ESTIMATION OF PALEOEARTHQUAKE MAGNITUDE FROM THE QUATERNARY FAULTS IN KOREAN PENINSULA

The Korean peninsula is located on the eastern margin of the Eurasian plate. There are two major faults, one is the Yangsan fault which running to NNE direction with about 200 km length, and the other is the Ulsan fault which is NNW direction with about 60 km length. Although the Korean peninsula belongs to a stable continental region and low seismicity area, there were discovered the Quaternary faults at about 30 sites along the two major faults. It is not easy to predict accurately the potential of earthquake that may occur in their sites. It has been debated that some of the Quaternary faults near the nuclear power plant site in SE Korean peninsula are active or not since early 1990's. In case that those are the active fault, estimating the size of earthquakes from the fault is one of the most important issues in the seismic hazard analysis for nuclear power plants. In this study, we discussed the methods for evaluation of the maximum potential earthquake, and estimated the size of paleoearthquake from the fault source parameters.

In this study, five methods were carried out to scale earthquake size, which are the fault rupture length, rupture area, fault displacement, seismic moment and moment release rate methods. On the basis of the fault parameters we could obtain M 5.36, M 6.98, M 5.34, M 6.40 and 6.46 by using the surface rupture length, displacement, rupture area, seismic moment and moment release rate methods, respectively. We think that these different results of paleo-magnitude attributed to the various factors of over- and under-estimation in evaluating the earthquake potential, and may not fully identify the detailed geometry and dynamics of fault system. Because the Quaternary faults in Korea are not only relatively old in comparison with that of the active area but also are the reverse sense of slip, however, it is not easy to define the surface rupture length on the basis of fault geometry alone. And, because it is also difficult to obtain material suitable for dating prehistoric earthquakes, it is not easy to evaluate the recurrence interval and the slip rate of faulting. To evaluate the adequate earthquake potential the characteristics of fault system and more detail data acquisition using the trench survey should be considered in analyzing the seismic hazard.