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

Compilation and Processing of Historical Data 14 1Historical information is normally not suitable for statistical processing becauseits homogeneity can be rarely guaranteed. However, its main importance is inevidencing the long term variation of earthquake activity and the existence of fociwhich become active after a time span of several hundred years or more, sometimesin regions which would not be considered as potentially active using other evidence.The occurrence of large, damaging events of very low probability of occurrence isthe phenomenon which must be taken into account, particularly when consideringearthquake hazard for important structures, such as nuclear power plants. In thisrespect, every effort must be made for improving historical information.