G12A-01
Thin-plate modeling of interseismic deformation, application to the observed asymmetric deformation across the Altyn Tagh fault zone
Geodetic measurements of interseismic velocities are key data for assessing seismic hazard and plate motion in actives fault zone. Creeping dislocations in a homogeneous elastic half-space are commonly used to model these deformations. However it would be more realistic to replace that half-space by elatic plates with finite thickness and rigidity variations. Here, we propose a new modeling approach, which includes a thin-plate model sheared at its base by opposite velocities on both sides of a burried creeping zone. We compare both models for vertical strike-slip fault. Our results suggest that the use of a half-space model to interpret interseismic velocity field leads to underestimate the fault locking depth or to overestimate its slip rate if the far-field velocity is neither available nor well- constrained. Next we apply these two approaches to interpret InSAR data across the Altyn Tagh fault system near longitude 94°E, which shows a significant asymmetry of interseismic velocity. Our results suggest a locking depth of 7- 9 km and a present-day geodetic slip rate of 8-10 mm/yr. We interpret the asymmetric pattern as the joint effect of a rigidity decrease from the Tarim basin north of the fault to the Qaidam basin south of it and of a 5-7 km southward offset of strain concentration from the quaternary active fault. This suggests that the rigidity contrast as well as most of the strain accumulation at depth occurs on the southern geological fault.
G12A-02 INVITED
Continental Microplate Tectonics
During the past decade, methods of space geodesy have demonstrated that the kinematics of the intra- continental deforming zones that lie between the large global plates can be usefully described as relative motions among small elastic blocks or microplates. At the same time, kinematic models that assume a smoothly varying deformation field have been developed and applied to the same data. Both models generally fit the data comparably well and there is much debate about which approach—blocks or continuum—is ‘better'. However, there is really no disagreement about the existence of crustal blocks in deforming zones and only their size and number are contentious. Therefore a perhaps more useful way of framing the debate is to examine the purpose of each modeling approach, its success in meeting that purpose, and its limitations. Continuum modeling approaches are typically a prelude to dynamic modeling of continental deformation, thus far usually using a thin viscous sheet rheology for the lithosphere. The purpose of continuum modeling is then to quantify the forces driving and resisting motions and understand their relation to the observed deformation. This approach has been notably successful in determining the relative importance of plate boundary tractions and internal buoyancy forces (gravitational potential energy) in driving intra-continental deformation, particularly in central Asia and western North America. Continental deformation is block-like because major faults are weak and block interiors are much stronger. The main purpose of simple rigid plate kinematics is to quantify the rate and sense of slip across major faults and mountain belts, with applications to active tectonics and earthquake hazard assessment. Where available, late Quaternary and Holocene fault slip rate estimates, with few (but notable) exceptions, agree with geodetically- estimated rates obtained from the block models. Where block rotations are sufficiently large, late Cenozoic rotation rates can be determined paleomagnetically and these rates commonly agree with the space geodetic estimates. Despite several similarities, continental block kinematics differs in notable ways from global plate tectonics. First, microplates are much smaller, typically ~100-1000 km in size. Departures from block rigidity are small but measurable and represent either heterogeneous internal deformation or a more complex but unresolved block structure. While major oceanic plates may persist for tens or 100s of Ma, continental microplates change and evolve over much shorter timescales, particularly near their often geometrically irregular boundaries. The depth to which discrete block structures extend is uncertain. While some major faults probably extend through the crust into the upper mantle as narrow ductile shear zones, blocks elsewhere may be at least partially decoupled from the mantle lithosphere by pervasive ductile flow of weak lower crust. Continental blocks must ultimately be subject to the same forces that drive and resist global plate motions. However, the role and importance of local forces is often evident from the observed patterns of continental block motion. These local forces include internal buoyancy due to lateral density gradients in continental lithosphere and block boundary forces such as those caused by slab roll-back, trench suction, and resistance to subduction of buoyant lithosphere. The importance of basal tractions that may drive or resist block motions is uncertain and controversial.
G12A-03
Reconciling interseismic strain and geologic deformation across convergent plate boundaries
The advent of space geodesy has revolutionized our ability to measure surface displacements and crustal strain along plate boundaries or across deforming continental areas. Some examples from Taiwan, the Nepal Himalaya and Sumatra where geologic, geodetic and plate motion data can be compared will be reviewed in the presentation. It will be shown that the geodetic data can generally be explained reasonably well from a conventional approach within the framework provided by Plate Tectonics and the ‘Seismic Cycle' theory. We will point to clear inconsistencies revealing potential problems with the data or with some the basic assumptions made in this kind of analysis. Temporal strain variations are generally observed in addition to the secular strain expected from the conventional theory of interseismic strain. We will show evidence for seasonal variations probably due to external seasonal forcing, and evidence for longer term temporal variations probably due to internal dynamics. It will be argued that the assumption that interseismic strain is stationary might in fact need be reconsidered. Also we will discuss ways to reconcile geodetic measurements with the build up of geological deformation in the long run, and how geologic studies can help understand better the mechanical processes governing geodetic strain.
