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

ANALYSIS OF HISTORICAL SEISMOGRAMS OFLARGE MEXICAN EARTHQUAKES (M, 2 7.0):SUMMARY OF IMPORTANT RESULTSS. K. SinghInstituto de Geofisica, U.N.A.M.C.U., Mexico 04510, D.F., MexicoABSTRACTAnalysis of historical seismograms recorded at teleseismic distances aswell as at stations of the Mexican Seismological Service has greatly improvedour understanding of the tectonics and the seismic risk along theMexican subduction zone. Important results include (1) homogeneousearthquake catalog for Ms 2 6.5 since 1906, (2) strong evidence of bulgein log N - Ms plot suggesting characteristic earthquakes with Ms e 7.7,(3) complexity/simplicity of P-waveforms of large earthquakes along theMexican subduction zone, (4) discovery that 15 January 1931 Oaxacaearthquake (Ms = 8.0) was an inland normal fault which probably brokethe entire subducted COCOS plate, (5) seismic moments of large earthquakes(Ms > 7.0) since 1906, (6) evidence that the great Jalisco earthquakeof 3 June 1932 (Ms = 8.2) broke the Rivera-North American plateinterface showing that small, young plates subducting with small relativeplate velocity are capable of generating great earthquakes, and (7) reexaminationof seismic gaps and recurrence periods of large earthquakesalong the Mexican subduction zone. We summarize these results whichshow the enormous value of the historical seismograms. Even though thequality and the quantity of historical seismograms as compared to themodern ones, is limited, there is a wealth of extremely useful informationwhich can be extracted from them. Of great value are the seismogramsfrom stations which have kept the old seismographs in continuous operation.1. IntroductionThe purpose of this paper is to summarize some of the important recent resultsrelated to tectonics and seismic risk in Mexico obtained from the analysis of historicalseismograms. By historical seismograms we shall mean seismograms recordedprior to the installation of World-Wide Standard Seismograph Stations (WWSSN)in 1962. In much of our analysis we have used seismograms from the stations ofthe Mexican Seismological Network and some key stations from Europe (Uppsala,Gottingen, DeBilt, Stuttgart, and Copenhagen, among others).2. A Catalog of Shallow Mexican EarthquakesA homogeneous catalog is essential in the statistical analysis of earthquakes.Although a catalog of Mexican earthquakes was prepared by Figueroa (1970), themagnitudes in this catalog are not reliable because of (a) inconsistent use of anuncalibrated local and regional magnitude scale, (b) clipping of seismograms forHistorical Seismogramsand Earthquakes of the World70Copyright (c) 1988 by Academic Press, Inc.All rights of reproduction in any form reserved.