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

118 Refi Cheng, Yushou Xie, and Zhifeng Ding3.3. The Xujiahui (Zi-ka-wei) seismic station has published bulletins since 1909.Mr. Sun Quinxuan revised the bulletin by reanalyzing twice as many original seismogramsin 1979 and 1982. The calibration curve is shown in Figure 3 and theformulas for magnitude determination are:MS = log ($) + 2.29 logAo + 2.53 (13' < A 5 40°),where A is the maximum displacement (in pm) of a single horizontal component ofthe ground motion. The calibration curve is composed of two segments with obviouslydifferent slopes, and may be caused by the fact that the maximum amplitudeat different epicentral distances correspond to different types of seismic waves. Thescatter in the data points gives a r.m.s. error of f 0.3.3.4. For earthquakes without amplitude data, magnitudes listed in related catalogueswere used and their values were extrapolated from the G-R magnitude scale.3.4.1. For earthquakes in the western part of China, mainly U.S.S.R. catalogueswere used. For earthquakes before 1962, magnitudes were revised by using formula(3) in New Catalogue of Strong Earthquakes in USSR (Kondorskaya and Shebalin,1977). Statistical results show that for Chinese earthquakes the magnitudes givenare near the G-R magnitudes, with a correction of +0.08.3.4.2. In Richter's (1958) catalogue, unified magnitude based upon surfacewaves, i.e. modified magnitude scale, is used. Statistical results show that themagnitude value is 0.22 too high, and thus can not be used.6.0 - ..-5.0 --I4.01 I 1 I l l I I 1 I4 5 6 7 a g i o 15 20 30 40 A"Figure 3. Calibration curve of Xujiahui (Zi-ka-wei) station.