Present-day Velocity Field Research Articles

Geodynamic model of the interaction between the continental lithosphere and the active continental margin in East Asia

We propose a concept for a geodynamic model of East Asia, taking into account the mechanism of the
 interaction between the stable part of the regional continental lithosphere and the active continental
 margin along the Kuril-Kamchatka and Japan island arcs. The concept involves upper mantle
 convection combined with keyboard-block mechanism explaining seismic cycle patterns in the active
 continental margin and provides grounds to resolve the paradoxes of the present-day velocity field
 observed by satellite geodesy methods. These paradoxes are associated with enormous variation in the
 velocity field, such as sometimes contradict directions of surface motions in the adjacent portions of the
 earth’s surface. We propose a model that attribute the observed motion pattern to the superposition of
 the long-term subduction-driven convection regime beneath the continent causing the ocean-ward
 lithosphere extension and the shorter-term cyclic motion of seismogenic blocks with alternating
 directions. The model contributes to the development of the physically-based theoretical concepts of
 modern plate tectonics and eliminates the contradictions between the observed data and classical plate
 tectonics in East Asia region.

Present-day velocity field and block kinematics of Tibetan Plateau from GPS measurements

Present-day velocity field and block kinematics of Tibetan Plateau from GPS measurements

Present‐day deformation of northern Pakistan from Salt Ranges to Karakorum Ranges

AbstractEpisodic GPS measurements are used to quantify the present‐day velocity field in the northwestern Himalaya from the southern Pamir to the Himalayan foreland. We report large postseismic displacements following the 2005 Kashmir earthquake and several mm/yr thrusting of the central segment of the Salt Ranges and Potwar Plateau over the foreland, westward thrusting of Nanga Parbat above the Kohistan Plateau, and ~12 mm/yr SSE velocities of the Karakorum Ranges and of the Deosai and Kohistan Plateaus relative to the Indian Plate. Numerical simulations allow to determine a first approximation of slip along active faults: (1) substantial creep of ~87 mm/yr between 2006 and 2012 along the flat northeast of the Balakot‐Bagh Thrust affected by the 2005 earthquake; (2) ~5 mm/yr slip of the central segment of the Salt Ranges and Potwar Plateau, whereas their western boundaries are clearly inactive over the time span covered by our measurements; (3) 13 mm/yr ductile slip along the Main Himalayan Thrust modeled by a dislocation dipping 7° northward, locked at a depth of 15 km; and (4) ~20 mm/yr slip along the shear zone forming the western boundary of Nanga Parbat, between depths of 1.6 and 6.5 km. Residuals velocities suggest the existence of left‐lateral strike slip along the Jhelum Fault.

Spectral-finite element approach to viscoelastic relaxation in a spherical compressible Earth: application to GIA modelling

SUMMARY The choice of the physical model of postglacial rebound plays a decisive role to derive information about the mantle rheology and viscosity from observed data. In models for the mantle rheology, an incompressible Maxwell material is often assumed in spite of seismic observations showing that the Earth's mantle is composed of compressible material. In this study, in order to assess the influence of compressibility on glacial isostatic adjustment (GIA), the spectral-finite element approach proposed by Martinec is extended to incorporate the effect of compressibility. Using this approach, the present-day velocity field is computed for Peltier's ICE5G/VM2 earth-model/glaciation-history combination considering the sea level equation in the formulation of Hagedoorn et al. The results show that the effect of compressibility on the vertical displacement rate is small whereas the horizontal rates are markedly enhanced. For example, the rate around Fennoscandia and Laurentide becomes twice as large when compressibility is considered. This large difference between the compressible and incompressible models can be reduced by adjusting the elastic rigidity of the incompressible model so that the flexural rigidity becomes approximately the same as that in the compressible model. However, differences of ∼1 mm yr−1 still remain for wavelengths longer than 8000 km. These findings show that when modelling horizontal motion induced by GIA, the influence of compressibility cannot be neglected.

Structure of the present-day velocity field of the crust in the area of the Central-Asian GPS network

The structure of the vector field of present-day horizontal velocities on the Earth’s surface is investigated with the use of the territory covered by the Central-Asian GPS network as an example. A method of identification of groups of GPS points (statistically rigid clusters), for which the rate of change in distances between them is virtually zero, is proposed and realized. Sites of the Earth’s surface (regions) containing such groups of GPS points, within the required measurement accuracy, move in the horizontal plane as two-dimensional rigid bodies. The clustering algorithm, which is based on the Student t statistics in determining the so-called statistical “sample cost,” is developed and carefully tested. The results of identification of regions take into account possible random errors in velocity measurements and do not depend on the chosen frame of reference. The method of identification of regions is sufficiently stable with respect to variations in the number of GPS sites used for clustering. Of all the sites of the Central-Asian GPS network, 323 points were selected for clustering. These sites were measured from 3 to 11 times over an 11-year interval of observations (1995–2005). The estimates of errors of velocity measurement for these sites must not exceed 1.0 mm/yr. As a result, 29 statistically rigid clusters, containing from 3 to 17 GPS sites, were identified, and the kinematic regimes of motion of regions corresponding to these clusters were determined with respect to the stable part of Eurasia. With the general direction of the translatory motion of regions toward the north, the majority of them rotate counterclockwise. Nearly one third of GPS sites do not participate in the formation of clusters; these points fall into the interregional space (IRS), which is characterized by increased strain rates. The IRS structure is partitioned into zones with four directions, of which two directions virtually coincide with directions of the principal axes of the regional strain rate tensor, and the two other directions are oriented diagonally to the principal axes. The axis of the maximum rate of the regional shortening has a north-northwestern orientation. It is in this direction (mainly along the IRS) that the crust’s contraction takes place. There is no spatial correlation between IRS zones and geological faults; however, their angular distributions and the directions of strike-slips on them are interrelated. The resulting patterns of regional motions and IRS deformations consistently reflect the dynamic action of the Indian plate on the territory under investigation.

