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

APPLICATION OF MODERN TECHNIQUES TOANALYSIS OF HISTORICAL EARTHQUAKESS. Stein and E. A. OkalDepartment of Geological SciencesNorthwestern University, Evanston, IL 60201, USAD. A. WaensDepartment of Earth and Planetary SciencesWashington University, St. Louis, MO 6SlS0, USAABSTRACTAnalysis of historical earthquakes is essential for interpretations of seismicityin regions away from recognized plate boundaries, due to longearthquake recurrence periods. We have applied a number of techniquesdeveloped for WWSSN data to a variety of instrumental records datingback to the 1930’s. Basic focal mechanism constraints can be obtainedfrom P- and S-waves polarities and S-wave polarizations. Surfacewave amplitudes and body wave modeling provide further mechanismconstraints and allow seismic moment estimation. Body wave modelingalso yields source depth and time function information. These techniquescan be combined to find a solution which simultaneously fits the differentdata types when only a few records are available. In one approach, themoment variance reduction technique, the model space is systematicallysearched for the mechanism which provides the best fit to P-, SH-, Love,and Rayleigh wave amplitudes. We present three examples where significanttectonic processes were identified from analysis of historical earthquakedata. Such data first demonstrated the large magnitude strike-slip“intraplate” seismicity along the “aseismic” Ninetyeast Ridge, showed thecomplexity and extent of the deformation in the Chagos region, and providedthe key constraints for a new tectonic model in which a diffuseplate boundary runs east from the Central Indian Ridge to the NinetyeastRidge, and northward along the Ninetyeast Ridge to the SumatraTrench. Similarly, historical earthquakes along the eastern Canadian passivemargin demonstrate that such margins can remain tectonically active,especially in the presence of stresses due to deglaciation or rapid sedimentloading. Finally, the historical seismicity of the young lithosphere in thesoutheast Pacific provides important information on the complex stressrelease mechanism in such areas.1. IntroductionWith the advent of the WWSSN in the early 1960’s and of digital networks in thelate 1970’s, the past 25 years have seen dramatic increase in the quantity and qualityof seismological data. As a result, many investigations relying on compilations ofseismicity restrict themselves to the recent data, and overlook the existence andavailability of useful seismograms predating 1962, despite the fact that teleseismicinstrumental recording dates back to 1889. In particular, historical seismologicalHislorical Seismogramsand Earthquakes of the World85Copyright 0 1988 by Academic Press, Inc.All rights of reproduction In any form reserved.