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

Digitization and Processing of J.M.A. Strong Motion Records 391and analysed the 17 seismograms (including 29 components which are all horizontal)from 9 great earthquakes around the Japan arcs recorded by the J.M.A. (orC.M.O.) low-magnification seismograph network. J.M.A. means the Japan MeteorologicalAgency and C.M.O. means the Central Meteorological Observatory. C.M.O.is the predecessor of J.M.A.In the following sections we describe in some detail the digitization and correctionprocedures of the records and then compare the response spectra of the records withthe Japanese seismic design specifications for large structures.2. RecordsTable 1 shows the list of the seismograms used in this analysis, and Figure 1shows the locations of the stations and the epicenters of the earthquakes. As shownin Table 1, these earthquakes occurred from 1933 to 1983 and their magnitude, Ms,are between 7.6 and 8.5. The epicentral distances of the records axe between 49 and501 km.The type of seismograph which has been operating since about 1950 at the J.M.A.stations is the J.M.A. strong motion seismometer, and those which operated duringthe period from about 1930 to about 1950 at the C.M.O. stations are Omori’s,Imamura’s and C.M.O. strong motion seismographs. Hereafter we will call thelatter the old type seismographs in this paper. Table 2 shows the characteristicsof each seismograph for each record. The range of the natural period of thoseseismographs is from 3.5 to 7.1 sec.Table 1. List of the SeismogramsNo. Date Earthquake Lat. Long. D Ms Station A ION OEEl 1933. 3. 3 Sanriku-Oki 39.25 144.50 - 8.5 Sendai 332 5E2 1938. 5.23 Iwaki-Oki 36.50 141.00 - 7.6 Mito 49 5E3 1938.11. 5 Fukushima-Ken- 36.75 141.75 60 7.7 Mito 122 5Toho-Oki( 1st .) Sendai 184 5E4 1938.11. 5 Fukushima-Ken- 37.25 141.75 60 7.7 Mto 150 4Toho-Oki(2nd.)E5 1938.11. 6 Fukushima-Ken- 37.25 142.25 60 7.6 Sendai 164 4Toho-0 ki( 3rd .) Mito 186 4E6 1944.12. 7 Tonankai 33.75 136.00 - 8.0 Murotomisaki 178 4Mishima 313 4E7 1946.12.21 Nankaido 32.50 134.50 - 8.2 Sumto 208 5Miyazaki 296 4E8 1968. 5.16 Tokachi-Oki 40.90 143.40 9 8.1 Akita 246 4Skata 377 3Niigat a 501 3E9 1983. 5.26 Nihonkai-Chubu 40.36 139.08 14 7.7 Mori 231 4Niigata 272 3Tomakomai 328 2No. = Earthquake number; Lat. = Epicenter latitude; Long. = Epicenter longitude;D = Depth in km; A = Epicentral Distance in km; I = Intensity (J.M.A. scale).