Wim Simons

Insight into the 2004 Sumatra–Andaman earthquake from GPS measurements in southeast Asia

Data collected at ∼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.

A decade of GPS in Southeast Asia: Resolving Sundaland motion and boundaries

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.

India and Sunda plates motion and deformation along their boundary in Myanmar determined by GPS

Using a regional GPS data set including ?190 stations in Asia, from Nepal to eastern Indonesia and spanning 11 years, we update the present?day relative motion between the Indian and Sundaland plates and discuss the deformation taking place between them in Myanmar. Revisiting measurements acquired on the Main Boundary Thrust in Nepal, it appears that points in southern Nepal exhibit negligible deformation with respect to mainland India. Including these points, using a longer time span than previous studies, and making an accurate geodetic mapping in the newest reference frame allows us to refine the present?day Indian motion. Our results confirm that the current motion of India is slower than predicted by the NUVEL?1A model, and in addition our India?Eurasia motion is significantly (?5 mm/yr) slower than previous geodetic determinations. This new Indian motion, combined with a refined determination of the Sundaland motion, gives way to a relative India?Sunda angular velocity of 20.2°N, 26.1°E, 0.370°/Myr in ITRF2000, predicting a relative motion of 35 mm/yr oriented N10° at the latitude of Myanmar. There, the Sagaing Fault accommodates only 18 mm/yr of right?lateral strike slip, only half of the shear component of motion. We present two models addressing how and where the remaining deformation may occur. A first model of distributed deformation implies convergence on the Arakan subduction (the northern continuation of the now famous Sumatra?Andaman Trench) and wrench faulting in the Arakan wedge. The second model uses localized deformation, where deformation observed west of the Sagaing Fault is entirely due to elastic loading on a faster and oblique Arakan subduction (23 mm/yr). This latter model predicts that a major earthquake of Mw 8.5 may occur every century on this segment of the subduction.

Crustal motion and block behaviour in SE-Asia from GPS measurements

Results acquired using global positioning system (GPS) data taken over a large part of SE-Asia, indicate that Sundaland, i.e. Indochina along with the western and central part of Indonesia, constitutes a stable tectonic block moving approximately east with respect to Eurasia at a velocity of 12 ˛ 3 mm yr 31 . With respect to India and Australia this block moves due south. Significant motion has not been detected along the northern boundary to South China i.e. along the Red River Fault, whereas nearly 50 mm yr 31 of right lateral motion has to be accommodated between India and Sundaland in the Andaman^Burma region. fl 2001 Elsevier Science B.V. All rights reserved.

Observing plate motions in S.E. Asia: Geodetic results of the GEODYSSEA Project

This paper presents the final geodetic results of the GEODYSSEA project. The GPS data from a 42 station network observed during two field campaigns (1994/1996) were analyzed by four groups using different software packages and analysis strategies. The precision of both campaign coordinate solutions was found to be 4–7 mm for the horizontal, and 1 cm for the vertical component. The campaign solutions were merged into one unique solution, which was accurately mapped into the ITRF-96 reference frame. The global accuracy of this solution with respect to ITRF-96 is ±1 cm, while the resolution of the relative horizontal velocities is estimated to be at the level of 2–3 mm/yr. This solution was used as the basis for all scientific interpretations, which are published in separate papers. The velocity estimates of a part of the network provided the first direct measurement of a relative motion of the Sundaland block with respect to Eurasian plate.

Microblock rotations and fault coupling in SE Asia triple junction (Sulawesi, Indonesia) from GPS and earthquake slip vector data

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.

Postseismic GRACE and GPS observations indicate a rheology contrast above and below the Sumatra slab

More than 7 years of observations of postseismic relaxation after the 2004 Sumatra-Andaman earthquake provide an improving view on the deformation in the wide vicinity of the 2004 rupture. We include both Gravity Recovery and Climate Experiment (GRACE) gravity field data that show a large postseismic signal over the rupture area and GPS observations in the back arc region. With increasing time GPS and GRACE show contrasting relaxation styles that were not easily discernible on shorter time series. We investigate whether mantle creep can simultaneously explain the far-field surface displacements and the long-wavelength gravity changes. We interpret contrasts in the temporal behavior of the GPS-GRACE observations in terms of lateral variations in rheological properties of the asthenosphere below and above the slab. Based on 1-D viscoelastic models, our results support an (almost) order of magnitude contrast between oceanic lithosphere viscosity and continental viscosity, which likely means that the low viscosities frequently found from postseismic deformation after subduction earthquakes are valid only for the mantle wedge. Next to mantle creep, we also consider afterslip as an alternative mechanism for postseismic deformation. We investigate how the combination of GRACE and GPS data can better discriminate between different mechanisms of postseismic relaxation: distributed deformation (mantle creep) versus localized deformation (afterslip). We conclude that the GRACE-observed gravity changes rule out afterslip as the dominant mechanism explaining long-wavelength deformation even over the first year after the event.