A full Stokes-flow thermo-mechanical model for firn and ice applied to the Gorshkov crater glacier, Kamchatka

AbstractThe Gorshkov crater glacier at Ushkovsky volcano, Kamchatka, is characterized by a large aspect ratio and special thermodynamic conditions at the bedrock caused by a locally enhanced and spatially varying geothermal heat flux. Furthermore, large parts of this glacier consist of firn rather than pure ice, which alters the rheological properties (such as viscosity and compressibility) of the glacier. We present a newly developed, thermo-mechanically coupled, three-dimensional flow model based on the finite-element (FE) modeling software Elmer, and apply it to the Gorshkov crater glacier. By assuming steady-state conditions, the present-day velocity field, temperature field, basal melting rate and age distribution are simulated. We find that flow velocities are generally small (tens of centimeters per year). Horizontal and vertical velocities are of comparable magnitude, which shows that the shallow-ice approximation is not applicable. Owing to the spatially variable volcanic heat flux, the thermal regime at the ice base is cold in the deeper parts of the glacier and temperate in the shallower parts. The measured temperature profile and age horizons at the K2 borehole are reproduced quite well, and remaining discrepancies may be attributed to transient (non-steady-state) conditions. Firn compressibility is identified as a crucial element for the modeling approach.

An interpretation of the active deformation of southern Taiwan based on numerical simulation and GPS studies

The fold-and-thrust belt of Taiwan results from oblique convergence between the Eurasia and the Philippine Sea plate, and its front structures are strongly influenced by the presence of large horsts and basins in the foreland. Using a 2-D numerical modelling (finite-element and distinct-element methods), we evaluate the influences on the active deformation of southern Taiwan of: (1) the foreland structural highs; (2) the major fault zones in the belt; and (3) the presence of a subduction zone to the south. To constrain our models, we take into account for the first time the present-day velocity field of southern Taiwan estimated based on new geodetic data obtained through the Global Positioning System (GPS). Particular attention is paid to the role of geological discontinuities, through an evaluation of the presence and role of mechanical decoupling along major faults, which plays an important role in the distribution of the regional and local velocity and stress patterns. This particular analysis of the behaviour and influence of weak shear zones in Taiwan is carried out by using, for the first time, the distinct-element method. Additional 3-D distinct-element modelling allows better consideration of oblique shearing, such as for the Longitudinal Valley Fault of eastern Taiwan. We conclude that the active velocity field and tectonic stress pattern in southwestern Taiwan strongly depend on: (1) the presence and shape of the Peikang High; (2) the presence of the major active regional discontinuities (the Longitudinal Valley Fault and the major thrusts of western Taiwan); and (3) the neighbouring weakness zone of the accretionary prism of the northern Manila subduction zone, and cannot be explained by any of these factors taken solely.

Testing the gravitational instability hypothesis?

We challenge a widely accepted assumption of observational cosmology: that successful reconstruction of observed galaxy density fields from measured galaxy velocity fields (or vice versa), using the methods of gravitational instability theory, implies that the observed large-scale structures and large-scale flows were produced by the action of gravity. This assumption is false, in that there exist nongravitational theories that pass the reconstruction tests and gravitational theories with certain forms of biased galaxy formation that fail them. Gravitational instability theory predicts specific correlations between large-scale velocity and mass density fields, but the same correlations arise in any model where (a) structures in the galaxy distribution grow from homogeneous initial conditions in a way that satisfies the continuity equation, and (b) the present-day velocity field is irrotational and proportional to the time-averaged velocity field. We demonstrate these assertions using analytical arguments and N-body simulations. If large-scale structure is formed by gravitational instability, then the ratio of the galaxy density contrast to the divergence of the velocity field yields an estimate of the density parameter Omega (or, more generally, an estimate of beta identically equal to Omega(exp 0.6)/b, where b is an assumed constant of proportionality between galaxy and mass density fluctuations. In nongravitational scenarios, the values of Omega or beta estimated in this way may fail to represent the true cosmological values. However, even if nongravitational forces initiate and shape the growth of structure, gravitationally induced accelerations can dominate the velocity field at late times, long after the action of any nongravitational impulses. The estimated beta approaches the true value in such cases, and in our numerical simulations the estimated beta values are reasonably accurate for both gravitational and nongravitational models. Reconstruction tests that show correlations between galaxy density and velocity fields can rule out some physically interesting models of large-scale structure. In particular, successful reconstructions constrain the nature of any bias between the galaxy and mass distributions, since processes that modulate the efficiency of galaxy formation on large scales in a way that violates the continuity equation also produce a mismatch between the observed galaxy density and the density inferred from the peculiar velocity field. We obtain successful reconstructions for a gravitational model with peaks biasing, but we also show examples of gravitational and nongravitational models that fail reconstruction tests because of more complicated modulations of galaxy formation.