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

Analysis of Historical Seismograms of Large Mezican Earthquakes 754. Complexity/Simplicity of P- Waveforms of Large Earthquake alongthe Mexican Subduction ZoneIt was pointed out by Chael and Stewart (1982) and Astiz arid Kanamori (1984)that P-waveforms of large subduction zone earthquakes of Mexico are very simpleat teleseismic long period (LP) WWSSN stations with only few exceptions. Becauseof its implications to the tectonics and rupture mechanics, we decided tostudy the complexity/simplicity of P-waveforms of older earthquakes using historicalseismograms. This presented difficulties because the response characteristicsof the older seismographs (mostly Wiechert and Galitzins) are very different fromLP WWSSN. A comparison of LP WWSSN seismograms at ESK of large Mexicanearthquakes with the corresponding Wiechert and Galitzin seismograms at someEuropean stations suggested that, in spite of different responses of the seismographs,qualitative and quantitative inferences regarding complexity/simplicity canbe drawn from Wiecherts and Galitzins, respectively (Figure 6). Figure 7 showsWiechert and Galitzin seismograms of large Oaxaca earthquakes recorded at Europeanstations. With the exception of 1931 and 1950 earthquakes all earthquakesappear simple. We have now checked Galitzin seismograms from DeBilt and ourprevious conclusion remains valid. Note that the polarity of 1931 earthquake isinverted with respect to other earthquakes. The polarity at European stations forlarge thrust earthquakes in Mexico are always compressional. A dilatational firstmotion, then, suggests an anomalous earthquake. This led to the suggestion thatthe 1931 earthquake was a great (Ms = 8.0) normal or strike-slip faulting event(Singh et al., 1084b).Study of historic seismograms showed that large earthquakes in Oaxaca aremostly simple, in Guerrero and hlichoacin simple or complex, whereas in Jaliscothey are complex (Singh et al., 1984b). [The recent great earthquake of Michoach(19 September 1985; Ms = 8.1) consisted of 2 subevents as seen on LP WWSSN].Synthetic and observed P-waves of some earthquakes itre shown in Figure 8. Notethat we have assumed a typical thrust focal mechanism based on recent studiesfor the synthetic modeling. Because of the location of European stations on thefocal sphere, small changes in focal parameters do not affect the synthetics. Sourcetime functions and depths estimated from just one European stations are consideredreliable (Singh et al., 1984a).5. The Great Oaxaca, Mexico Earthquake of 15 January 1931: LithosphericNormal Faulting in the Subducted Cocos PlateAs mentioned earlier the first motion at European stations for the 1931 earthquakesuggested that the event was not a thrust fault (Figure 7). Location of the1031 earthquake is shown in Figure 9. The focal mechanism constructed from manyhistorical seismograms is shown in Figure 10 which suggests a normal faulting ona E-W plane dipping 56' to the north. Figure 11 shows synthetic and observedP-waves for Galitzin seismograms at DeBilt and LaPaz. The estimated depth ofthe source is about 25 to 40 km. A section through AA' in Figure 10 is shown inFigure 12. It appears that the 1931 earthquake broke the entire subducted Cocosplate. Such great normal faults generally occur near the trench (Sanriku type). Theearthquake of 1931 occurred inland and destroyed the city of Oaxaca. Great normalfaulting earthquake may occur elsewhere in Mexico and add another elementin seismic risk estimation. Further details are given in Singh et al. (1985a).