Deformation of Thailand as Detected by GPS Measurements due to the December 26th, 2004 Mega-Thrust Earthquake

Abstract The Mw 9.3 mega-thrust earthquake on December 26th, 2004 off the coast of North Sumatra, Indonesia has resulted in large co- and post-seismic motions throughout SE Asia. As a result, the geodetic network of Thailand has been deformed. The THAICA network has been regularly observed with GPS since 1994. Therefore the continuous tectonic motions in Thailand, which is located on the Sundaland block are well known. The last GPS campaign prior to the mega-thrust earthquake took place in October 2004 as part of the EU-ASEAN funded SEAMERGES project. Shortly after the earthquake, the GPS campaigns were repeated in January 2005 and February 2005 to determine the displacements. The GPS data in Thailand were processed, using the Precise Point Positioning strategy of the GIPSY software package. Data from the International GPS Service (IGS) were included, to obtain the positions and velocities of the Thai GPS sites in the International Terrestrial Reference Frame (ITRF) solution of 2000. Since the coordinate time series of the Thai geodetic network spanning up to a decade are available, the absolute steady-state velocities prior to the earthquake are known with uncertainties below 1 mm/yr. Results obtained from the latest campaign indicated that the earthquake has resulted in the horizontal displacements, ranging from 33 cm in the south, 9 cm in the centre, to about 3 cm in the north and east of Thailand. In addition, it was found that the post-seismic motion due to the earthquake has increased further the displacement at the PHUK station.

Modelling post-seismic displacements in Thai geodetic network due to the Sumatra-Andaman and Nias earthquakes using GPS observations

Abstract It is evident that the 2004 Sumatra-Andaman, 2005 Nias and 2007 Bengkulu earthquakes caused significantly large co-seismic and post-seismic displacements all over the South-East Asian region. The Thai geodetic network has been severely affected by the 2004 and 2005 earthquakes. Largest post-seismic horizontal displacements were observed in the southern part of Thailand, while moderate and small displacements were seen in the central and northern parts of Thailand. This paper will analyse the GPS observations obtained by the Royal Thai Survey Department GPS campaigns up to the end of 2009. The post-seismic displacements have been shown to follow a main direction which is towards the rupture area. A simple filtering technique is proposed to remove noise from the post-seismic displacements prior to the fitting of the post-seismic displacements with the logarithmic decay function. As a result, the τ log values in the logarithmic decay function at each point were found to be more consistent in both north and east directions. The new fitting results can therefore be used to estimate the coordinate of the zero-order Thai geodetic network to any epoch with millimetre accuracy.

Updating Thai Reference Frame to ITRF2005 Using GPS: Diversion Between ITRF2000 and 2005 in Southeast Asia

Abstract The Thai geodetic network has been regularly observed with Global Positioning System (GPS) since 1994 through several collaborative EU-ASEAN projects such as GEODYSSEA, SEAMERGES and RTSD-Delft. This geodetic network has long been served as a reference frame for Thailand. Previous realisations of the Thai coordinate reference frame were therefore tied to the global International Terrestrial Reference Frame (ITRF) at epochs 1994, 1996 and 2000. After the occurrence of the 9.2 Mw Sumatra-Andaman earthquake on the 26th December 2004, horizontal displacements were evident at different magnitudes in many surrounding countries. The geodetic network within Thailand was also significantly deformed during the earthquake at the centimetre to decimetre level. Large co-seismic horizontal displacements were observed in the southern part of Thailand, while moderate and small displacements were seen in the central and northern parts of Thailand. The Royal Thai Survey Department (RTSD) has been carrying out multiple GPS field campaigns to monitor the post-seismic displacements. This paper will analyse the GPS observations obtained from the RTSD GPS campaigns up to the end of 2008 using the Precise Point Positioning (PPP) strategy of the GIPSY-OASIS II software. It has been demonstrated that by employing the state of the art PPP technique, the users could achieve mm-level of repeatability in the horizontal components and centimetre precision in the vertical direction, for a 24-hr data span from a static site occupied by a geodetic-quality receiver. Coordinate results obtained from each campaign are then mapped to ITRF2000 and ITRF2005 using a number of well-determined global International GNSS service (IGS) sites. By comparing coordinate results between ITRF2000 and ITRF2005, it is evident that there is a significant diversion in the north component at a rate of 1.7 mm per year over Southeast Asia region. Finally, ITRF2005 coordinate results obtained from the latest RTSD GPS campaign (November 2008) will be served as a new coordinate reference frame for Thailand.