Abstract.We have inverted velocity solutions from nine geodetic networks distributed across New Zealand to derive present-day continuous horizontal velocity and strain rate fields at the Earth's surface throughout the country.
Summary Evidence from geomorphology, the distribution of large earthquakes, and geodetic measurements suggests that the active faulting in mainland Greece and the north Aegean Sea is concentrated into a small number of discrete, linear zones that bound relatively rigid blocks. On land, the zones are most clearly identified where the faulting is associated with large escarpments, particularly in hard footwall rocks. The zones can become indistinct and diffuse near their ends, sometimes because they enter high, unstable topography where landsliding obscures the fault morphology, but mostly because the extension rates decrease along strike as a consequence of the relative rotation of the blocks they bound. The main graben systems of Chalkidiki, southern Thessaly and the North Gulf of Evia all seem to connect with the strike‐slip faulting in the offshore Aegean and to die out in the west, a configuration related to clockwise rotations in east‐central Greece. By contrast, the Gulf of Corinth, which is the fastest opening graben system in Greece, opens more rapidly in the west than the east. Both these features of the Gulf of Corinth are consequences of the motion of the south Aegean and Peloponnese as a single block. Late Pliocene and Quaternary geology and geomorphology indicate that the boundaries of the rigid blocks in central Greece have changed over that time, with faulting migrating into the hanging walls, sometimes changing in orientation. These changes are probably related to the way faulting in the seismogenic layer adapts to block rotation, so as to maintain the general features of a velocity field that is controlled by larger scale effects, such as buoyancy forces and the seaward migration of the subducting slab beneath Crete. The image of Greece as a mosaic of rigid blocks whose boundaries change with time is a useful framework for seismic hazard evaluation, but we emphasize that some moderate‐sized earthquakes do occur away from the main fault zones, within the relatively rigid blocks and especially near their diffuse ends.
Continental convergence between Arabia and Eurasia is taken up by distributed deformation in Iran.At wavelengths large compared with the thickness of the lithosphere this deformation is best described by a continuous velocity field. The only quantitative source of information on the spatial distribution of strain rates within Iran is the record of earthquakes. We find that we can reproduce the style of deformation observed in the seismicity by simply minimizing the rate of work in a continuous viscous medium that has to accommodate the Arabia-Eurasia plate motion between the defined shapes of Iran's rigid borders. When, in addition, we specify central Iran, Azerbaijan, and the southern Caspian basin to be relatively rigid blocks within the deforming zone, then the fit to the style of the observed strain rate distribution is even better. We conclude that much of the pattern of deformation in Iran is predetermined by the shape of its rigid borders and by the disposition of relatively rigid blocks within it. This is likely also to have been a common occurrence in older orogenic belts. We confirm earlier suggestions that earthquakes between 1909 and 1992 can account for only a small part (,-•10-20%) of the total deformation required by the convergence between the Arabia and Eurasia plates. We then show that the whole plate motion can be accommodated by a velocity field with the same orientations and relative magnitudes of principal strain rates seen in the earthquakes but with larger absolute magnitudes. There is therefore no requirement that the large proportion of aseismic deformation in Iran is substantially different in style, orientation, or distribution from that released seismically in the earthquakes. Introduction The deformation in most regions of active continental tectonics is distributed over areas several times wider than the lithosphere thickness. Such deformation can reasonably be described by a continuous velocity field [e.g., England and McKenzie, 1982; Vilotte et al., 1982; McKenzie and Jackson, 1983; England and Jackson, 1989] provided one considers only length scales that are large compared with the thickness of the seismogenic upper crust, within which deformation is mostly discontinuous and concentrated on faults. In these terms, a proper description of the kinematics requires the answers to two questions: (1) what does the velocity field look like, and (2) how is it accommodated by Copyright 1995 by the American Geophysical Union. Paper number 95JB01294. 0148-0227/95/95JB-01294505.00 faulting in the upper crust? It is then possible to ask further questions that may be related to the dynamics (i.e., to the forces responsible for the deformation), such as why the velocity field has the characteristics it does and what the relation might be between the discontinuous deformation in the upper crust and the more distributed flow that is likely to characterize the creeping deformation in the lithosphere beneath [e.g., Jackson et al., 1992; Lamb, 1994].In this paper we investigate these questions in Ir...
Kinematic modelling utilising the method of Haines & Holt extended to the case of cubic Bessel interpolation on curvilinear grids, allows analysis of presentday horizontal motions occurring in the Hikurangi margin, North Island, New Zealand. The velocity field solutions are derived from first order geological data; that is, rates and orientation of extension in the Taupo Volcanic Zone and rates and orientation of motion on the North Island Dextral Fault Belt, against the background of the pattern of uplift and subsidence in the margin.A basic (preferred) velocity field is presented, with four other solutions with different input data, to explain what controls features in the main solution. Every one of the five solutions is the best fitting solution for the input data in each case. All velocity fields are shown relative to the western boundary of the model, which is considered fixed, as part of the assumed non-deforming Australian plate.The velocity field in the main solution includes a strong clockwise rotation of the Hikurangi margin east of the Taupo Volcanic Zone in the north. Farther south, shear across the North Island Dextral Fault Belt facilitates the southwestward motion of the eastern part of the margin. An important boundary condition for the deformation in the North Island appears to be the higher rate of dextral shear in the Marlborough region, which accommodates the relative motion of the Australian and Pacific plates immediately south of the Hikurangi margin.The extension in the Taupo Volcanic Zone, with rates onshore of 5-10 mm/yr north of where the Taupo Volcanic Zone terminates in the centre of the North Island, and the strike-slip component of shear on the North Island Dextral Fault Belt, of c. 20 mm/yr in the south and <5 mm/yr in the north, account for most of the margin-parallel plate motion in the Hikurangi margin. No other major geological strains are required to be occurring in the North Island out of compatibility with these strains.
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