Fractal size and spatial distributions of fault zones: An investigation into the seismic Chelungpu Fault, Taiwan

Abstract   The fractality of fault zones of thickness T for the 248-m-long core from a borehole penetrating the northern segment of the seismic Chelungpu Fault, Taiwan, was analyzed. The frequency curve of T shows that the fractal dimension is normal when T is smaller than a characteristic thickness T _c, and it becomes abnormally large when T exceeds T _c. The fractal dimensions of size and spatial distributions of T increase as the mean distribution density of T increases, which is inconsistent with the evolution laws for ordinary brittle faults. This discrepancy implies that the thickening rate of T when T is more than T _c is not constant, but a decreasing function of fault displacement. The slow thickening rate is related to the elastohydrodynamic lubrication which was effective on the fault when T exceeds T _c. This slip instability mechanism can explain the large, fast and smooth slip on the northern segment of the Chelungpu Fault during the 1999 Chi-Chi earthquake.