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

~~228 M. DeBecker and T. CamelbeeckTable 2. Stations and Parameters Used for Locationof the Earthquake of April 3, 1949 at 12h 33mStations A PN 0-C residues(km)HEEPARSTRBASSTUNEUJRSZURRAVCHU140.9216.7308 .a411.7412.3437.5468.4474.4499.7566.212h 34m 6.0s12h 34m 15.0s12h 34m 30.0s12h 34m 39.2s12h 34m 38.5s12h 34m 42.1s12h 34m 47.0s12h 34m 46.9s12h 34m 50.0s12h 34m 58.9s-0.10-0.45+3.17-0.33-1.10-0.61+0.48-0.37-0.40+0.30Magnitudes were determined in two ways: 1) Ms from the Prague formula(KQrnik, 1969); and 2) mbLg from the maximum amplitude of the vertical componentof the Lg-waves (Camelbeeck, 1985), or from the vector sums of the maximumamplitudes of the Lg-waves recorded on the horizontal components when nodata are available from the vertical component (in this case the amplitude value isdivided by a factor 2.5).t n b = ~ ~ 2.60 + 0.833 loglo (A/lO) + 0.0011A + log,, A(A) (2)The values of the function .(A) given in Table 3 are derived from KArnik (1969).Period and amplitude data for the earthquake of 11 June 1938 are summarized inTable 4. Table 5 shows the limits of distance and period within which the Pragueformula holds.It is interesting to compare the values of magnitude obtained from different stations.The scatter of both Ms and mbLg values from the mean and the variation ofMs with epicentral distance are plotted in Figure 3. Considering the date at whichthe records were made, the scatter is gratifyingly small.Figure 4 shows the maximum ground displacements for Lg-waves of one secondperiod as a function of epicentral distance for both events, together with curvesfor shocks of different magnitude. There does not appear to be any systematicdependence on epicentral distance, suggesting that historical seismograms can bemade to produce consistent results for studies of this kind, and therefore have aspecial value for the study of regions with low seismicity.Table 3. a(A) for LgHA" I 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 5.0 6.0 8.0 9.0U(A) 1 2.2 2.7 3.0 3.3 3.6 3.8 4.0 4.1 4.4 4.7 4.80 4.86