Vol.2 No.3 2009
Research paper : Evaluation of earthquake occurrence from active faults (T. Yoshioka)−181−Synthesiology - English edition Vol.2 No.3 (2009) Faults in Japan was innovative as a nationwide assessment based on a uniform model, the assessment of co-movement among the behavioral segments was difficult, and we were unable to directly predict the scale of earthquakes that may occur in the future. Research is being continued in this aspect, but determining whether two behavioral segments ruptured at the same time in the past earthquake, or whether they ruptured with some time lag, are beyond the time resolution of geological survey. This is the limitation of this method. To solve this issue, another group in the Geological Survey is gathering the historical record of the earthquake that occurred recently and analyzing the relationship between the distribution form of faults and the propagation of ruptures using the numerical simulation of dynamic rupture.Looking at some recent earthquakes that caused damage, there are cases where no major slip occurred in the active fault although the earthquake occurred along the active fault, as in the Mid-Niigata Prefecture Earthquake in 2004 (magnitude 6.8), as well as the case of Iwate-Miyagi Nairiku Earthquake in 2008, where no clear active fault was previously found although the earthquake of magnitude 7.2 occurred. Such earthquakes could not be predicted with the conventional assessment method.Moreover, as a result of detailed survey of the active faults throughout Japan, there are data that show that the past rupture intervals of the active faults are not necessarily constant in some points. Although there are possibilities that there are flaws in the surveyed data or exceptions due to the uniqueness of the survey point, it will be necessary to review the model from the basic periodicity of the active fault ruptures.However, the purpose may be defeated if the model becomes complicated in order to offer explanation to exceptional cases like the Iwate-Miyagi Nairiku Earthquake of 2008, and evaluation becomes impossible for the faults that do not have sufficient data for such a complicated model. Exceptions are inevitable in natural phenomena, and it is extremely important to discern what is an exception. Time range of several thousands and several tens of thousands of years is necessary to investigate the long-term prediction of earthquake occurrence, and the future issue is to build a model that is simple and universal because verification is virtually impossible.5 ConclusionThe prediction of earthquakes that occur at active faults advanced dramatically in the past 10 years. However, we often feel the gap with what the society demands. For example, when an unpredicted earthquake occurs, society will not accept the explanation “this earthquake was unique and exceptional.” In that sense, the degree of achievement of ReferencesT. Matsuda: 1891nen Nobi jishin no jishin danso (Surface faults associated with Nobi (Mino-Owari) earthquake of 1891, Japan), Jishin Kenkyusho Kenkyu Sokuho (Special Bulletin of Earthquake Research Institute, University of Tokyo), 13, 85-126 (1974) (in Japanese).Y. Awata, Y. Kariya and K. Okumura: Kojishin chosa ni motozuku 1891nen Nobi jishin dansokei no segumento kubun (Segmentation of the surface ruptures associated with the 1891 Nobi Earthquake, Central Honshu, Japan, based on the paleoseismic investigations), Jishin Chosasho Sokuho (Interim Report on Active Fault and Paleoearthquake Researches in 1998 Fiscal Year (GSJ Interim Report)), EQ/99/3, 115-130 (1999) (in Japanese).T. Yoshioka, Y. Awata, K. Shimokawa, H. Ishimoto, M. Yoshimura and K. Matsuura: Torenchi chosa ni motozuku 1891nen Nobi jishin dansokei/Nukumi danso no katsudo rireki (Paleoseismicity of the Nukumi fault ruptured during the 1891 Nobi earthquake, Central Japan, revealed by a trenching study), Jishin (Journal of Seismological Society of Japan (Zisin)) (2), 55, 301-309 (2002) (in Japanese).Working Group on California Earthquake Probabilities: Seismic hazards in southern California: Probable earthquakes, 1994 to 2024, Bull. Seism. Soc. Amer., 85, 379-439 (1995).J. P. McCalpin: Application of paleoseismic data to seismic hazard assessment and neotectonic research, McCalpin ed. this study is low. Another issue is that the Japanese society tends to demand an osumitsuki or official endorsement that may lead to blaming the government, and therefore the government tends to publicize only things that are absolutely certain.The author feels that our social responsibility is to organize the intellectual foundation that can be used to prepare against earthquakes, by presenting the prediction of active fault ruptures as accurately and as comprehensively as possible to society. Although presenting the information with sufficient accuracy or complete comprehensiveness cannot be realized immediately, I believe that the publication of the results on a nationwide scale is significant as the first step. Whether this research methodology was effective or not will be evaluated by how the society responds.AcknowledgementsThis research is a continuation of the “Research on the Assessment for the Potential of Earthquake Occurrence by Observing Active Faults and others (FY 1994~1999),” a special research of the Agency of Industrial Science and Technology conducted at the Geological Survey, which was one of the predecessors of AIST, as well as earlier researches. The research evolved through repeated researches and discussions of numerous researchers during that time. I am grateful to all people involved, and shall emphasize that this research is the result of the collaboration with many people including my joint researcher Dr. Yasuo Awata.