No abstract

A unique GPS velocity field that spans the entire Southeast Asia region is presented. It is based on 10 years (1994–2004) of GPS data at more than 100 sites in Indonesia, Malaysia, Thailand, Myanmar, the Philippines, and Vietnam. The majority of the horizontal velocity vectors have a demonstrated global accuracy of ∼1 mm/yr (at 95% confidence level). The results have been used to (better) characterize the Sundaland block boundaries and to derive a new geokinematic model for the region. The rotation pole of the undeformed core of the Sundaland block is located at 49.0°N–94.2°E, with a clockwise rotation rate of 0.34°/Myr. With respect to both geodetically and geophysically defined Eurasia plate models, Sundaland moves eastward at a velocity of 6 ± 1 to 10 ± 1 mm/yr from south to north, respectively. Contrary to previous studies, Sundaland is shown to move independently with respect to South China, the eastern part of Java, the island of Sulawesi, and the northern tip of Borneo. The Red River fault in South China and Vietnam is still active and accommodates a strike‐slip motion of ∼2 mm/yr. Although Sundaland internal deformation is general very small (less than 7 nanostrain/yr), important accumulation of elastic deformation occurs along its boundaries with fast‐moving neighboring plates. In particular in northern Sumatra and Malaysia, inland‐pointing trench‐perpendicular residual velocities were detected prior to the megathrust earthquake of 26 December 2004. Earlier studies in Sumatra already showed this but underestimated the extent of the deformation zone, which reaches more than 600 km away from the trench. This study shows that only a regional Southeast Asia network spanning thousands of kilometers can provide a reference frame solid enough to analyze intraplate and interplate deformation in detail.

Data collected at approximately 60 Global Positioning System (GPS) sites in southeast Asia show the crustal deformation caused by the 26 December 2004 Sumatra-Andaman earthquake at an unprecedented large scale. Small but significant co-seismic jumps are clearly detected more than 3,000 km from the earthquake epicentre. The nearest sites, still more than 400 km away, show displacements of 10 cm or more. Here we show that the rupture plane for this earthquake must have been at least 1,000 km long and that non-homogeneous slip is required to fit the large displacement gradients revealed by the GPS measurements. Our kinematic analysis of the GPS recordings indicates that the centroid of released deformation is located at least 200 km north of the seismological epicentre. It also provides evidence that the rupture propagated northward sufficiently fast for stations in northern Thailand to have reached their final positions less than 10 min after the earthquake, hence ruling out the hypothesis of a silent slow aseismic rupture.

We present the crustal motion velocity field for the Indonesian archipelago based on Global Positioning System (GPS) field surveys conducted from 1991 to 1997, and 2001, totaling more than 150 sites, as well as on a reanalysis of global tracking data in the Scripps Orbit and Permanent Array Center archive from 1991 to 2001 in International Terrestrial Reference Frame 2000. We compute poles of rotation for the Australia, Eurasia, and Pacific plates based on our analysis of the global GPS data. We find that regional tectonics is dominated by the interaction of four discrete, rotating blocks spanning significant areas of the Sunda Shelf, the South Banda arc, the Bird's Head region of New Guinea, and East Sulawesi. The largest, the Sunda Shelf block (SSH), is estimated to be moving 6 ± 3 mm/yr SE relative to Eurasia. The South Banda block (SBB) rotates clockwise relative to both the SSH and Australia plate, resulting in 15 ± 8 mm/yr of motion across the Timor trough and 60 ± 3 mm/yr of shortening across the Flores Sea. Southern New Guinea forms part of the Australia plate from which the Bird's Head block (BHB) moves rapidly WSW, subducting beneath the Seram trough. The East Sulawesi block rotates clockwise about a nearby axis with respect to the Sunda Shelf, thereby transferring east‐west shortening between the Pacific and Eurasia plates into north‐south shortening across the North Sulawesi trench. Except for the Sunda Shelf, the crustal blocks are all experiencing significant internal deformation. In this respect, crustal motion in those regions does not fit the microplate tectonics model.

The island of Sulawesi, eastern Indonesia, is located within the triple junction of the Australian, Philippine, and Sunda plates and accommodates the convergence of continental fragments with the Sunda margin. We quantify the kinematics of Sulawesi by modeling GPS velocities and earthquake slip vectors as a combination of rigid block rotations and elastic deformation around faults. We find that the deformation can be reasonably described by a small number of rapidly rotating crustal blocks. Relative to the Sunda Plate, the southwestern part of Sulawesi (Makassar Block) rotates anticlockwise at ∼1.4°/Myr. The northeastern part of Sulawesi, the Bangai‐Sula domain, comprises three blocks: the central North Sula Block moves toward the NNW and rotates clockwise at ∼2.5°/Myr, the northeastern Manado Block rotates clockwise at ∼3°/Myr about a nearby axis, and East Sulawesi is pinched between the North Sula and Makassar blocks. Along the boundary between the Makassar Block and the Sunda Plate, GPS measurements suggest that the trench accommodates ∼15 mm/yr of slip within the Makassar Strait with current elastic strain accumulation. The tectonic boundary between North Sula and Manado blocks is the Gorontalo Fault, moving right laterally at about 11 mm/yr and accumulating elastic strain. The 42 mm/yr relative motion between North Sula and Makassar blocks is accommodated on the Palu‐Koro left‐lateral strike‐slip fault zone. The data also indicate a pull‐apart structure in Palu area, where the fault shows a transtensive motion and may have a complex geometry involving several active strands. Sulawesi provides a primary example of how collision can be accommodated by crustal block rotation instead of mountain building.