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

Analysis of Historical Seismograms of Large Mexican Earthquakes7s18.5 yr data mb has been converted to MS using 2 relations (shown by cross andsolid dots in Figure 3):Ms = 1.8 mf, - 4.3 (Wyss and Habermann, 1982),Ms = 1.142mb - 0.821 (Mexico; Singh et al., 1983).The data shows that the 18.5 yr data is representative of 75.5 yr data and thatthere is a "bulge" in log N - MS plot implying a deficiency of events in the range6.5 IS Ms 2 7.4. If the Oaxaca region is considered separately (Figure 4) the deviationof log N - MS plot from GR relation appears dramatic. In terms of seismic riskstudies this observation is of paramount importance since fitting a straight line, logN = a - b Ms, to the data is grossly inadequate. The data suggests a deficiencyof earthquakes in the range 6.4 IS Ms 7.4 which is seen more clearly in Figure 5.The earthquakes in Oaxaca appear to have a characteristic magnitude of Ms 2 7.7;1000N100X xe Ms 1.142 mb-0.821 (Mexico)XMsz1.8mb-4.3 9.(Wyss 8 Habermonn,l982) 6864810xMs , 63.5 -81, normalizedI'I1 II I1A M,, 1906 -1981 ,755 yrb I I I A A I13.5 4.5 5.5 6.5 7.5 8.5IAAFigure 3. N (cumulative frequency) vs. MS for the combined regions of Oaxaca,Guerrero, and Michoacin. Data from July 1963 to December 1981 (18.5 yr) have beennormalized to 75.5 yr (solid circles). Observed N(Ms) data (Ms > 6.4) from July 1906to December 1981 (75.5 yr) are shown by solid triangles. Note the bulge in log N - Msplot. (E'rom Singh et al., 1983).