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

44 Katsuyuki AbeIn the present study, the “Maximum Waves” data alone are used for estimatesof instrumental origin times. The origin time is estimated byiwhere t, is the origin time of an earthquake, t, is the reported time of ”MaximumWaves” at a particular station, Ai is the epicentral distance, U is the group velocityand n is the number of Milne stations used. Assuming that the group velocity isconstant for individual regions, we have determined the velocity from the datagiven in Figs. 3-6 by the least-squares fit. Excluding few pcints of apparentlylarge deviation (bracketed in Figs. 3 and 5), we obtain U = 3.0 km/s for Chineseearthquakes from Figs. 3 and 4, and U = 3.1 km/s for California eartliquskes fromFigs. 5 and 6. Using these values of U, single-station origin times are calculatedand averaged. Then, extreme values which are different by more than one standarddeviation are excluded, and the remainder values are re-averaged to obtain theorigin time of an earthquake. All the time data in this paper are given in GMT.From the above method, the origin time for the 1906 Sinkiang earthquake hasbeen estimated to be 18:21.0 (Figure 3); the data used are given later in Table 3.This time is equal to the time given by Gutenberg and Richter (1954). Similargood agreement is obtained for the 1906 San Francisco earthquake. Its origin timehas been estimated to be 13:12.5 from the Milne data (Figure 5). This time isclose to the time, 13:12.0, obtained by Gutenberg and Richter (1954), and 13:12:21,determined by Bolt (1968). In later sections, results are compared further with onlya few cases where origin times were formerly determined on an instrumental basis.It is considered from the comparisons that the estimates for well-observed shocksare accurate within one minute. For shocks of earlier date and small shocks of laterdate, the data are not sufficient and results are probably less accurate.3. Chinese EarthquakesThe primary sources of earthquake data are catalogs of Gu (1983) and Lee et al.(1978). Earthquake catalog of Utsu (1982) is useful for supplementing some earthquakesnear Taiwan. All the shallow shocks of 1898-1912 in these catalogs havebeen surveyed to assign magnitudes and origin times from the Milne data. Theresults for 32 shocks are listed in Table 2. The earthquakes for which no Milne datawere available are omitted from the table.No reports from any Milne station have been found for many shocks of magnitude6 or less in the Gu’s catalog. In the catalog of Lee et al. (1978)) 28 shallow shocksof magnitude 6 or greater are listed for the period 1900-1912. For three out of the28 shocks, no data have been found. It is suggested that the smallest earthquakesrecognizable from the Milne data alone are considered to be Ms = 6 or so.Following are some remarks and additional information concerning some of theearthquakes listed.A series of Taiwan earthquakes from CHI2 to CH17 in 1906, except CH13, startedwith the large earthquake of CH11; the reported loss of life is 1258 (Richter, 1958).The origin time of CH12 is given 02h by Lee et al. (1978), but the correct time hasbeen proved to be 03h in GMT. The origin time for CH14 has not been estimatedowing to the lack of the time data of “Maximum Waves”. Ekutsk (A - 31’) stationreports several earthquakes during the period from March to April; the smallest