Abstract

Sulawesi Island, eastern Indonesia, is located at the junction between the Pacific-Philippine, Indo-Australian Plates, and the Sunda Block, i.e., the southeastern edge of the Eurasian Plate (fig. 1). Its peculiar shape results from an on-going complex history of collision and rotation of continental slivers, island arcs, and oceanic domains with respect to the Sunda Block. Seismic network document a high level of seismicity in its northern boundaries, corresponding to deformation along the North Sulawesi trench and within the Molucca Sea subduction (fig. 1). Seismic activity is lower in central and south Sulawesi (fig. 4). It represents the activity of the NE, SW and SE arms thrust and the left-lateral Central Sulawesi Fault System, which comprises the Palu-Koro and Matano fault zones. This system connects, from northwest to southeast, the North Sulawesi Subduction zone to the Sorong fault (through th Sud Sula fault, after, Hinschberger et al. [2000] and the Tolo thrust in the North Banda Sea, Silver et al., [1983] proposed a deformation model that implies a clockwise rotation of the Sula block that is limited to the west and south by the Central Sulawesi Fault System. Paleomagnetic [Surmont et al., 1994] and GPS [Walpersdorf et al., 1998a] studies confirm and measure this rotation. In order to discus the present day kinematics and deformation of Sulawesi area, we performed a seismotectonic study, using focal mechanism of moderate and large (Mw ≥ 5) shallow earthquake (≤ 60 Km), collected from the Harverd CMT database (period 1976 to 2001) and complemented by Fitch [1972] and Cardwell [1980] (period 1964–1976). From these focal mechanisms and the known structural context, we defined ten homogeneous deformation domains (fig. 3 et fig.5). For seven of these, focal solution and moment tensors were inverted (Carey-Gailhardis and Mercier method [1987Carey-Gailhardis and Mercier method [1992]) and summed, in order to obtain stress and deformation tensors and rate estimates (Brune [1968] or Kostrov [1974] methods). Results are presented in table I, on figure 2 and figure 3.

In northern Molucca Sea (north of equvator), the fast convergence slip rate (75 mm/a) is absorbed by the Sangihe subduction and accommodates the major part of the Philippines/Sunda plates motion. South of the equator, the estimated slip rate is only 2 mm/yr and represents the Sangihe slap subduction, which is affected by a torsion from NNE to E strike. Along the North-Sulawesi fault system, direction of the stress axes are not significantly different from east to west (average N356°±5E), but the determined slip rates increase from 20±4 mm/a to 54±10 mm/a, respectively. These values agree with the Sula block rotation pole previously proposed and located at the eastern extremity of the Northern Arm. The Palu-Koro fault, bounding the western Sula block, contributes to this rotaion because its trace fits well a small circle centered on the pole. However, seisicity document few moderate magnitude earthquake (fig. 4) related to the left lateral Central Sulawesi fault system, despite many identified active tectonic feature [Beaudouin, 1998]. Moreover, geologically determined Palu-Koro long-term slip rate of 35±8 mm/a, [Bellier et al., 2001] agrees with the far-field strike-slip rate of 32–45 mm/a proposed from GPS measurement [Walpersdorf et al., 1998b ; Stevens et al., 1999]. This confirms that is a fast slipping fault with a relatively low level of seismicity. The southeastern limit of the Sula block is represented by the ENE-trending Sorong strike-slip fault that extends from Irian-Jaya island to the east coast of Sulawesi where it connects to the Matano fault through the South Sula fault, This structure is particularly active south of the Sula island with a major Mw=7.7 earthquake (29/11/98). The inversion provides a strike-slip regime with respectively N220°E and N310°E-trending σ1. and σ3 stress axes.

This study also highlight the Sula block internal deformation that could explain in the GPS velocities model obtained by walpersdorf et al. [1998a] for the Sula block rotation. We evidence an extensional stress regime with a N030°E-trending σ3, in the southern part of the Tomini Gulf. The estimated extension rate is 9 mm/a toward a N036°E direction. Considering the location of the Tomini Gulf, this deformation could be interpreted as a back-arc spreading related to the North Sulawesi subduction. The Batui zone correspond to the domain of the collision wich occured in the early-middle Plicene [e.g., Velleneuve et al., 2000] between the NE arm and the Irian-jaya derived Banggaï-Sula block. This domain remains active (12 earthquake with a major one of Mw=7.6, 14/05/00, fig. 4) but is mainly affected by strike-slip deformation.

The Tolo thrust, lying off the SE arm east coast, absorbs the convergence to the west of the North Banda Sea, as attested by six moderate earthquake with reverse faulting focal mechanisms. This allows to distinguish a North-Banda block in SE Sulawesi, bounded by the South Sula segment of the Sorong fault, the Tolo thrust and the Hamilton fault (fig. 5) and moving westward at a lower rate than the Sula block. The SW arm of Sulawesi is also characterised by a compressional stress regime with N099°E-trending σ1 and an estimated convergence rate of 8.5 mm/a toward a N080°E direction. This is the consequence of the Majene-Kalosi thrust activity and could represent the most western accommodation of the Philippines/Sunda plates motion.

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