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

His t o r ical Mate rials of Chin e s e Ea I t h qua ke s 169A catalogue of instrumentally determined parameters of earthquakes of the 20thcentury is included in the Compilation. Considering the detecting capability ofthe networks of seismic stations and the precision of magnitude determination, 4.7is taken as the lower limit of magnitude. The origin time and the location ofearthquake foci were adjusted with computer whenever possible. For the revisionof magnitudes, all available amplitude and period data were collected and collated.In order to unify the magnitude scale, Gutenberg and Richter's formulas of 1945(Gutenberg, 1945) and 1956 (Gutenberg and Richter, 1956) were used as standardsfor the calculation of surface wave and body wave magnitudes respectively. Asa supplement, the 1962 Moscow-Prague formula (KArnik et al., 1962) was usedin determining magnitudes from surface waves with shorter periods. Calibrationcurves of Xujiahui (Zi-ka-wei) seismic station was established. An emperical formulafor the estimation of magnitude from maximum distance of registration of P-waveswas found. Regression curves of magnitudes listed in various catalogues relative toour revised magnitudes were obtained. On these basis, a new catalogue of Chineseearthquakes from 1900 to 1948 with uniform magnitude scale was compiled. It willbe published in volume IV, part 2 of the Compilation. In the 1949-1980 cataloguepublished in volume V of the Compilation, the unification of the magnitude scalewas not well done, and we are now revising it.Although affected by local conditions, macroseismic effects of earthquakes basicallyreflect the intensity of ground shaking. The medium through which a seismicwave propagates influences its dynamic and kinematic properties as well as the attenuationof seismic intensity. To inquire into the regional differences and to give aquantitative description of the attenuation of intensity with distance, we used theisoseismals given in volume V of the Compilation to find the intensity attenuationfactor "k" of different earthquakes. The regionalization map delineated with "k" asthe criterion agrees well with those of intensity attenuation, Q-value, tectonic andcrustal structure zonations given by others.The assessment of seismic intensities is sometimes not so simple as it appears tobe. This is especially true for very high intensities. Besides the usual insufficiencyof macroseismic phenomena observed in meizoseismic regions, the main difficulty isthe indefiniteness of the criteria at very high intensities. At times, the assessmentof intensity in the meizoseismic region is not convincing.Historical materials were written by scholars with various styles of writing andgreat differences in detail. In most cases, only scattered descriptions of casualtiesand damages in individual living centers were given. As a matter of fact, vastamount of ancient historical materials simply recorded: "the Earth quaked" in acertain region. Thus, only in rare occasions can isoseismals be drawn.In recent years, strong earthquakes occurred successively in most of our majorseismic regions. Both macroseismic materials and instrumentally determinedparameters of these earthquakes were obtained. We are now investigating the empericalrelations between magnitude and pattern of isoseists, maximum intensity,felt range, rate of attenuation of intensity, length of surficial faulting, etc. of regionswith different intensity attenuation factors, to be used in the estimation of magnitudesof historical earthquakes occurred in the corresponding regions. On thisbasis, we believe, the magnitude of historical earthquakes can be determined morereliably.