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

392 R. Inoue and T. MatsumotoINIAki taic-. .IXElFigure 1. Locations of the stations and the epicenters of the earthquakes.The seismogram at Akita from the 1968 Tokachi-Oki event was previously digitized3 years ago with 16 other seismograms from that event (JSCE's Tank Committee,1982). However, because the levels of response spectra and peak groundmotions at Akita were the largest among the 17 seismograms from that event, weincluded this record in this analysis to compare the levels of ground motions withother seismograms in Table 1, which were newly digitized.We collected more than 1000 J.M.A. (or C.M.O.) strong motion records duringthe large or great earthquakes from about 100 J.M.A. stations. The records onlyfrom great earthquakes were first selected. Among them, the records which are notextremely off-scale were finally used for digitization. Figure 2 shows an example ofa seismogram recorded on smoked paper, however this example is a negative of theoriginal.Because the standard strong motion accelerograph (SMAC etc.) cannot accuratelyrecord the ground motions whose periods are longer than about 4 sec, thisdata base is very useful for seismic design of large-scale structures which have naturalperiods around 10 sec.