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Monitoring Active Tectonic of Sulawesi Island using GPS Method
D.A. Sarsito Institute of Technology Bandung (ITB), Bandung, Indonesia W.J.F. Simons, and A. Socquet Department of Earth Observation and Space Systems (DEOS), Delft, Netherlands Abstract Studying the geodynamic of Eastern Indonesia has been conducted under corporation between Dept of Geodetic Engineering (ITB) and DEOS (TU Delft) since 1997 until now. The purpose of these research are to study the geodynamic processes of micro-plate boundaries and to monitor motion of the active fault systems in Sulawesi Island, which is located near the triple junction of Eurasian, Philippine and Australian plates. One of space-geodetic tool for these types of studies is using Global Positioning System. The repeated GPS campaigns will give the positions and velocities/motion of the station networks, and these results can be used to improve the geophysical model of that area. Base on the capability of GPS tools, this island has been repeatedly observed with GPS during a number of Sulawesi campaigns and since 2000 is added by GPS permanent stations monitoring at the nearby main active fault system. A validated kinematics model for the present motions is derived, indicating steady state velocity or non-linear displacements of several micro-plate boundaries. 1. Introduction The convergence between three major lithospheric plates (the south-southeast-moving Eurasian plate, the northward-moving Australian plate and the westward-moving Philippine plate) produces a very complex deformation at eastern Indonesia archipelago since Mesozoic (figure 1). The collision zone between southeast Sunda Platform and Sula domain (a micro continental fragment derived from the Australian Plate) create Sulawesi Island (Silver et al, 1983). This collision is accommodated by convergence between the Southeast Arm and the northern part of Banda Sea along the Tolo Thrust where Palu-Koro fault locate as a southwest limit and the Matano fault as a southern limit and by subduction of oceanic crust from Sulawesi Sea under the North Sulawesi Arm at northern limit (Rangin, 1989; Walpersdorf et al, 1998). Sulawesi can be divided into five distinct geologic regions (Hamilton, 1979): continental fragments in the easternmost region (including Banggai and Sula Island), an east-central ultramafic and ophiolite belt, a central metamorphic belt, the western volcanic belt which continues through the northern arm and Minahasa volcanic belt. The tectonic phenomena of Sulawesi area become a very interesting object of geodynamic studies. One of space-geodetic tool for these types of studies is using Global Positioning System. The repeated GPS campaigns will give the positions and velocities/motion of the station networks, and these results can be used to improve the geophysical model of that area. Base on the capability of GPS tools, this island has been repeatedly observed with GPS during a number of Sulawesi campaigns since the original 8 GEODYSSEA sites in 1994, up to more than 30 in 2003 and 25 additional faults transect points ( Palu-Koro and Gorontalo Faults). Since 2000, 6 permanent stations have been installed mainly to study the Palu-Koro fault. The purpose of these activities is to study the complex geo-dynamic processes in Sulawesi and to monitor the motion of an active fault system in Sulawesi, namely Palu-Koro, which is responsible for the main part of tectonic processes in Central Sulawesi.  Figure 1. Tectonic setting of Sulawesi region and the GPS networks 2. GPS Measurements Activities The Sulawesi networks points (figure 1) are separated into three groups, the first group is the main Sulawesi network (IGRS site: BLKP, main GEODYSSEA sites: MALI, REDO, KEND, AMPA, AMBO, MANA, SANA, TERN, TOAR, TOMI, UJPD, TAWA, KUAL, and GEODYSSEA densification sites: WUAS, TOBO, LUWU, KAMB, BAUB, BARA, WATA), the second group is Palu-Koro Transect (consist of 11 points in between Watatu-Palu-Toboli) and the third group is Gorontalo transect consist of 7 points. To accomplish the positioning of the Sulawesi network (in a global reference frame), 10 stations (YAR1, TIDB, TSKB, KIT3, KARR, GUAM, IISC, LHAS, NTUS and WUHN) were added. These supplement stations are coming from the International GPS Service for Geodynamics (IGS) tracking network. The campaign coordinate solutions were computed using GAMIT-GLOBK software and mapped in the latest ITRF solution of 2000 (ITRF-2000) using a regional IGS