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

Magnitudes and Origin Times from Milne Seismograph Data 474. California EarthquakesEarthquakes listed in Toppozada et al. (1978), Richter (1958) and Coffman andvon Hake (1973) have been surveyed. The results for 21 earthquakes are summarizedin Table 4.Most of the Milne stations are distributed far from the California region, andonly five stations are at distances shorter than 70': Victoria (A - 15"), Toronto(A - 30°), Baltimore (A - 30'), Honolulu (A - 35'), and Trinidad (A - 60').The reports from Toronto started in September, 1897, and those from Victoriastarted in October, 1898. The data from these two stations are complete for 1899and later years, and are very useful. The reports from other three stations started in1901, and their reportings are so discontinuous or infrequent that the data are onlypartly useful, particularly for large shocks. Owing to such a biased distribution, thedata for small shocks are insufficient. Magnitudes with a dagger symbol in Table 4indicate that they have been determined from only one or two stations.Richter (1958) lists three large shocks with magnitude 6 or 6;: September 20,1907, November 4, 1908 and May 15, 1910. If these magnitudes are correct, theshocks are large enough to be recorded at many stations. However, no Milne datahave been reported from any stations, not even Victoria and Toronto. In addition,no data have been found for five shocks of magnitude 5.5 listed in Toppozada et al.(1978). Probably these shocks may have smaller magnitudes on the instrumentalbasis.Table 4. Magnitudes of Earthquakes in California RegionEVENT YEAR MO DY HR:I" ON ' W MyCAO 1CA02CA03CA04CA05CAOGCA07CA08CA09CAlOCAI1CA12CA13CA14CA15CAl6CA17CA18CA19CA20189818991899189919011902100319031903190619061906190619071908190919091910191019104 157 67 2212 253 37 281 246 118 33 34 184 194 238 118 185 1810 293 113 198 507:06.720:14.520:31.112:24.907:42.3O6:57*05:27.813:13.906:50.720:23.813:12.500:27.709:10.312:19.310:59*01:19*06:48.506:54.200:09.401 :31.2393734.533.53634.631.537.637.33337.732.54140.840.84140.536.94040.8124121.5117.5116.5120.5120.4115.5121.8121.8117122.5115.5124124.2124124124.2121.8125124.25.575.55.54.58.36.05.06.05.56.06.8CA21 1911 7 1 21:59.7 37.3 121.8 6.6 6.5* = Richter (1958) and Toppozada et al. (1978).# = Surface-wave magnitude from Milne data.t = Determination from only one or two station data.6.7t4.7t5 .6t6.46.45.0t6.65.415.3t5.3t7.86.26.45.0t4.9t4.9t5 .a5.86.06.6