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

Magnitudes and Origin Times from Milne Seismograph Data 49The origin time for CA13 is assigned 08:48 by Toppozada et al. (1978), but 0910by Coffman and von Hake (1973). The time data from 10 Milne stations indicate09:10.3 zt 1.1. Data of instrumentally determined origin times are available onlyfor three California shocks of 1904-1912. The time for CAI1 has been noted before.Gutenberg and Richter (1954) give the times for CA20 and CA21. The differencebetween their times and the present results is 0.3 and 0.4 min, respectively.5. ConclusionUsing the amplitude and time data of surface-wave maxima measured by undampedMilne seismograms, we have determined the surface-wave magnitudes Msand origin times of shallow earthquakes in China and California for 1898-1912. Thestation corrections have been calculated from a number of the Milne data, and incorporatedto make the magnitude estimate more reliable. A comparison betweenthe Ms values and the catalog magnitudes is shown in Figure 7, plotted from Tables2 and 4. The scatter is considered to be mainly due to an inhomogeneity ofmagnitude scales used in the previous catalogs. The estimates of origin times fromthe Milne data are considered to be accurate within one minute for well-observedshocks. The use of the Milne data permits the fairly accurate determination of Msand origin times in shallow earthquakes where the Milne data are available, andextends the period of instrumental seismology.IM6/O00I d/6080Figure 7. Comparison between surface-wave magnitudes derived from Milne data andmagnitudes given in previous catalogs. The data are taken from Tables 2 and 4.