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

DIGITIZATION AND PROCESSING OF THEJ.M.A. STRONG MOTION RECORDSIN THE PERIOD OF 2 TO 20 SECFROM NINE GREAT EARTHQUAKESR. Inoue and T. MatsumotoFaculty of Engineering, Ibaraki UniversityIbaraki-Ken 916, JapanABSTRACT17 seismograms (including 29 horizontal components) from 9 greatearthquakes (Ms = 7.6 to 8.5) around the Japan arcs recorded by theJ.M.A. (or C.M.O.) low-magnification seismograph network within an epicentraldistance of several hundred kilometers were digitized and analysed.Because the standard strong motion accelerograph (SMAC etc.) cannotaccurately record the ground motions whose periods are longer thanabout 4 sec, this data base is very useful for seismic design of large-scalestructures with natural periods around 10 sec.The digitized data were corrected for instrument characteristics, andthe high and low frequency noise in the data was removed by applying abandpass filter (the upper cutoff period is 20 sec, and the lower is 2 sec).Time history of corrected ground motions were obtained. Maximumground displacement, velocity and acceleration are about 23 cm, 12 kineand 28 gal, respectively.Velocity response spectra, Sv, for the period range of 2 to 20 sec withdamping factors, h, of 0.1% and 2% were computed (h = 0.1% correspondsto oil storage tanks and 2% to steel structures such as suspension bridges).Here we denote ( SV)~ as the peak value of Sv for each component. Theresults can be described as follows: (1) the maximum value of (SV)~ withh = 0.1% is about 190 kine, and there are 7 components whose ( SV)~ withh = 0.1% are larger than 110 kine (the value 110 kine is the upper limitfor Sv of the seismic design regulations for oil tanks in Japan); (2) themaximum value of (SV)~ with h = 2% is about 70 kine, and there are 16components whose (SV)~ with h = 2% are larger than 40 kine (the value40 kine is the upper limit for Sv of the seismic design specifications forsuspension bridges in Japan). These results suggest that those seismicdesign specifications for large structures should be reexamined.1. IntroductionIn view of the recent increase of large-scale structures such as large oil tanksand long span suspension bridges whose natural periods are around 10 sec, theknowledge of the characteristics of ground motions in those period range from greatearthquakes is becoming increasingly important for seismic design of such structures.For this purpose, three years ago we digitized the J.M.A. strong motionrecords (so-called for their low magnification) from the 1968 Tokachi-Oki event(17 seismograms, including 50 components) at the Public Work Research Institute(JSCE’s Tank Committee, 1982). Following this work, we recently digitizedHistorical Seirmogramsand Earthquakes of the World390Copyright 0 I9Xh by Academic Press, Inc.All rights of reproduction in any form reserved.