Roland Bürgmann

Continuous deformation of the Tibetan Plateau from global positioning system data

Global positioning system velocities from 553 control points within the Tibetan Plateau and on its margins show that the present-day tectonics in the plateau is best described as deformation of a continuous medium, at least when averaged over distances of .;100 km. Deformation occurs throughout the plateau interior by ESE-WNW extension and slightly slower NNE-SSW shortening. Relative to Eurasia, material within the plateau interior moves roughly eastward with speeds that increase toward the east, and then flows southward around the eastern end of the Himalaya. Crustal thickening on the northeast- ern and eastern margins of the plateau occurs over a zone ;400 km wide and cannot be the result of elastic strain on a single major thrust fault. Shortening there accommodates much of Indias penetration into Eurasia. A description in terms of movements of rigid blocks with elastic strain associated with slip on faults between them cannot match the velocity field.

Contemporary crustal deformation around the southeast borderland of the Tibetan Plateau

[1] We derive a detailed horizontal velocity field for the southeast borderland of the Tibetan Plateau using GPS data collected from the Crustal Motion Observation Network of China between 1998 and 2004. Our results reveal a complex deformation field that indicates that the crust is fragmented into tectonic blocks of various sizes, separated by strike-slip and transtensional faults. Most notably, the regional deformation includes 10–11 mm/yr left slip across the Xianshuihe fault, � 7 mm/yr left slip across the Anninghe-Zemuhe-Xiaojiang fault zone, � 2 mm/yr right slip across a shear zone trending northwest near the southern segment of the Lancang River fault, and � 3 mm/yr left slip across the Lijiang fault. Deformation along the southern segment of the Red River fault appears not significant at present time. The region south and west of the XianshuiheXiaojiang fault system, whose eastward motion is resisted by the stable south China block to the east, turns from eastward to southward motion with respect to south China, resulting in clockwise rotation of its internal subblocks. Active deformation is detected across two previously unknown deformation zones: one is located � 150 km northwest of and in parallel with the Longmenshan fault with 4–6 mm/yr right-slip and another is continued south-southwestward from the Xiaojiang fault abutting the Red River fault with � 7 mm/yr left slip. While both of these zones are seismically active, the exact locations of faults responsible for such deformation are yet to be mapped by field geology. Comparing our GPS results with predictions of various models proposed for Tibetan Plateau deformation, we find that the relatively small sizes of the inferred microblocks and their rotation pattern lend support to a model with a mechanically weak lower crust experiencing distributed deformation underlying a stronger, highly fragmented upper crust.

Kinematics of the India‐Eurasia collision zone from GPS measurements

We use geodetic techniques to study the India-Eurasia collision zone. Six years of GPS data constrain maximum surface contraction rates across the Nepal Himalaya to 18 ± 2 mm/yr at 12°N ±13° (1σ). These surface rates across the 150-km-wide deforming zone are well fitted with a dislocation model of a buried north dipping detachment fault striking 105°, which aseismically slips at a rate of 20 ± 1 mm/yr, our preferred estimate for the India-to-southern-Tibet convergence rate. This is in good agreement with various geologic predictions of 18 ± 7 mm/yr for the Himalaya. A better fit can be achieved with a two-fault model, where the western and eastern faults strike 112° and 101°, respectively, in approximate parallelism with the Himalayan arc and a seismicity lineament. We find eastward directed extension of 11 ± 3 mm/yr between northwestern Nepal Lhasa, also in good agreement with geologic and seismic studies across the southern Tibetan plateau. Continuous GPS sites are used to further constrain the style and rates of deformation throughout the collision zone. Sites in India, Uzbekistan, and Russia agree within error with plate model prediction.

Time-dependent land uplift and subsidence in the Santa Clara valley, California, from a large interferometric synthetic aperture radar data set

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Evidence of power-law flow in the Mojave desert mantle.

