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

168 Yushou Xieof Chinese Earthquakes (State Seismological Bureau, 1979) were published later.The latest edition of Catalogue of Chinese Earthquakes (Gu, 1983) was publishedat the end of 1983. The method of determination of magnitude remains the same.From materials of historical earthquakes, we investigated the division of seismicactive zones and their temporal variations of seismic activity (see e.g. Shih et al.,1974). Generally speaking, the cycle of seisniic activity in the eastern part of Chinamainland is comparatively long. It is of the order of hundreds of years, or evenmore than 1,000 years. For example, in the Valley of the River U’eihe, where themost disastrous historical earthquake with a death toll of more than 830,000 peopleoccurred in 1556, tens of felt to destructive earthquakes occurred in the easternpart of tlie graben during the few decades before and after this earthquake, but ithas remained quiet since 1568. As this region was an important political, culturaland econortiic center and has long recorded history, we believe that no destructiveearthquake could be overlooked during this period. Since the establishment ofa regional network of seismic stations in the late 1970’~~ seismicity in the wholegraben has been very low. Only several small earthquake swarms were recorded inthe central part of the graben, where strong earthquakes rarely occurred in history.Earthquakes with magnitude around 3 occurred once in several years. On tlie otherhand, the cycle of activity in Yunnan and Xinjiang is only several tens of years.That seismic activity is closely related to geological conditions was noticed longago. Strong earthquakes concentrate in regions with active Neotectonic movementsand almost no destructive earthquakes occur in stable platforms, such asTalimu Basin and Erdos Platform. For example, the central eastern part of YunnanProvince is one of the most active seismic regions in China. Geologically, itis characterized by two sets of surface faults with obvious Neotectonic iiiovements:one striking nearly meridionally, the other trending N 60’ W. Almost all epicentersof the 46 medium to strong earthquakes occurred before 1940 are located on knownfault zones. Their isoseismals are usually elongated in form with their major axesparallel to the local surficial faults (see Hsieh, 1958). Thereafter, several destructiveearthquakes occurred in this region. Generally speaking, they agreed well with theabove-mentioned correlation between seismic activity and geological conditions.As a reference for the assessment of seismic risk of construction sites, “The Mapof Seismicity of China” (Lee, 1957) was conipiled in the mid-1950’s. The followingtwo principles were applied: in a given region, earthquakes of the same intensityas those which occurred previously may recur; and regions with similar geologicalconditions can also be regarded as of similar seismicity. Such a map simply reflectsthe maximum possible intensity of different regions, without any indication of time.In the 1970’s, our colleagues conipiled a new map of scisinic zoning valid for thecoming 100 years.5. Seismological Analysis of Historical MaterialsIn the Compilation of Historical Materinls of Chinese Earthquakes, all historicalmaterials were quoted directly from their original records. Thus, seismological analysisis indispensable for it to be used conveniently by seismologists. The conversionof calendars and the identification of localities have been done by paleonstronomersand historical geographers, respectively. The estiiiiatiori of rriagnitudes of historicalearthquakes with macroseismic data only is much more complicated.