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

28 Hiroo Kanamori9. Difficulties in the Use of Historical Seismograms.The quality of old seismograms varies greatly from station to station as a functionof time, and difficulties are encountered frequently in analyzing historical seismograms.Most common problems include: (1) unknown instrument constants, (2) missingor uncalibrated time marks; (3) large solid friction between the stylus and paper,(4) 110 damping device attached to the seismograph; (5) cross coupling between theN-S and E-W components, and (6) unknown polarity.Because of these difficulties, there is no standard method for the analysis of oldrecords. Sometimes, the signal duration provides useful information on the size ofthe earthquake even if the instrumental constants are unknown. Even when thetime marks are missing, some distinct phases on the record serve as “time marks”.The problem of the solid friction becomes particularly serious when an attempt touse long-period (longer than 100 sec) waves for the seismic moment determinationis made. When damping is not provided or is very weak, extreme caution must beexercised for the use of the amplitude data. For seismographs in which one pendulumis used to record vectorial horizontal motions, (e.g. Wiechert seismograph),cross coupling between the two horizontal components sometimes occurs. In thiscase, the conventional method of rotating seismograms into transverse and radialcomponents cannot be applied.10. ConclusionAlthough the quality of historical seismograms is non-uniform and it is moredifficult to use them than standardized modern seismograms, old seismograms, ifproperly interpreted, provide invaluable information on earthquakes in the past, andevery effort should be made to save them, regardless of their quality, from possibleloss and to make copies in an easily readable form.REFERENCESAbe, K. (1974a). Seismic displacement and ground motion near a fault: The Saitamaearthquake of September 21, 1931, J. Geophys. Res., 79, 4393-4399.Abe, K. (197413). Fault parameters determined by near- and far-field data: The Wakasabay earthquake of March 26, 1963, Bull. Seism. SOC. Am., 64, 1369-1382.Abe, K. (1977). Mechanisms of the 1938 Shioya-Oki earthquakes and their tectonic implications,Tectonophys., 41, 269-289.Abe, K. (1978). Dislocations, source dimensions and stresses associated with earthquakesin the Izu peninsula, Japan, J. Phys. Earth, 26, 253-274.Abe, K. (1979). Size of great earthquakes of 1873-1974 inferred from tsunami data, J.Geophys. Res., 84, 1561-1568.Abe, K. (1981). Magnitudes of large shallow earthquakes from 1904 to 1980, Phys. EarthPlanet. hat., 27, 72-92.Abe, K., and S. Noguchi (1983). Revision of magnitudes of large shallow earthquakes,1897-1912, Phys. Earth Planet. Int., 33, 1-11.Bakun, W. H., and T. V. McEvilly (1984). Recurrence models and Parkfield, California,earthquakes, J. Geophys. Res., 89, 3051-3058.Ben-Menahem, A. (1975). Source parameters of the Siberian explosion of June 30, 1908,from analysis and synthesis of seismic signals at four stations, Phys. Earth Planet.Int., 11, 1-35.Ben-Menahem, A. (1977). Renormalization of the magnitude scale, Phys. Earth Planet.Int., 15, 315-340.