Studies of the Earths response to large earthquakes can be viewed as large rock deformation experiments in which sudden stress changes induce viscous flow in the lower crust and upper mantle that lead to observable postseismic surface deformation. Laboratory experiments suggest that viscous flow of deforming hot lithospheric rocks is characterized by a power law in which strain rate is proportional to stress raised to a power, n (refs 2, 3). Most geodynamic models of flow in the lower crust and upper mantle, however, resort to newtonian (linear) stress–strain rate relations. Here we show that a power-law model of viscous flow in the mantle with n = 3.5 successfully explains the spatial and temporal evolution of transient surface deformation following the 1992 Landers and 1999 Hector Mine earthquakes in southern California. A power-law rheology implies that viscosity varies spatially with stress causing localization of strain, and varies temporally as stress evolves, rendering newtonian models untenable. Our findings are consistent with laboratory-derived flow law parameters for hot and wet olivine—the most abundant mineral in the upper mantle—and support the contention that, at least beneath the Mojave desert, the upper mantle is weaker than the lower crust.

The motion and active deformation of India

Measurements of surface displacements using GPS constrain the motion and deformation of India and India-Eurasia plate boundary deformation along the Himalaya. The GPS velocities of plate-interior sites constrain the pole of the angular velocity vector of India with respect to Eurasia to lie at 25.6±1.0°N 11.1±9.0°E, approximately 6° west of the NUVEL-1A pole of <3 Ma plate motion. The angular rotation rate of 0.44 ±0.03°Myr−1 is 14% slower than the long-term rate of 0.51° Myr−1. Insignificant velocities between plate interior sites indicate that the exposed Indian plate is stable to within 7 · 10−9 yr−1. The observed contraction vector across the Himalaya (≤20 mm/yr) veers from ∼N20°E in the northwest Himalaya to ∼N25°W in east Nepal, consistent with east-west extension of southern Tibet.

Mobility of continental mantle: Evidence from postseismic geodetic observations following the 1992 Landers earthquake

The crust around the rupture zone of the 1992 Landers earthquake has continued to deform in the years following the earthquake at rates ∼3 times greater than pre-earthquake rates. We use a combination of Global Positioning System (GPS) and synthetic aperture radar (InSAR) data collected during a ∼3-year epoch following the earthquake in order to investigate postseismic mechanisms responsible for the high transient velocities. In order to maximize the potential signal from viscoelastic relaxation we evaluate and model postseismic relaxation following the first few months of documented accelerated deformation. The combination of GPS and InSAR data allows us to establish viscoelastic relaxation of the lower crust and upper mantle as the dominant postseismic process and to discriminate among possible viscoelastic models. The data particularly require the presence of a highly ductile uppermost mantle beneath the central Mojave Domain, with temperature between the wet and dry basalt solidus. This is consistent with independent seismic and geochemical inferences of a regionally warm uppermost mantle. Further consideration of seismic velocity variations in conjunction with faulting patterns within the Mojave Desert suggests that the primary faulting characteristics of the Mojave Desert, namely, the pervasive late Cenozoic deformation within the Eastern California Shear Zone versus the near absence of faults in the Western Mojave Domain, are controlled by the rheology of the uppermost mantle.

Slip distributions on faults: effects of stress gradients, inelastic deformation, heterogeneous host-rock stiffness, and fault interaction

Abstract Fault slip distributions are commonly assumed to be symmetrical about a central slip maximum, however, slip distributions in nature are often asymmetric. Although slip along an idealized fault is expected to follow an elliptical distribution after a single slip event in an elastic material, the slip distribution may be modified if the fault propagates or if additional slip events occur. Analytically and numerically computed fault-slip distributions in an elastic medium indicate that: (1) changes in the (frictional) strength along a fault; (2) spatial gradients in the stress field; (3) inelastic deformation near fault terminations; and (4) variations of the elastic modulus of the host rock can cause strong deviations from idealized symmetrical distributions along single-slip event faults. A relatively stiff body adjacent to or cut by a fault will tend to reduce fault slip in its vicinity and tends to flatten the slip profile where it is cut by the fault. Sharp slip gradients develop near the interface between relatively soft and stiff materials. The interaction of faults within about one fault radius of one another can strongly influence slip gradients. Inelastic processes, caused by stress perturbations in the stepover region of echelon faults, may link individual segments and thereby create a slip distribution resembling that of a single fault.