G12A-04
The Sundaland Block in SE Asia: A Tectonic Entity Surrounded by Earthquake Hazards
The present-day crustal deformation of SE Asia results from the convergent motion between the Sundaland (SU) block, which covers most of SE Asia, and the adjacent Philippine (PH), Australian (AU) and Indian (IN) tectonic plates in highly active subduction zones. To the north, SU is disconnected from the ‘stable' Eurasian (EU) plate by the IN-EU collision which shaped the crustal deformation zones in and around the Himalayan Region. The oblique relative motion between IN/AU and SU caused the 2004 Sumatra-Andaman earthquake/tsunami disaster. A unique (100+ points) GPS velocity field (in ITRF2000 and ITRF2005) that spans the entire SE Asia region, based on a decade of regional measurements (1994-2004), was exploited to resolve Sundaland's motion and boundaries and perform a detailed study of the inter-seismic intraplate deformation [1]. This revealed previously undetected deformation patterns on Java, North Borneo and North Thailand and the much larger subduction deformation zone extending (>600 ~km) from the Sumatra trench towards Peninsular Malaysia and South Thailand. There far-field data implies a ~220~km wide locked trench fault plane below SU at shallow (≤gslant 13°) dip angle and full coupling on the subduction. This matches the very large magnitude of the 26/12/2004 earthquake and the subsequently recorded wide-spread co-seismic deformations throughout the network also provided the first unambiguous evidence of the rupture plane length [2]. The post-seismic motions (both at spatial and temporal scales) of ~50 GPS sites (2005-2006) provide information on the contribution of various mechanisms (aftershocks/afterslip, visco-elastic relaxation and poro- elasticity). These post-seismic motions are very significant in the far-field, e.g. the island of Phuket additionally moved ~1.5 times the initial 2004 and 2005 co-seismic displacements. References: [1] Simons, W.J.F, et al al.(2007), J .Geophys. Res., 112, B06420, doi:10.1029/2005JB003868. [2] Vigny, C. et al.(2005), Nature, 436, 201-206, doi: 10.1038/nature03937 http://www.deos.tudelft.nl/~wims/sunda.html
G12A-05
Relative plates Motions and associated deformation across the Sunda-Sumatra-Arakan trenches
Using a regional GPS data set, we assess the relative motion between the Indian, Australian and Sundaland plates and discuss the deformation taking place between them from the Sumatran trench to Myanmar. 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 Sunda plate motion, predicts a relative motion of 35 mm/yr oriented N10 at the latitude of Myanmar, and 35 mm/yr oriented 17° at the latitude of the Andaman Basin. The relative motion between Australia and Sunda however is much faster and reaches 53mm/yr oriented N9° at the latitude of northern Sumatra. In Myanmar, the Sagaing Fault accommodates 18 mm/yr, representing only half of the shear component of the motion. The remaining deformation (23 mm/yr) is likely accommodated elastically on the Arakan oblique subduction, which could produce a major earthquake every few hundred years. At the latitude of Sumatra, interseismic deformation extended ~400km below Sumatra and the Malaysian peninsula within the Sunda plate, attesting for a deeply locked subduction zone before the earthquake. We analyse the deformation associated with the 2004 earthquake that broke the Sumatra-Andaman trench. Tsunami modelling, validated against independent Jason-1 altimetry data and tsunami arrival time data, was used to retrieve the most likely co-seismic slip distribution based on geodetic data inversions. Coulomb stress increase computations show unclamping associated to right lateral increased shear stress on the Sumatra fault, north of the equator. On the trench, coulomb stress is increased at both ends of the rupture, on the Sumatra trench south of the equator, and the Arakan trench to the north.
