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

Catalogue of Chinese Earthquakes (M 2 6.5) 1193.4.3. The magnitudes given in Duda’s (1965) catalogue are quite complicated.Most of the values were adopted from Gutenberg (1954, 1956) and Richter (1958).For Chinese earthquakes, excluding those adopted directly from Gutenberg (1954) ,statistical results show that the magnitude value is 0.25 too high on the average(n = 30, (T = k0.25).3.4.4. For the Taiwan region, the earthquake catalogues used are: History ofTaiwan earthquakes (Taipai Observatory, 1936), Catalogue of major earthquakeswhich occurred in the vicinity of Japan (1885-1950) (Central Meteorological Observatoryof Japan, 1952), and Seismicity of Taiwan and some related problems(Hsu, 1971). In History of Taiwan earthquakes, earthquakes are divided into “obvious,”“moderate,” “smaU,” and ‘local” according to their strength. Statisticalstudy shows that they correspond to 7, 6+, 5$, and 4; of G-R magnitude scalerespectively. In Catalogue of major earthquakes which occurred in the vicinity ofJapan (1885-19501, magnitude values for earthquakes before 1930 were convertedfrom Kawasumi’s magnitude, MK. They differ greatly from the G-R scale valuesand were not adopted. For earthquakes after 1931, average magnitudes obtainedfrom data of six Japanese stations were added and can be used for reference.HSU’S (1971) catalogue is composed of two parts. In Appendix 11, the magnitudesfor earthquakes before 1935 are mostly Kawasumi’s magnitude or HSU’S estimation.Statistics show that for the years before 1925, the magnitudes given differ greatlyfrom the G-R scale with a correction of +0.43 (n = 12, o = &0.34); for 1925to 1935, the deviation is smaller, with a correction of +0.1 (n = 9, o = *0.17).In Appendix I, the magnitudes of earthquakes after 1936 were determined fromobservational data by Hsu using complicated magnitude scales. For earthquakesbetween 1936 and 1948, the result of least square regression isMs = 1.49 MH - 2.97 (n = 63, 7 = 0.80, CT = f 0.29), (6)where MH is HSU’S estimation of magnitude. It can be seen from Figure 4 thatMH = 5.2, Ms = 4;; and when MH < 6, the correction is negative.3.4.5. In the International Seismological Summary, magnitude, amplitude andperiod data are not given. For these earthquakes, we estimate their magnitudesfrom the maximum distance of P-wave observation, Ap. Considering the probabledifference in P-wave attenuation for earthquakes occurring in mainland China andin Taiwan Province and its neighbouring seas, they were treated separately (Figure5). For Ap < 105”, the following emperical formulas were obtained:for mainland China, andMs = 0.0119Ap” + 4.89 (7)Ms = 0.0179Ap” + 4.23, (8)for Taiwan Province and its neighboring seas (- 5 < Ms < - 6).