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

22 Hiroo Kanamorievents often have a very distinct first motion while thrust events have an emergentbeginning. This difference can be used to distinguish normal-fault mechanism fromthrust mechanism. Recently, Kanamori and McNally (1982) applied this method tothe records from the 1906 Ecuador-Colombia earthquake (Mw = 8.8) and concludedthat it is a thrust event.Singh et al. (1985s) found that the great Oaxaca, Mexico, earthquake of 1931,which had been generally considered a large interplate thrust event, is a lithosphericnormal-fault earthquake in the subducted COCOS plate (Figure 4). This result hasan important bearing on the evaluation of seismic potential along the Mexicansubduction zone.3. Rupture Process of Large Earthquakes.The rupture process of large earthquakes has been one of the important researchsubjects since the beginning of instrumental seismology. Imamura (1932, p.267)examined the seismograms of the great Kanto earthquake of 1923 and identifiedseveral discrete events during the main shock sequence. This type of multiple shockhas been studied by various investigators, particularly for several large earthquakessuch as the 1960 Chilean earthquake (Nagamune, 1971), and the 1964 Alaskanearthquake (Wyss and Brune, 1967).Many recent studies have demonstrated that mechanical heterogeneities on thefault plane have important effects on generation of strong motions, precursory seismicitypatterns, and fault mechanics. In this context, Hartzell and Heaton (1985)examined Benioff long-period seismograms of large subduction-zone earthquakes tostudy the regional variation of the nature of subduction zones. Since the occurrenceof great earthquakes is infrequent, the use of historical seismograms is crucial tothis type of study.NCoostooxocoE100 km0 COMPRESSION0 DI-ATATIONFigure 4. First-motion data of 1931 Oaxaca earthquake on the equal area projectionof the lower focal hemisphere (left). The SW-NE cross section perpendicular to the coastline near Oaxaca. The mechanism of the 1931 event is schematically shown (right). (AfterSingh et al., 1985a)