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

38 Kat su y uki A beaddition to this, a technique is presented to estimate the origin times from arrivaltime data of surface waves. The present study is an extension of the previous workon the quantification of large earthquakes of early date, and small earthquakes aswell as large ones are treated here.2. Materials and Methods2.1. Materials UsedIn this study, reported amplitudes and times based on records of undampedMilne seismographs are used. These data are published in Report (BAAS, 1899)and Circulars (BAAS, 1900-1912). The former covers readings for 1897-1899, andthe latter for 18941912.The maximum trace amplitudes on the Milne seismograms are reported in unitsof mm. The minima of the reported amplitudes are 0.1 mm at most stations, andoften below 0.1 mm at Guildford, Victoria and Toronto. In Report and Circulars,arrival times are regularly reported in units of min or the tenth of min. The natureof the time data is discussed later.2.2. MagnitudesAdhering to the original definition of Gutenberg (1945), Abe and Noguchi (1983)calculated surface-wave magnitudes, Ms, byMs = log (A,/G) + 1.656 log A + 1.818 + s (1)where At is the maximum trace amplitude in pm on the single component of Milneseismograms, G is the effective magnification of Milne instruments, A is the epicentraldistance in degrees, and s is the station correction. Assuming that s = 0, theyadjusted G to make the calculated magnitude equal to Ms derived from amplitudedata based on various types of seismographs and experimental data based on anewly-built Milne seismograph. They concluded that G = 15.5 is most satisfactoryover a wide range of magnitude. We apply this result to the present study.Gutenberg and Richter (1954) calculated magnitudes (denoted here by MCR) for49 world earthquakes of 1910-1912. For comparison, the values of Ms have beenrecalculated by using the original method of Gutenberg (1945) and the amplitudedata given in the Gutenberg and Richter’s worksheets (for details, Abe, 1981); herewe call these magnitudes Ms(GR). Figure 1 shows a comparison of Ms(GR) withMs from the Milne data for 49 shocks. It is seen that MS from the Milne datais essentially equal to Ms(GR) over a wide range; Ms is only 0.02 f 0.19 loweron average than Ms(GR). Figure 2 shows a comparison between Ms derived fromthe Milne data and MGR taken directly from Gutenberg and Richter (1954). It isevident that MGR deviates from Ms, particularly for smaller and deeper shocks.This deviation originates from a somewhat ill definition of the MGR scale (Abe,1981). It should be emphasized here that Ms used in this study is different fromMGR.In the course of the previous study and the later work, it has been observedthat the calculated magnitudes from the Milne data are above average at certainstations, below at others. This observation is considered to be related chiefly to