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

~~~24 Hiroo KanamoriThatcher et al. (1975) analyzed several seismograms of large earthquakes whichoccurred on the San Jacinto fault, California, to determine the spatial-temporalpattern of seismic moment release pattern along the San Jacinto fault. On the basisof this analysis, Thatcher et al. identified a seismic slip gap where the amount ofseismic slip for the previous few decades is significantly smaller than in the adjacentsegments. This seismic slip gap is located near Anza, southern California, and isextensively monitored by various seismic instruments.Recently several investigators examined the possibility of large subduction-typeearthquakes along the plate boundary between the Juan de Fuca plate and the NorthAmerican plate (along the Oregon-Washington coast) (Figure 6). Although no largeearthquakes are known to have occurred here at least for the past 150 years, comparisonof this subduction zone with other subduction zones in the world suggeststhat this subduction zone may be capable of generating large thrust earthquakes(Heaton and Kanamori, 1984).One important feature of this subduction zone is the very young (about 10 My)subducting Juan de Fuca plate. Can a subduction zone with such a young subductingplate generate a large earthquake? To answer this question, it is important toexamine seismicity of subduction zones with similar characteristics.A recent study by Singh et al. (1985b) has an important bearing on this problem.A large (Ms = 8.2) earthquake occurred in Jalisco, Mexico, in 1932. The epicenterof this earthquake is very close to the triple junction between the COCOS, the NorthAmerican, and the Rivera plates. However, the exact location and the nature ofthis event is not well known until recently. Singh et al. (1984, 1985b) examinedold seismograms from several Mexican stations to determine the aftershock areaof this earthquake, and concluded that the 1932 Jalisco earthquake occurred onthe boundary between the Rivera and the North American plates (Figure 6). Thegeometry and age of this plate boundary are very similar to those of the Juan deFuca plate boundary. It is important to note that a large earthquake did occur ata subduction zone very similar to the Juan de Fuca boundary.1932 Jalisco Earthquake300 kmll AFigure 6. Comparison of the geometry of subduction zone along the coast of Oregonand Washington (left), and Jalisco (right). The rupture zone of the 1932 Jalisco earthquakeis indicated (After Singh el al., 1985b).