2002 年 55 巻 1 号 p. 11-18
The author investigated fractal structure of hypocentral distributions before the 2000 Tottoriken Seibu earthquake (MJ=7.3), by estimating temporal changes in the spatial fractal dimension D of the hypocenter distributions, and comparing these results with the seismic activity in and around the focal region. The correlation integral method and the running time-window technique were applied to the D-value calculation for the JMA Earthquake Catalogue data during the period of January 1983 -February 2001. The results obtained are summarized as follows; (1) The D-value decreased drastically prior to the main shock in and around the focal region. The precursory durations are estimated as 1-3 years, which may be different with different sizes of the time-window. (2) The D-value decrease must be caused by hypocenter clusterings, or unusual seismic activity, which occurred over the wide area in and around the focal region. (3) Consequently, temporal variations in the D-values associated with a large earthquake may be effective as a precursor to the earthquake.