Vol.2 No.3 2009

Research paper : Evaluation of earthquake occurrence from active faults (T. Yoshioka)−178−Synthesiology - English edition Vol.2 No.3 (2009) established by the Japanese government, and the nationwide survey of active faults and the long-term assessment (probability prediction) of active faults were started. In an attempt unseen elsewhere in the world, the scale of the earthquakes that may occur in the next 30, 50, and 100 years and the probability of their occurrences were assessed for each major fault zone (98 fault zones at the time) in Japan, and the figures were publicized. This was one way for the Product Realization of the results of active fault research.2 Relationship of active fault and earthquake scaleIn predicting future earthquakes, it is important to predict the scale as well as the place and time of the occurrence. The size of an earthquake is proportional to the length of the active fault that ruptures during the earthquake and the amount of slip. Also, the amount of slip is proportional to the length of the active fault. Therefore, to predict the scale of the earthquake that may happen in the future, it is necessary to estimate the length of the active fault related to that earthquake.Considering the length of an active fault, it is not easy to judge from what point to where should be considered one fault, since numerous active faults are distributed like a network throughout the Japanese Archipelago. Moreover, according to the surveys so far, there are several geological and geomorphological evidences that indicate that a slip of certain magnitude has been occurring repeatedly at the same interval from the past in a certain point of a certain active fault. However, according to the historical records of earthquakes and careful geological survey, the range of active faults that rupture during earthquakes that occur repeatedly in a region may be different for each earthquake, and the active faults may not necessary rupture within the same range.In the Nobi Earthquake (magnitude 8.0) that occurred in 1891, the Nobi Fault System that crossed the Gifu and Fukui Prefectures ruptured, and a slip of about 6 m high occurred in Midori, Neo-mura (current Motosu City) located in the center of the fault zone. From the distribution form of the fault, it is determined that this fault zone is composed of the Nukumi Fault, Neodani Fault, Umehara Fault, and other smaller active faults (Fig. 2). It is recorded that during the earthquake of 1891, the western half of the Nukumi Fault, Neodani Fault, and Umehara Fault ruptured together and caused a slip in the earth surface[1]. However, looking at the past ruptures of this fault, it was found that the past rupture periods of Nukumi, Neodani, and Umehara faults differed[2][3]. This means that in certain periods in the past, the Nukumi and Neodani faults, and the Neodani and Umehara faults did not rupture in unison.One problem arises. If the ranges of fault destruction when the earthquake occurs (i.e. length of the fault) are different, the amount of slip at a certain point should be different each time because the length and the amount of slip are proportional. Moreover, since the rate of an average slip of a fault (average rate of displacement) is thought to be constant over a long term, if the amount of slip differs every time, the rupture interval of the fault (interval of earthquake occurrence) must be different each time accordingly. However, as mentioned earlier, from the geomorphological and geological studies, it has been concluded that the amount of slip did not change greatly in the past except in certain points. If the amount of slip and the rupture interval of the faults differ for each earthquake, the cyclic nature of active fault rupture mentioned earlier will be negated and future prediction of earthquake occurrence becomes impossible.The author and others looked at the Cascade Earthquake Model as a model that offers rational explanation without Fig. 1 Example of trench investigation of an active fault (Nukumi Fault of the Nobi Fault zone, Ohno City, Fukui Prefecture).This active fault is known to have ruptured in the Nobi Earthquake in 1891. The bump on the earth surface is the slip that occurred in 1891. Since the lower layer (older layer) show more pronounced horizontal displacement than the top layers, it can be seen that the slippage of the fault accumulated repeatedly.Fig. 2 The Noubi Fault zone and the distribution of surrounding active faults.The heavy lines show the faults that ruptured in the Nobi Earthquake of 1891. Dashed lines are inferred faults.GifuNagoyaFukui020 kmNukumi FaultNeodani Fault36.0N137.0ELake BiwaUmehara Fault


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