Pergamon zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA J. Geodynamics Vol. 22. No. 112, 1-9. 1996 pp. Copyright 0 1996 Elsevier ScienceLtd Printedin Great Britain. All tights reserved PII: zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH SO264-3707(%)00015-4 0264-3707196 $15.00+0.00 FRACTAL GEOMETRY OF THE SUMATRA ACTIVE FAULT SYSTEM AND ITS GEODYNAMICAL IMPLICATIONS SIGIT SUKMONO,‘*+ M. T. ZEN,’ W. G. A. KADIR,’ L. HENDRAJAYA,’ D. SANTOSO’ and J. DUBOIS2 ’ Teknik Geofisika-Institut Teknologi Bandung, Jl .Ganesha 10 Bandung 40132, Indonesia * Inst. de Physique du Globe de Paris, 4 place Jussieu, B89,75252 Paris Cedex, France (Received 30 November 199.5; accepted in revisedform 9 March 19%) Abstract-The Sumatra active fault system is a 1650 km long northwest trending dextral active strikeslip fault which accommodates the oblique convergence between the Indo-Australian and the Eurasian plates. It consists of 11 fault segments connected northward to the Andaman extensional back arc basin and southward to the extensional fault zone of the Sunda Strait. The geometries of the 11 segments are quantified using a fractal approach and it is found that their fractal dimensions (D) range from 1.00+0.03 to 1.24~0.03. Larger D values are associated with more irregular fault geometry. There are six discontinuities, reflected by sharp changes of fractal dimensions, observed along the segments. The locations of the discontinuities correspond to sites of major structural breaks in Sumatra mainland and its fore arc and back arc. The discontinuities and variations of D values suggest that the Sumatra mainland is not rigid. Instead it appears to be segmented into several blocks which may correlate to the segmentation in the Sumatra fore arc. This segmentation may explain the large discrepancy among the displacements and velocities of the Andaman Sea opening, the Sumatra fault motion and the Sunda Strait opening. This research also demonstrates the applicability of the fractal approach for analyzing the variations of fault geometry due to geodynamic processes. Copyright 0 1996 Elsevier Science Ltd INTRODUCTION The Sumatra fault system (SFS) is a 1650 km long northwest trending dextral spike-slip fault zone which cut the entire length of Sumatra-Indonesia. The SFS accommodates the oblique convergence between the Indo-Australian and Eurasian Plates and is widely known as one of the greatest active faults in the world (Fig. 1). It extends in a succession of en echelon segments and is connected northward to the Andaman extensional back arc basin and southward to the extensional fault zone of the Sunda Strait. There is a correlation in time between the Sunda Strait and the Andaman Sea opening during the Pliocene (Lassal et al., 1989). However, the total displacement measured within the Andaman Sea is 460 km (Curray et al., 1979) and cannot be * Author to whom correspondence should be addressed. ’ Tel. 62-22-2509167; Fax: 62-22-2509169; e-mail: sukmono@tnelsa.net.id 1