Earth Planets Space, Vol. 56 (No. 3), pp. 401-405, 2004LETTER
Hot Springs Research Institute of Kanagawa Prefecture, 586 Iriuda, Odawara, Kanagawa, 250-0031, Japan
(Received November 30, 2003; Revised March 1, 2004; Accepted March 8, 2004)
Changes in form of hypocenter distribution preceding the 2003 Tokachi-oki Earthquake (MJ = 8.0) were investigated by using the analysis of temporal variation in spatial fractal dimension D. In this study, it was found that the D value began to decrease in 1998, and had been very small for about one year before the main shock occurrence. Such a D decrease before the main shock occurrence is a characteristic of some recent large earthquakes. Therefore, the D decrease may be an earthquake precursor. The D value decrease is yielded by both of seismic activation and quiescence which have often been reported as an earthquake precursor, due to a property of the calculation method. Therefore, the D change can be detected, even if the seismic activation and quiescence occur simultaneously in which case the number of earthquakes does not change significantly. On account of this property, using the D value is advantageous to detect the precursory change of seismic activity before a large earthquake.
Key words: The 2003 Tokachi-oki Earthquake, hypocenter distribution, fractal dimension, precursory change.