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

Pattern Analysis of Small Earthquakes and Explosions357+0 KM10203040500 1 2 3 A 5 6 7S-P TIME [SECIFigure 6. Plot of differential S-P travel-times versus apparent angle of incidence. Thebold lines are depth contours. The plus symbols are for compressional and open circles arefor dilatational first-motions, reqectively. Notice the concentration of dilational events atshallow depths between 0 and 20 km.represents an upper bound on the focal depth for a given i, and ts-p. Eight eventslie below the h = 20 km contour and these events are candidates for deep focusearthquakes.The apparent angle of incidence, azimuth, and first-motion were determined fromthe three-component SHS recording. The first-motion polarities are plotted on alower hemisphere stereographic projection in Figure 7. When drawing nodal planesthrough the first-motion plot, it is assumed that the earthquakes in the region havea dominant mechanism. Although this composite plot has poorly constrained nodalplanes, with a few compressional first-motions in the dilatational quadrants, theinterpretation of a strike-slip mechanism is consistent with the local tectonics andwith the few previous results of fault plane solutions for this region (Bolt et al.,1968; Simila, 1981). The composite mechanism has a compressive stress orientationof about N 17' W and an associated east-west extension in a N 73' E direction.