It has been noted that the characteristics of earthquakes occurring on an active fault are closely related to the irregularity of fault geometry. Because of their rough appearances over many length scales, faults can be regarded as fractal sets and their geometrical irregularity can be quantified using fractal dimension values. Previous observations show that the Sumatra fault system (SFS) consists of 11 active fault segments with geometric fractal dimension values (D) ranging from 1.00 ± 0.03 to 1.24 ± 0.03. In this article, the relationships between D values and large shallow earthquakes (5.0 ≦ mb ≦ 7.0, depth ≦ 50 km) occurring between 1965 and 1994 are analyzed. The results show that there is a repetitive pattern, which we call the fractal pattern, of the SFS seismicity, correlating the times of earthquake recurrence with fault segment geometric D values. Based on this fractal pattern and the relationship between segment D values and seismogenic crustal models along the SFS, the 11 segments of the SFS can be divided into two groups of three classes in which each segment class has a specific earthquake recurrence interval. The recurrence interval then may be used to predict future large earthquakes in the segment classes as we have done before in predicting the 7 October 1995 Kerinci earthquake and 10 October 1996 Toru earthquake.

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