Dynamics of İzmit Earthquake Postseismic Deformation and Loading of the Düzce Earthquake Hypocenter

We have developed dynamic finite-element models of Izmit earthquake postseismic deformation to evaluate whether this deformation is better explained by afterslip (via either velocity-strengthening frictional slip or linear viscous creep) or by distributed linear viscoelastic relaxation of the lower crust. We find that velocity-strengthening frictional afterslip driven by coseismic shear stress loading can reproduce time-dependent Global Positioning System data better than either linear viscous creep on a vertical shear zone below the rupture or lower crustal viscoelastic relaxation. Our best frictional afterslip model fits the main features of postseismic slip inversions, in particular, high slip patches at (and below) the hypocenter and on the western Karadere segment, and limited afterslip west of the Hersek Delta (Burgmann et al., 2002). The model requires a weakly velocity-strengthening fault, that is, either low effective normal stress in the slipping regions or a smaller value for the parameter describing rate-dependence of friction ( a - b ) than is indicated by laboratory experiments. Our best afterslip model suggests that the Coulomb stress at the Duzce hypocenter increased by 0.14 MPa (1.4 bars) during the Izmit earthquake (assuming right-lateral slip on a surface dipping 50° to the north), and by another 0.1 MPa during the 87 days between the Izmit and Duzce earthquakes. In the Marmara Sea region (within about 160 km of the Izmit earthquake rupture), this model indicates that the Coulomb stresses increased by 15%-25% of the coseismic amount during the first 300 days after the earthquake. Three hundred days after the earthquake, postseismic contributions to Coulomb stressing rate on the Maramara region faults had fallen to values equal to or less than the inferred secular stress accumulation rate. Our estimates of postseismic Coulomb stress are highly model dependent: in the Marmara region, the linear viscous shear zone and viscoelastic lower crust models predict greater postseismic Coulomb stresses than the frictional afterslip model. Near-field stress and fault-zone rheology estimates are sensitive to the Earth9s elastic structure. When a layered elastic structure is incorporated in our model, it yields a Coulomb stress of 0.24 MPa at the Duzce hypocenter, significantly more than the 0.14 MPa estimated from the uniform elastic model. Because of the higher near-field coseismic stresses, the layered elastic model requires a higher value of velocity-strengthening parameter ( A - B ) ([ a - b ] times effective normal stress r ′) to produce comparable postseismic slip. ( A - B ) is estimated at 0.4 and 0.2 MPa, respectively, for the layered and uniform elastic models. These results highlight the importance of understanding the Earth9s elastic structure and the mechanism for postseismic deformation if we wish to accurately model coseismic and postseismic crustal stresses.

Time-Dependent Distributed Afterslip on and Deep below the İzmit Earthquake Rupture

Surface deformation transients measured with the Global Positioning System during the 87 days between the 17 August 1999 Izmit earthquake and the 12 November 1999 Duzce earthquake indicate rapidly decaying aseismic fault slip on and well below the coseismic rupture. Elastic model inversions for time-dependent distributed fault slip, using a network inversion filter approach, show that afterslip was highest between and below the regions of maximum coseismic slip and propa- gated downward to, or even below, the base of the crust. Maximum afterslip rates decayed from greater than 2 m/yr, immediately after the I zmit earthquake to about 1.2 m/yr just prior to the Duzce earthquake. Maximum afterslip occurred below the eastern Karadere rupture segment and near the I zmit hypocenter. Afterslip in the upper 16 km decayed more rapidly than that below the seismogenic zone. These observations are consistent with a phase of rapid aseismic fault slip concentrated near the base of the seismogenic zone. Continued loading from the rapid deep afterslip along the eastern rupture zone is a plausible mechanism that helped trigger the nearby, Mw 7.2, 12 November Duzce earthquake.