Search

A general framework for modelling morphological responses to perturbation is proposed, based on the underpinning principle that the rates of morphological response in alluvial channels are initially high and then decrease through time as the system relaxes following disturbance. The framework includes three morphological response models, each developed from the fundamental rate law, which has the form of an exponential decay function. These models consider the possibility that characteristic behaviours of the fluvial system, such as delayed response and/or cumulative effects, may affect morphological responses, making them capable of representing relaxation paths and times for a range of morphological response variables, whatever their initial states. To test their utility, the models in the framework were applied to simulate the sequence of geomorphological responses to disruption observed in selected rivers with well-documented histories of morphological perturbation, adjustment and recovery. The results demonstrate that the models in the general framework can successfully simulate temporal and spatial patterns of morphological response in the fluvial system under a range of different circumstances, while also indicating how their reliability could be further improved. [ABSTRACT FROM PUBLISHER]

SUMMARY In 2001 November a magnitude 7.8 earthquake ruptured a 400 km long portion of the Kunlun fault, northeastern Tibet. In this study, we analyse over five years of post-seismic geodetic data and interpret the observed surface deformation in terms of stress relaxation in the thick Tibetan lower crust. We model GPS time-series (first year) and InSAR line of sight measurements (years two to five) and infer that the most likely mechanism of post-seismic stress relaxation is time-dependent distributed creep of viscoelastic material in the lower crust. Since a single relaxation time is not sufficient to model the observed deformation, viscous flow is modelled by a lower crustal Burgers rheology, which has two material relaxation times. The optimum model has a transient viscosity 9 × 1017 Pa s, steady-state viscosity 1 × 1019 Pa s and a ratio of long term to Maxwell shear modulus of 2:3. This model gives a good fit to GPS stations south of the Kunlun Fault, while displacements at stations north of the fault are over-predicted. We attribute this asymmetry in the GPS residual to lateral heterogeneity in rheological structure across the southern margin of the Qaidam Basin, with thinner crust/higher viscosities beneath the basin than beneath the Tibetan Plateau. Deep afterslip localized in a shear zone beneath the fault rupture gives a reasonable match to the observed InSAR data, but the slip model does not fit the earlier GPS data well. We conclude that while some localized afterslip likely occurred during the early post-seismic phase, the bulk of the observed deformation signal is due to viscous flow in the lower crust. To investigate regional variability in rheological structure, we also analyse post-seismic displacements following the 1997 Manyi earthquake that occurred 250 km west of the Kokoxili rupture. We find that viscoelastic properties are the same as for the Kokoxili area except for the transient viscosity, which is 5 × 1017 Pa s. The viscosities estimated for the Manyi and Kokoxili areas are consistent with constraints obtained from other earthquakes in the northwest and south central parts of the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Abstract: The 1600-km-long Kunlun Fault striking E–W to WNW–ESE had long been recognized as one of the major left-lateral strike-slip faults bounding the Tibetan Plateau, and ranked one of the most active faults in China continent. During the past hundred years, over twenty strong earthquakes occurred along and near the Kunlun Fault, including six large earthquakes (M >7). Since some major highly-populated and industrialized cities are close to the Kunlun Fault, understanding of stress transfer and earthquakes migration along the Kunlun Fault is most important for assessing seismic hazard in this region. In this study, by integrating coseismic effect, viscoelastic relaxation and tectonic loading, we studied the evolution of the regional Coulomb stress field by analyzing a sequence of strong earthquakes along the Kunlun Fault. We studied the stress evolution over one century by analysing a sequence of five earthquakes (M ≥7) that occurred along the Kunlun Fault since 1937. The model of dislocation sources embedded in a mixed elastic/inelastic layered half-space was used, and the layered model and relevant parameters were constrained by seismic studies. Fault rupture locations and geometry, as well as slip distribution of earthquakes were taken from field observations and seismic studies. Numerical results showed a good correlation between stress transfer, accumulation and earthquakes occurrence. All four studied earthquakes occurred after the 1937 Tuosuo Lake quake were encouraged by the preceding earthquakes with positive stress loading. In subject to the choice of the earthquake source parameter two or three out of four events occurred in regions that experienced previous coseismic and postseismic stress changes of at least 0.01 MPa, suggesting that earthquake triggering due to stress transfer has occurred along the Kunlun Fault. The total stress change since 1937 of the Kunlun Fault region has lead to high levels of stress accumulated on the Xidatan–Dongdatan segment and Maqin–Maqu segment, which have not experienced any significant large earthquake over at least several hundred or several thousand years. The accumulated stress raises the potential earthquake hazard in these areas. Our study demonstrated the crucial importance of postseismic viscoelastic relaxation in the stress transfer and accumulation following large earthquakes. [Copyright &y& Elsevier]