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

404 S. MiyamuraOrigin times of the following earthquakes in the Japanese area are not given inGMT and should be reduced by 9 hours:(1) “1909 Oct 31 19:18” should be “10:18”,(2) “1913 Apr 13 15:40” should be ‘06:40”,(3) “1915 Aug 05 22:16” should be “13:16”,(4) “1915 Nov 18 13:04” should be “04:04”,(5) “1925 May 27 11:30” should be “02:30”,(6) ‘1931 Mar 09 12:49” should be “03:49”.Origin times of the following earthquakes should also be corrected as follows:“1935 Apr 20 2O:Ol” should be “22:01”, “1941 Nov 24 11:46” should be “21:46”. Allof these errors were found in RN.GR is probably one of the main data sources for RN and SE22, but they do notinclude 29 earthquakes with MGR 2 7.0 for the period 1904-1917, 8 of which areMGR 2 7.5 (see Table 2). On the other hand, they include many earthquakeswhich are not found in GR, DU, A, AN, and AN’. They are mostly earthquakesin and near Japan taken from CMO (1950), but more than 20 are earthquakesoutside the Japanese area (see Table 3). For example, the October 10, 1932 SolomonIslands earthquake (M = 7.7) and the March 19, 1951 Philippine Islands earthquake(M = 7.8) appear to be too large to be missed by GR. Their magnitudes mayactually be less than 7.0, but we need amplitude data for verification.Magnitudes in RN or SE22 are the largest of the values from referred sources.However, MGR of many earthquakes are larger than M in SEZ2, which indicatesthat GR was not the data source for those earthquakes (see Table 4).The Glover and Meyers (1982) catalog [hereafter SESS] combined the global andUnited States catalogs in SE22, However, due to the lack of visual examination,SE33 failed to include 15 events during October 16, 1906 to April 10, 1907, 6 eventsduring December 26., 1949 to February 2, 1950, and 2 events during November 16,1925 to January 25, 1926. Moreover, 3 earthquakes in SE22 (November 5, 1904 inTaiwan; May 4, 1907 in Bonin Island; and May 30, 1929 in Argentina) are omittedin SE3.9, the reason of which is unclear. There are also minor errors in that severalsuccessive events axe placed reverse in time, e.g., 1908 May 12 20:22 and 20:36; 1909Jul 07 21:37 and 21:39, 1909 Aug 14 06:30 and 06:39. These errors again suggestthe necessity of examining a computer prepared catalog carefully.,92333 supplemented several earthquakes not included in SEZZ (Table 2) andrevised many magnitudes in SE22, but the corrections are mostly less than 0.2,except for “1929 Mar 07 Aleutian Island” where M = 8.1 is revised to M = 8.6.2.3. Geographical Inhomogeneity of Magnitude ThresholdThe common procedure for selecting earthquakes having magnitude larger thana certain threshold introduces a regional bias of the number of selected events.GR lists 15 earthquakes with M 2 7.75 in the Seismic Region 19 (Kamchatka - NEHonshu) and 6 events in Region 24 (Indonesia), but only 2 events in Regions 3 and 4(California and Gulf of California), and 1 event in Region 29 (Iran - Ural). Further,no earthquakes with M 2 7.75 are given in Region 32 (Atlantic Ocean), Region 44(East Pacific Ocean), and Region 45 (Indian- Antarctic Swell). Therefore, it seemsreasonable to change the magnitude threshold from region to region in selectingearthquakes. For GR, optimum levels of two alternatives are suggested as follows: