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

Catalogue of Chinese Earthquakes (M 2 6.5) 115For surface waves with periods less than 17 sec, the Moscow-Prague formula(KBrnik et al., 1962), indicated by MV in this paper, was used:MV = log($) + 1.66 log A’ + 3.3 + SV (2’ 5 A 5 160’). (3)Here, the vector sum of amplitudes of the horizontal components of surface waveswas taken for A. When the epicentral distance is less than 5’, care is taken todifferentiate between surface wave and S wave.Although the magnitudes calculated using formulas (1) and (3) were obtainedfrom seismic waves in different frequency bands, statistical results show they aremutually consistent for earthquakes during the period 1900 to 1948, as shown inFigure 1. The average deviation of MV from Ms is +0.02 and the standard deviationis k 0.18. The station corrections are given in Table 1. As data are limited, thestandard deviation is comparatively large. The corrections for different time periodsare not always the same, and in some cases the differences are relatively large.3. Data Analysis and Magnitude DeterminationDuring the period 1900 to 1948, seismic stations were established, instrumentswere improved, and the determination of seismic parameters was steadily improved.Thus, the magnitude scales used in different earthquake catalogues differ greatly. Inthis paper, the Gutenberg-Richter magnitude scale (Gutenberg, 1945a, Gutenbergand Richter, 1956) is taken as the standard. Amplitude and period data givenby different seismic stations were used to calculate earthquakes magnitudes. Forearthquakes with no amplitude data, magnitudes given in different catalogues wereextrapolated to our standard scale through careful investigation.3.1. From the amplitude and period data of the 82 Chinese earthquakes givenin Gutenberg and Richter’s unpublished original worksheets for Seismicity of theEarth and Associated Phenomena, Ms and mg were calculated using formulas (1)and (2), respectively. The results show that: for h 5 40 km, the magnitude valuesin that book are calculated using formula (1); for 40 < h 5 100 km, mainly mg wascalculated, and in rare occasions MS; and for h > 100 km, all magnitudes are mg-(as shown in Figure 2). In Figure 2, the average deviation of body-wave magnitudeAM = 0.07. It may be due to the difference between mg and MB. This conclusionagrees with that of Abe (1981). For class “d” earthquakes in Gutenberg’s book, noreliable magnitudes were assigned. Results of our calculation show that their meanvalue is 5.6 & 0.22.3.2. In addition to Gutenberg’s worksheets, amplitude and period data from otherseismic stations were used. For earthquakes before 1935, the most important dataused are from the stations listed in Table 1. As shown in Table 1, there was a largechange in the Osaka station correction around 1920, and in the Strasbourg stationcorrection around 1930. Undamped Omori seismographs were used at Osaka in itsearlier stage. According to Abe (1979), the station correction is -0.1, which agreeswith the -0.06 value for the years 1902 to 1920, as shown in Table 1. After 1934,data from Tashkent, Vladivostok, Moscow, Pulkovo, Sverdlovsk, Irkutsk, Baku, andother U.S.S.R. stations were used frequently. Their station corrections are shown