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 collisions and rotations of continental slivers, island arcs, and oceanic domains with respect to the Sunda Block. Seismic networks document a high level of seismicity in its northern boundaries, corresponding to deformations 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 thrusts 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 the 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 discuss the present day kinematics and deformation of Sulawesi area, we performed a seismotectonic study, using focal mechanisms of moderate and large (Mw greater than or equal to 5) shallow earthquakes (less than or equal to 60 Km), collected from the Harvard CMT database (period 1976 to 2001) and complemented by Fitch [1972] and Cardwell [19801 (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 solutions and moment tensors were inverted (Carey-Gailhardis and Mercier method [1987, 1992]) and summed, in order to obtain stress and deformation tensors and rate estimates (Brune [1968] or Kostrov [19741 methods). Results are presented in table 1, on figure 2 and figure 3. In the northern Molucca Sea (north of equator), the fast convergence slip rate (75 mm/a) is absorbed by the Sangihe subduction and accommodates the major part of the Philippine/Sunda plates motion. South of the equator, the estimated slip rate is only 2 mm/yr and represents the Sangihe slab subduction, which is affected by a torsion from NNE to E strike. Along the North-Sulawesi subduction, directions of the stress axes are not significantly different from east to west (average N356degrees+/-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 bloc, contributes to this rotation because its trace fits well a small circle centered on the pole. However, seismicity documents few moderate magnitude earthquakes (fig. 4) related to the left lateral Central Sulawesi fault system, despite many identified active tectonic features [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 measurements [Walpersdorf et al., 1998b; Stevens et al., 1999]. This confirms that it 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 islands with a major Mw=7.7 earthquake (29/11/98). The inversion provides a strike-slip regime with respectively N220degreesE and N310degreesE-trending sigma(1) and sigma(3) stress axes. This study also highlights the Sula block internal deformation that could explain residuals in the GPS velocities model obtained by Walpersdorf et al. [ 1998a] for the Sula block rotation. We evidence an extensional stress regime with a N030degreesE-trending sigma(3), in the southern part of the Tomini Gulf. The estimated extension rate is 9 mm/a towards a N036degreesE 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 corresponds to the domain of the collision which occurred in the early-middle Pliocene [e.g., Villeneuve et al., 20001 between the NE arm and the Irian-Jaya derived Banggai-Sula block. This domain remains active (12 earthquakes 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 earthquakes 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 westwards at a lower rate than the Sula block. The SW arm of Sulawesi is also characterised by a compressional stress regime with a N099degreesE-trending sigma(1) and an estimated convergence rate of 8.5 mm/a toward a N080degreesE direction. This is the consequence of the Majene-Kalosi thrusts activity and could represent the most western accommodation of the Philippine/Sunda plates motion.
