ff*24- 3I I I I I *ff f-80 200 T,CFigure 1. Envelopes of P-waves and P-codas of the Aleutian earthquake. (1,2) -04.02.65, (1) - M = 8.2, (2) - M = 7.3; (3) - 15.02.65, M = 5.0. Talgar (TCHISS stations1.0-1.6 Hz); arrows - levels in mm/s.