Vol.2 No.3 2009

Research paper−177−Synthesiology - English edition Vol.2 No.3 pp.177-183 (Dec. 2009) diffusion of the plate tectonics theory that states that the Japanese Archipelago is being compressed as it is pushed by the Pacific Plate. The landforms such as mountain ranges and basins of Japan were created by fault activities. The study of active faults started as an attempt to answer one of the main themes of geomorphology and geology: “Why is a mountain high?” This can be positioned as Type 1 Basic Research.On the other hand, since great earthquakes are caused by the rupture of active faults, the study to predict future earthquake occurrences from the past rupture and their frequency started in the latter half of the 1970s. The method involves digging a trench across an active fault, and the past dates of fault ruptures are investigated from the slippage and the age of the geological layers (Fig. 1). This method is called trench survey, and such investigations were conducted in many places in the 1980s. With the diffusion of trench survey, various data on the past ruptures (past earthquakes) of active faults were accumulated, and they aggregated into a discipline called “paleoseismology.” While conventional seismology concentrates on the observation of current earthquakes, paleoseismology offers possibility of predicting future occurrences of great earthquakes by studying the cycle of ruptures from geological history. In other words, the study of active faults evolved into its utilization by society, or Type 2 Basic Research.The importance of active faults became widely known to the public in the Great Hanshin-Awaji Earthquake (Hyogoken-nanbu Earthquake) of 1995. The fact that this earthquake was caused by the rupture of active faults was widely publicized through the mass media, and the awareness for “active fault” rose sharply in society. Immediately after this earthquake, the Headquarters for Earthquake Research Promotion was 1 Objective and background of the researchJapan is frequently struck by earthquakes and experiences damages almost every year. Particularly, the earthquakes that are caused by the rupture of active faults occur in the shallow part of the inland region and can cause major damages. Therefore, from the viewpoint of effective earthquake damage prevention, it is extremely important to be able to accurately predict the earthquakes that are caused by active faults. Recently, the survey of active faults has progressed throughout Japan and scores of data have been obtained. However, the predictions based on these data are not sufficient. We conducted this research because we believe that even if the information is not sufficiently accurate, the transmission of information based on the most rational and most uniform method at this point is extremely important, as a researcher and as a research institution whose mission is geological survey.An “active fault” is a fault that has repeatedly ruptured in the past and possesses possibility of causing a major earthquake in the future. The interval of cyclic rupture is extremely long, normally from a thousand to several tens of thousands of years. Although the slip of a fault in one earthquake may be only a few meters, when the slippages are repeated in the same direction over several thousand or several million years, a displacement of several tens or hundreds of meters may accumulate. As a result, if the displacement is in the vertical direction, the upthrust side becomes a mountain range and the downthrust side becomes a plain or a basin. If the displacement is in a horizontal direction, valleys and peaks become bent. The study of active faults from such geomorphological perspective advanced rapidly in the 1960s to the first half of the 1970s. The background was the - Evaluation of rupture probabilities of active faults using the Cascade Earthquake Model based on behavioral segmentation -Toshikazu YoshiokaActive Fault and Earthquake Research Center, AIST Tsukuba Central 7, 1-1-1 Higashi, Tsukuba 305-8567, Japan E-mail : Original manuscript received March 5, 2009, Revisions received July 6, 2009, Accepted July 7, 2009In order to assess the probability of the occurrence of future large earthquakes based upon the past activities of active faults, we divided active faults into behavioral segments, and adopted the Cascade Earthquake Model, that is, a model that considers that an earthquake is sometimes caused by a single segment and sometimes caused by multiple segments. Using this model, we can evaluate the rupture probability of active faults by a uniform standard without any inconsistencies with field data. The result was published as the Rupture Probability Map of Major Active Faults in Japan.Evaluation of earthquake occurrence from active faultsKeywords : Active fault, earthquake, assessment, rupture probability, behavioral segment[Translation from Synthesiology, Vol.2, No.3, p.194-200 (2009)]


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