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

Catalogue of Chinese Earthquakes (M 2 6.5) 1274.2. From Table 2, it can be seen that after careful revision the magnitudes ofsome strong earthquakes differ greatly from their previously adopted values. Forexample: for the August 22, 1902 Xinjiang earthquake, the Ms calculated usingdata from Gutenberg's worksheet was 8.2. After calibration, it was revised to 7.7.The epicenter of the August 31, 1902 Xinjiang earthquake was redetermined. Accordingto Gutenberg's worksheet, its Ms is 7.4. Abe (1983a) changed it to 7.0 bycorrecting the effective amplification of the undamped seismograph, and later hefurther revised it to 6.8, which agrees well with the revised magnitude of USSR.The magnitude of the August 30, 1904 Sichuan earthquake was estimated to be 6from macroseismic observations. By using the data of Gutenberg's worksheet and10 other stations, Ms = 7.5 was obtained. It was further revised to 6.8 after calibration.In addition, the epicenter determined from instrumental data is 1" away fromthe macroseismic location. The calculated Ms for the December 23, 1906 Xinjiangearthquake was 7.9. After calibration, it was reduced to 7.2. The magnitude of theDecember 21, 1913 Yunnan earthquake was estimated to be between 62 and 7 frommacroseismic observations in 1971 and 1983 editions of the Catalogue of ChineseEarthquakes respectively. The result of our calculation based upon instrumentaldata is Ms = 7$, which is very close to Abe's revised magnitude of 7.2. In thetwo editions of the Chinese catalogues, the magnitude of the July 31, 1917 Yunnanearthquake (the instrumentally determined epicenter is at Sichuan) was givenbetween 6; and 6: respectively. Abe corrected it to 7.5. According to the datafrom several seismic stations, we obtained Ms = 74. Similar circumstances existfor the Taiwan earthquakes, although the correction is less. As the instrumentaldata increased gradually after 1918, such large corrections of magnitude becamerare. However, the accuracy and reliability were improved through careful revision.For example: the magnitude of the May 23, 1927 Gansu earthquake was taken as8.0 by Gutenberg. Among the magnitudes calculated by using formula (1) from thedata of 15 stations, only one attained 8.1, with all the others lower than 8. Theaverage value is 7.8, near Abe's value of 7.9. We revised it to 7.9. Based upon thedata of 10 seismic stations, we revised the magnitudes of the two August 1, 1937Shandong earthquakes to 6.8 and 6.7 respectively by using formula (3).4.3. During the period from 1900 to 1948, the number of stations was insufficient,the seismogram quality was poor, the station distribution was not optimal, andthe data collected were insufficient. The fact that the frequency-magnitude curve(Figure 7) bends below Ms = 6.5 indicates that many earthquakes with magnitudesless than 6.5 were not recorded. The b-value for the straight line portion of the curveis 0.84. It is close to the value b = 0.90 f 0.02 obtained by Gutenberg for worldwideshallow shocks, but differs greatly from the value given in Report of Seismic Zoningof China (State Seismological Bureau, 1982).4.4. Figure 8 shows the variation in annual frequency, energy and cumulative strainrelease for earthquakes with M 2 6.5. Obviously, the seismicity had a maximum in1920, when the Ms = 8.6 Haiyuan earthquake and the Ms = 8.0 Taiwan earthquakeoccurred. The year 1902 was the next largest and includes the Ms = 7.7 Atush,Xinjiang shock. In addition, there are small jumps in the years 1906, 1927, 1931,and 1947.