HR: 0800h
AN: T51D-0770    [Abstracts]
TI: Maximum magnitudes of earthquakes and the geothermal structure in Japanese inland area
AU: * KUDO, T
EM: kudo@isc.chubu.ac.jp
AF: Department of Natural Science and Mathematics, Chubu University, 1200, Matsumoto-cho, Kasugai, 487-8501, Japan
AU: TANAKA, T
EM: tanaka@tries.jp
AF: Tono Research Institute of Earthquake Science, 1-63, Yamanouchi, Akeyo-cho, Mizunami, 509-6132, Japan
AU: FURUMOTO, M
EM: furumoto@eps.nagoya-u.ac.jp
AF: Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan
AB: Information on the potential maximum magnitude of earthquake in an area is valuable for not only the engineering aspect of seismology but also the scientific one. If we had a sufficient record of earthquake data, the upper limit of magnitudes in a given region might be determined simply by the record. Our historical record on large earthquakes does not have an enough length compared with the average interval of large crustal earthquakes. Although some previous studies estimated the possible maximum magnitude using historical earthquake catalogues, geological information, and active fault distributions, their results are not sufficient to discuss the spatial variation of the magnitude. We here report a possibility that the maximum magnitude in an area can be evaluated from the geothermal structure in the crust. The maximum width of the fault in the crust, which is constrained by the thickness of the brittle layer in the crust, gives the maximum earthquake magnitude. The thickness, in turn, should be generally related to the geothermal structure in the upper crust. Therefore, we can expect that the geothermal gradient in the upper crust is used to estimate the maximum magnitude of earthquakes. We examine a relationship between the maximum magnitude of earthquakes and the geothermal gradient in the crust. Two catalogs of large earthquakes (M ≥ 6.5) which occurred in the inland area of the Japanese Islands are used: (1) earthquakes occurring in 1927- 2005 and (2) historical earthquakes in 1500- 1927. The spatial distribution of the geothermal gradient is estimated by low (L ≥ 100km)-pass-filtering of gradient data observed in deep (D ≥ 1000m) boreholes. It is found that for each catalog there is a clear curve defining the upper-limit of the magnitude distribution as a function of the geothermal gradient. Combining the curves and the spatial distribution of the geothermal gradient, we depict maps of the maximum magnitude over the Japanese Islands for two different ranges of about 100 and 500 years.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8123 Dynamics: seismotectonics
SC: Tectonophysics [T]
MN: 2007 Fall Meeting