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

Analysis of Historical Seismograms of Large Mexican Earthquakes 77OAXACAWIECHERTGALlTZlNIUPP E1GOT EIGOT z1COP zSTU Z--I I I II9 Oct 1928 --c/dcMs'7.8 , I I 115 Jan 1931 ~Ms= 8.023 Dec 1937- - .Ms= 7.7--JL.n-I I I I I I I I 1P-P 2.5cm)4 Dec 1950 c_LMs=7.3 , I23 A q 1965 A -4- + WWSSN ,STU SIMPLEM,= 7.8 , , I I2 Aug 1968 ~__rv_Ms=74 , , , I I I-,.--. -.A- WWSSN SIMPLE29 NOV 1978 --firMs=784----JL-- W WSSN SIMPLEWWSSNSIMPLEWWSSN- SIMPLEFigure 7. P-waves from Oaxaca earthquakes recorded on Wiecherts (at UPP, GOTand COP) and Galitzin-Wilip (at STU). Note that all P-waves, except for the 1931 andthe 1950 earthquakes, are simple. The earthquake of 1931 is complex with first motion atGOT Z and COP Z being up. Compare it with first motion of other earthquakes whichare down. (Polarity of these stations is reversed). Thus, the 1931 event has a drasticallydifferent focal mechanism. The 1950 earthquake is complex at UPP as well as STU. Theearthquakes of 1982 are not big enough to generate visible P-waves on the Wiecherts.(From Singh et al., 1984b).