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

26 Hiroo Kanamo riIf relatively complete data sets of old seismograms for major events are available,this type of study can be made more easily and thoroughly. There are still manymore important events which have not been examined thoroughly. Examples arethe 1957 Fox Island (Aleutian Islands) earthquake, the 1923 Kamchatka earthquakeand the 1906 Valparaiso (Chile) earthquake.7. Strong-motion Seismology.Since near-field strong motion records of very large earthquakes are seldom available,strong motion records from historical earthquakes are very important.In an attempt to estimate the amplitude of strong ground motions generatedby the 1906 San Francisco earthquake, Jennings and Kanamori (1979) examinedold seismograms recorded by Ewing duplex pendulum seismographs (Figure 7).Although the quality of these seismogranis is rather poor, Jennings and Kanamoricalibrated one of the old seismographs which recorded the San Francisco earthquake,and estimated its local magnitude to be in the range 6; to 7. Despite the largeuncertainty involved in this determination, it is the only ML value for earthquakeswith Ms = 8.Some long-period strong motion seismographs developed in the early days of seismologyrecorded near field strong motions of several Japanese earthquakes (e.g. 1943Tottori earthquake; 1930 Izu earthquake; 1948 Fukui earthquake). These recordshave been used to estimate the gross rise time of fault motion (Kanamori, 1972a,1973; Abe, 1974a, 197413, 1978). Although these results are subject to large uncertaintiesbecause of the very limited number of records, they provide useful informationon the mechanics of faulting.8. Tsunami Earthquakes and Other Unusual EventsThe 1896 Sanriku and the 1946 Aleutian Islands earthquakes generated verylarge tsunamis despite their relatively small earthquake magnitude, about 7.5 to 8(Kanamori, 1972~). Although several mechanisms have been proposed (slow slip,shallow steep fault, landslide, etc.), the problem is still unresolved.The 1929 Grand Banks, Canada, earthquake is not only the largest historicalearthquake in Atlantic Canada, but also a very unique event in that it cut Trans-Atlantic cables in 28 places, and generated large tsunamis. Heezen and Ewing (1952)suggest that a large submarine landslide caused by this earthquake transformed intoa turbidity current which ruptured the Trans-Atlantic cables. The details of theseismological nature of this event, however, are still uncertain, and more detailedstudies of this unusual event using historical seismograms are desirable.Ben-Menahem (1975) analyzed a few seismograms of the 1908 Siberian explosion,and concluded that this explosion consists of an air explosion at a height of 8.5 kmand a ballistic wave. This is probably the only event of this nature that was recordedby seismic instruments.The Spanish deep focus earthquake of 1954 (mc, = 7.1; depth = 630 km) is avery unique event, because it is the only large deep focus earthquake that is notassociated with a well defined Benioff zone. In view of its uniqueness, Chung andKanamori (1976) made a detailed study of this event using historical seismograms