G12A-06
A Block Model for the Interseismic Deformation in the Taiwan Arc-Continent Collision Zone
The southern Taiwan is a key region for understanding the Taiwan arc-continent collision. The active collision between the Eurasia and Philippine Sea plates results in numerous folds and thrust faults in the area. A converging rate of about 80 mm/yr is observed across the collision zone. Approximately half of the convergence is accommodated in the fold and thrust belt of southwestern Taiwan and another half is taken up in the Longitudinal Valley and the Coastal Range in eastern Taiwan. In this area, a continuous GPS array of about 100 stations is now in operation and most of the 210 densely-deployed survey-mode sites have been occupied at least once annually since 1994. A detailed new velocity field is derived from GPS time series analysis of all available survey-mode and continuous GPS data in southern Taiwan from 1994 to 2006. With respect to Paisha (S01R), Penghu, located in the Chinese continental margin, the Luzon arc (Coastal Range, Lutao and Lanhsu) is moving northeastward at rates of 60-83 mm/yr. Across the Longitudinal Valley, a plate suture zone, a major velocity discontinuity of about 30 mm/yr is obvious. Crustal motion in the Chiayi-Tainan area of southwestern Taiwan shows nearly in the west direction with maximum rates of about 37-43 mm/yr and decrease to zero near the western coast. Further south in the Kaohsiung-Pingtung area, velocities increases southward and reaches 52-57 mm/yr in the southwest direction. Extrusion tectonics is significant here. Based on the new velocity field, a block model consisting of several blocks bounded by major active faults is employed to study the interseismic deformation in the Taiwan arc-continent collision zone. The block model includes the effects of block rotation and strain accumulation in an earthquake cycle. The block boundaries used are the Chukou Fault, Chishan Fault, Chaochou Fault, and Longitude Valley Fault. From this modeling study, we may estimate the present-day fault-slip rates of major active faults and detect the possible locations of blind faults. The results can be utilized in seismic hazard analysis.
G12A-07
Reconciling Geodetic and Geologic Rates in Fold and Thrust Belts
Although the scales of observation are not the same, thrust fault slip rates based on dated sediments often yield values an order less than those derived from geodesy (cm vs mm/yr). Moreover, the patterns of present surface motions compared to those accumulated over millions of years are somewhat different. For instance, results from the Idaho-Wyoming-Utah thrust belt and the Andes suggest that the initiation of thrusting progresses from hinterland to foreland. Certainty worsens with age because complexity of deformation increases with age, i.e., initiation of younger, deeper thrust fault rejuvenates older, higher ones and thrust sheets may move several times. Although preservation degrades toward the hinterland, thrusting appears to finally turn off from foreland to hinterland. Evidence for out-of-sequence thrusting is not common. Where timing data are unusually good (e.g., Pyrenees), displacement was evidently not constant in either space or time. This episodicity at times scales of 106 yrs may extend to individual thrust sheets. By contrast, geodetic data - which include both elastic strain accumulation and creep - indicate that all structures in southern Taiwan are moving. The frontal portion is slightly more active than areas to the east. Current geodetic motions are largely normal to local thrust fault traces, even coseismic slip such as that of the Chi-Chi earthquake. Fold growth may be nearly as important as fault slip, although greater data density is required. The discrepancy between geodetic and geologic rates may be attributed to the fact that in the two cases different quantities or aggregates of them are being measured. The difference in patterns is attributable to the different time scales of observation.
G12A-08
Rigid Block Motion, Interseismic Strain, and Backarc Deformation in the Aegean
We combine available GPS data in and around the Aegean region to model plate boundary deformation and earthquake cycle effects in the observed velocity field. Typically, GPS data in the region have been used as evidence that a southern Aegean micro-plate behaves coherently and rigidly. These first-order models match the observed data quite well suggesting little intra-plate or plate-boundary deformation. However, the M >8 Crete earthquake of AD 365 and some other historic earthquakes may have occurred along the Hellenic subduction zone, which implies that a substantial portion of the subduction thrust may be locked. Depending on the locking required to generate earthquakes of this magnitude, a measureable elastic strain signal reaching far into the overriding plate should be evident in the surface velocity field. Alternatively, slow accumulation of elastic strain and dominantly aseismic creep on the subduction thrust may generate very little deformation across the southern Aegean plate. It is also possible that the surface velocities generated by convergence at a locked subduction zone are masked by simultaneous back-arc extension, creating the illusion of rigid plate motion. We consider a range of possibilities in an attempt to interpret the current geodetic signal in the region and its implications for earthquake hazard assessment. We use a block modeling approach that considers both rigid rotations and elastic strain fields along block boundaries to examine the possible trade-off between these components. As many of the stations are located away from the plate boundaries in question it is difficult to constrain boundary parameters, such as locking depth and dip, using only the GPS data. We generate multiple models to explore the solution space of all reasonable parameters. Our modeling suggests that it is possible for coeval extension (back-arc) and convergence (subduction) to occur masquerading as rigid motion. Eventually, precisely determined vertical motions of GPS stations above the Hellenic subduction zone are needed to resolve this important question.