site approach. In this analysis, we included data of 10 closest IGS stations GUAM, IISC, KARR, KIT3, LHAS, TIDB, TSKB, WUHN, NTUS and YAR1. For each of the measurement campaigns, daily solutions were calculated in 24-hours session with 30 seconds sampling rate and in each session the theoretical values for phase and pseudorange observable are modeled. Station coordinates, phase biases and zenith delay parameters are adjusted by a least squares method. The observables were examined in the ionosphere-free combination (LC), forming double differences to eliminate clock errors and tropospheric parameters were estimated using Saastamoinen model every 3 hours for each station. The antenna phase variations were modeled using the IGS table (Roctacher and Mader, 1996) and IGS orbits were kept fixed for each individual session. The phase ambiguities are estimated as real values in the first step and it was then attempted to resolve the phase ambiguity using a routine developed by King and Bock (1989). The quality assessment of the individual solution is based on the repeatability of the independent daily estimations for each baseline component. The repeatability for the Sulawesi and IGS stations for horizontal component are ranging between 1,3 to 2,8 mm and between 5,7 to 6,5 mm for the vertical component. These values show a very high internal precision of Sulawesi network campaigns, but the transect shows rather poorer values (2,0 to 5,6 mm for horizontal component and 9,4 to 12,2 mm for vertical component) due to data time coverage that less than 24 hours for each transect stations. And finally, the multisession free network solutions with loose constraints on positions and fixed IGS orbits (Herring et al, 1990) were established and Kalman filter method were applied to the analysis of the solution vectors and associated covariance matrices generated during daily solutions. The campaign coordinate solutions were mapped in the latest ITRF solution of the 2000 (ITRF-2000), by constraining the IGS stations included in the analysis to their positions in ITRF-2000. The results indicate that accuracy with respect to ITRF-2000 are better than 1 cm for both horizontal and vertical position, and an accurate estimation of Sulawesi motions could be obtained. The displacement rates were computed by subtracting the ITRF-2000 that mapped the campaign solutions. The preliminary results until 2002 are shown in figure 2.  Figure 2 Horizontal station velocities 1997-2002 in ITRF-2000 3. Discussion Figure 2 show basically that the island can be divided into two or three different domains behaving independently base on GPS measurement results. In the Sunda reference frame, the Makassar Block (southern part), displays a small but significant motion with respect to the Sunda Block and rotates anticlockwise. The Euler vector describing the motion of the South Sulawesi Block with respect to Sunda is located at 4.6°S, 116.4°E and rotates anticlockwise at 1.17°/Myr The Sula block (northern part), moves quickly toward the NNW and seems to rotate quickly clockwise. The pole minimizing the velocities of the Sula Block sites with respect to Makassar Block is located at 0.5°N, 125°E and rotates clockwise at 3.65°/Myr. Most of the relative deformation between these two blocks is accommodated on the Palu-Koro fault zone accommodating left-lateral strike-slip of about 35 mm/yr. The eastern part of the northern arm of Sulawesi Island, the Manado Block, seems to have an independent motion from the Sula Block. These two latest entities are separated by a fault evidenced by geology and called the Gorontalo Fault. To get better understanding about kinematics and active tectonics on the region, now we are still constructing our GPS velocities model as a combination of rigid rotation and elastic deformation around the main faults. The simultaneous inversion results in the Euler vectors describing the blocks motions, the faults locations, velocities and their locking depth. Acknowledgement This work is a continuation of the DEOS, ENS and ITB research activities in Sulawesi. Thanks and appreciation to GFZ and BKG for making the Sulawesi part of GEODYSSEA-98 data available and to DSMM for the extending Sulawesi measurements at the Malaysian GEODYSSEA sites. Special recognition is given to all aother people at DEOS, ENS, ITB, ABKOSURTANAL and UC who contribute to and enable the organization and execution of the Sulawesi field campaign. Reference:
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