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

Application of Modern Techniques to Analysis of Historical Earthquakes 89BODY WAVE MODELING -FEBRUARY 29, 1944SH WAVEFORMSUC C LEDE BlLT20 40 60 80 20 40 60 80P WAVEFORMSDE BlLT VERTICAL DE BlLT RADIAL CHRISTCHURCHE EUCCLE RADIALDATA1 2 tDEPTH24k u 20 40 i20 40 60 8020 40 60 20 40 60secondssecondssecondssecondsFigure 2. Synthetic body waves and data for the 1944 M = 7.2 Chagos-LaccadiveRidge earthquake (Wiens, 1985). The waveforms are well modeled for a focal depth of 12km for a variety of stations and instruments.filtering, to exclude periods below 100 s, the Love/Rayleigh ratio is about 1.5.Synthetic seismograms show that the Love/Rayleigh ratio is extremely sensitive tothe slip angle on the north-south nodal plane. The best match is obtained for aslip angle of 55O, a mixture of thrust and left-lateral motion. The power of themethod resides in that in many cases, the Love-to-Rayleigh ratio at a given stationcan span several orders of magnitude as a function of an unconstrained parametersuch as slip angle, thus reducing considerably the influence of uncertainties in thegains of the various instruments used (which are inherent in the method since Loveand Rayleigh waves are usually studied on different components). This is illustratedin Figure 4, for another earthquake near the Nintyeast Ridge. One fault plane isconstrained by P-wave first motions, Love/Rayleigh ratios at Resolute and De Biltuniquely determine the other nodal plane.