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

92 S. Stein, E. A. Okal and D. A. WiensIMOMENT AS A FUNCTION OF STRIKEASSUhllNG DIP 45 SLIP 90CHRISTCHURCH9 ...._ . I _.. .:._CALCULATED FROM----- RAYLEIGH..........FEBRUARY 29, 1944& 200 220 240 260 280 300 320 340 360k-N-3-rvL5w2 3.00 I20DE EIL T10STRIKE 318 DIP 45 SLIP 100M 7.2 Moment 1.5 xdyne-cm200 220 240 260 280 300 320 340 360STRIKEFigure 5. Mechanism determination for the 1944 M = 7.2 Chagos-Laccadive Ridgeevent (Wiens, 1985). All high quality first motions show compression; the final mechanismwas constrained by minimizing the variation in seismic moments determined from bodyand surface waves. This process is shown for two stations.On the other hand, until recently, present day global plate motion models (LePichon, 1968; Chase, 1972, 1978; Minster et al., 1974) assumed a single rigid Indianplate. This assumption was reasonable given the seismicity maps in generaluse showing only events beginning in the 1980's (e.g., Tarr, 1974). Stein and Okal(1978) pointed out that examination of the historical seismicity yielded a very differentpicture. Far from being "aseismic", the Ninetyeast Ridge area is a quiteactive seismic zone; four magnitude seven or greater earthquakes (including onewith Ms = 7.7) and ten magnitude six events have occurred in the general areasince 1913 (Figure 6). This level of activity is much greater than along any spreadingridge and comparable to that along the southern San Andreas fault. The largerearthquakes occur along the segment of the Ninetyeast Ridge north of about 10'5.Stein and Okal (1978) pointed out that this phenomenon had, in fact, been notedby Gutenberg and Richter as early as 1941 in the first edition of Seismicity of theEarth:"A peculiarly isolated group of shocks occurs near 2'S, 89'E ... Withother epicenters near 90°E north of the Equator, there is suggested aminor seismic belt following imperfectly known rises and ridges roughlynorth and south."