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

80 S. K. SinghDE BlLT (DBN) Z$I = 37", A= 84"LA PA2 (LPB) Z# = l38", A=43"t LJFigure 11. Observed (upper trace) and synthetic (lower trace) seismograms at DeBilt(DBN) and La Paz (LPB). Both DBN and LPB are Galitzin-Wilip seismographs (naturalperiod of seismometer and galvanometer -N 12 9). The largest down swing on LPB seismogramis clipped. Synthetics were generated with the focal mechanism shown in Figure 10and required three point sources (depths 40, 25, and 25 km). Trapezoidal source timefunctions for each source were taken as 2, 5, and 7 s. The seismic moments (M,) and thetime delays of the sources are indicated in the figure. (From Singh et al., 1985a).6. Seismic Moments of Large Mexican EarthquakesHistorical seismograms (mostly Wiecherts from Uppsala) have been used to estimateseismic moments of old large earthquakes at a period of about 40 to 60 sec(Espindola et al., 1981; Singh et al., 1982; Singh et al., 1984b). Since the focalmechanism of large eart,hquakes and the paths to Uppsala are essentially constant,the spectral amplitudes of surface waves at 40 to 60 sec, after calibration with respectto recent earthquakes with known M,, can be used to estimate M, of olderearthquakes. The basic assumption is that the 40 to 60 sec amplitude is on theflat part of the source spectra. This appears justified since the rupture lengths oflarge Mexican earthquakes are generally small (: 100 km). The M, estimates havebeen used in many studies. In particular, seismic potential in Acapulco-San Marcosregion was quantified (Singh et a/., 1982). The historic seismicity and this century'searthquakes suggested the validity time-predictable model (Shimazaki and Nakata,1980). There appears a high probability of a large earthquake in the region betweennow and 1993 (Singh et a!., 1982).