活断層の活動予測

Abstract

The nature of active faults is reviewed to estimate future activity. Criteria for the prediction of large earthquakes are given based on active faults and some earthquake-expectancy maps are shown.The activity of active faults is of a cyclic nature. The cyclic consists of the following four stages : (1) A long-continued quiet period of very low seismicity. The length of this period differs with each fault on the order of 100 year to more than 10, 000 years. (2) A p ecursory seismic period of relatively high seismicity for several years (or longer) before the next stage. (3) A foreshock period of several weeks (or less). About half of the Japanese inland active faults, that moved in the last one hundred years, had remarkable felt-foreshock activity before the main shock. (4) An aftershock period. The magnitude of the seismicity of each stage and time-length of the cycle differ with each fault, but have been nearly constant at every fault or every fault segment during the late Quaternary.The following four criteria are used for estimating future large earthquakes, from which seismic zoning maps and strong ground motion frequency maps have been prepared : I. An active fault indicates the site of the future destructive earthquakes. II. Longer faults can produce larger earthquakes. III. Every fault has its own size of seismic slip (or earthquake magnitude) and recurrence interval. W. Large earthquakes tend to occur successively from segments in a fault zone.