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

Evaluation of the Caracas Earthquake of October 24, 1900 205or 13.7 % of the critical damping. With this and the static gain of 6, the resonantgain = 6/(2() = 21.5, so that the dynamic gain used by Abe and Noguchi (1983) isjust 3.6 times larger than the static one, and this appears not to be a representativeresult for most Milne seismometers which are not equipped with a device for viscousdamping.Kanamori and Abe (1979) made important tests of a reconstructed Milne seismograph,the free period of which is 15 sec, and h = 0.05 or 5 % of the criticaldamping. The problem with the dynamic response is discussed in that paper.From the Kew seismogram (Figure 2), we obtain a zero-tepeak trace amplitudeof about 8 mm for the maximum of the 20 sec Rayleigh waves recorded between9h 49m to 9h 52m. Taking into consideration of the fact that theoretically thevertical component of a pure Rayleigh wave has an amplitude 1.4 times larger thanthe horizontal one, and that a cosine has to be applied to the trace amplitude,projecting it into the plane of the true oscillation of ground particles, and puttingthe result into the relation:Ms = log A + 1.66 log A’ + 2.0where A is the ground amplitude measured in microns and A’ is the epicentraldistance in degrees, we obtain with a dynamic gain of 33 and the ground motion of400 p, Ms = 7.6 which is about equivalent to mb = 7.1.The seismogram of the Caracas earthquake recorded on Mauritius Island (Figure1) is difficult to evaluate because of too many unknown constants and otherinformation. If the seismogram represents the E W component, the effective gainfor 20 sec Rayleigh waves is about 200, and since the maximum amplitude is 19 mmon the seismogram, we obtain Ms = 7.5.Magnitudes as a function of the duration of the earthquake record 7 (in minutes)were calculated by Kondorskaya and Shebalin (1977) by the relation (depth lessthen 70 km):Ms w M, = 2.4 log T + 1.6with the empirical result of t = 37 for Milne seismographs with an effective gainof 30. As the factor of T decreases if the gain increases, we may use for the mostresponsible gain of = 36, a factor 2.5 for log 7, and with a duration of about 150minutes, we get Ms = M, = 7.7, but also higher values could be obtained as longas ‘duration” is not well defined or adjusted to a certain level of ground motion.3. Summary of ResultsFor the Caracas earthquake of October 29, 1900, the preliminary results of thispaper are summarized together with a few data from other authors for comparison.