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

82 S. K. Singh7. The Great Jalisco, Mexico Earthquakes of 1932: Subduction of theRivera PlateThe Jalisco earthquake of 3 June 1932 (Ms = 8.2; Abe, 1981) was the largestearthquake that has occurred along the Mexican subduction zone since the instrumentalrecordings began. The seismograms at teleseismic distances suggest acomplex rupture (Figure 8). Singh et al. (1984b) modeled the Stuttgart Galitzinseismogram with 4 subevents with a total duration of 90 sec (Figure 8). The locationof the earthquake, however, has presented problems (Singh et al., 1985b; Eisslerand McNally, 1984). The earthquake was followed by another large earthquake on18 June 1932 (Ms = 7.8; Abe, 1981). A detailed study of the aftershock area ofthese earthquakes was essential since there was doubt whether they broke the Rivera(RIVE)- North American (NOAM) plate interface or the COCOS (COCO)- NOAMinterface. RIVE is a small, young plate subducting with a small relative velocityof about 2 cm/yr. It was important to know whether such plates are capable ofgenerating great earthquakes. The result was also likely to help in understandingthe seismic potential of other tectonically similar plates such as the Juan de Fuca.The aftershock area of the earthquakes was delineated using seismograms from 3stations of the Mexican Seismological Service (MNZ, GUM, TAC). Because MNZwas down from 3 to 8 June (following the mainshock) and S times could not be readfor larger aftershock at MNZ, a very careful analysis had to made of the polaritydata, (S-P) times at GUM, TAC, and MNZ (for smaller shocks after 8 June, 1932)to delineate the aftershock area shown in Figure 13 (Singh et al., 1985b). Theresult clearly shows that the great earthquakes of 1932 broke the RIVE-NOAMplate boundary. This conclusion is not affected by the uncertainty in the locationof the RIVECOCO plate boundary. The conclusion has obvious implications forthe Juan de Fuca subduction zone which has been suggested as a possible futuresite for large earthquake even though there is no historical evidence for such anevent (Heaton and Kanamori, 1984).8. Seismic Gaps and Recurrence Periods of Large Earthquakes along theMexican Subduction ZoneKelleher et a/. (1973) delineated seismic gaps along the Mexican subduction zone.Singh et al. (1981) compiled a catalog of last century’s large earthquakes of Mexicofrom historical descriptions of damages, relocated some events of this century andreexamined the seismic gaps and recurrence periods. Last three large earthquakes(25 October 1981, Ms = 7.3; doublet of 7 June 1982, Ms = 6.9,7.0; 19 September1985, Ms = 8.1) have occurred in the gaps designated by Singh et al. (1981) andKelleher et af. (1973). Of the remaining gaps the most outstanding one appearsto be the Guerrero gap. Data from last and present centuries suggest recurrenceperiods of 30 to 74 years for large earthquakes along the Mexican subduction zone.9. ConclusionsThis review has been intended to show the enormous amount of information thatcan be extracted from historical seismograms. These seismograms provide a basictool for fundamental research. At least for Mexican earthquakes the analysis ofhistorical seismograms has been very fruitful. We would be much worse off if we