No description available.

No description available.

Smectite clay minerals are among the most common minerals in subduction zone megathrusts faults at shallow depth and in landslide decollements. Consequently, deformation processes at seismic slip rates (ca. 1 m/s) in smectites contribute to control the mechanics of megathrust earthquakes and landslide collapses. To investigate the deformation processes, rotary shear experiments on smectite-rich gouge layers (70/30 wt.% Ca-montmorillonite/opal) were performed. The experiments were conducted at ambient temperature and at 5 MPa normal stress. The gouges were sheared under vacuum (<0.001 mbar) and room humidity (i.e., water depleted) or in the presence of liquid water (i.e., water rich) conditions, at slip rates of 0.0003 <V <1.5 m/s and displacements of 0.1 <d <30 m. The temperature evolution with slip was measured with thermocouples and modelled numerically. Permeability of the gouge layer was measured with the pore pressure oscillation method prior to the rotary experiments. Before and after the experiments, the mineral and amorphous material content in wt.% were determined via quantitative X-ray powder diffraction and the microstructures investigated via scanning and transmission electron microscopy. The activation of deformation processes was strongly controlled by the water content of the gouge layers. Under water depleted conditions, grainsize reduction producing nanoparticles controlled the evolution of the friction coefficient f at all slip rates. Coseismic dynamic weakening (f = 0.2 - 0.3) occurred by combined thermal decomposition or melting (with decreasing water content) and pressurization of water released by dehydration of smectite interlayer. Under water rich conditions, grain size reduction was minor and development of nano-foliations occurred. At all slip rates, the friction coefficient rapidly decreased at the onset of slip. The large initial weakening (to f <0.15) was due to the presence of a film of water lubricating the surfaces of the sub-parallel smectite grains forming the nano-foliation in combination with shear-enhanced water pressurization. Then, friction coefficient evolved depending on the balance between dissipation of pore pressures, dehydration of smectite interlayer and thickening of the nano-foliation layers. At higher displacement and slip rates, sustained dynamic weakening was aided by vaporization of pore water. Expulsion of water determined a switch to deformation processes typical of water depleted conditions. In nature, the presence of liquid water in smectites has a lubricating effect, pressurizes the slipping zone and renders the smectite-rich gouges prone to accommodate large seismic slips. During megathrust earthquakes, such lubricating effect may result in the easy propagation of seismic ruptures in smectite- and water-rich sediments at shallow depths. Similarly, the presence of water can promote large displacements during landslide collapse.

No description available.

This dissertation consists of two parts and investigates the crustal deformation associated with great subduction zone earthquake at two different spatial scales. At the small scale, I investigate the stress transfer along the megathrust during great earthquakes and its effects on the forearc wedge. At the large scale, I investigate the viscoelastic crustal deformation of the forearc and the back arc associated with great earthquakes. Part I: In a subduction zone, the frontal region of the forearc can be morphologically divided into the outer wedge and the inner wedge. The outer wedge which features much active plastic deformation has a surface slope angle generally larger than that of the inner wedge which hosts stable geological formations. The megathrust can be represented by a three-segment model, the updip zone (velocity-strengthening), seismogenic zone (velocity-weakening), and downdip zone (velocity-strengthening). Our dynamic Coulomb wedge theory postulates that the outer wedge overlies the updip zone, and the inner wedge overlies the seismogenic zone. During an earthquake, strengthening of the updip zone may result in compressive failure in the outer wedge. The inner wedge undergoes elastic deformation. I have examined the geometry and mechanical processes of outer wedges of twenty-three subduction zones. The surface slope of these wedges is generally too high to be explained by the classical critical taper theory but can be explained by the dynamic Coulomb wedge theory. Part II: A giant earthquake produces coseismic seaward motion of the upper plate and induces shear stresses in the upper mantle. After the earthquake, the fault is re-locked, causing the upper plate to move slowly landward. However, parts of the fault will undergo continuous aseismic afterslip for a short duration, causing areas surrounding the rupture zone to move seaward. At the same time, the viscoelastic relaxation of the earthquake-induced stresses in the upper mantle causes prolonged seaward motion of areas farther landward including the forearc and the back arc. The postseismic and interseismic crustal deformation depends on the interplay of these three primary processes. I have used three-dimensional viscoelastic finite element models to study the contemporary crustal deformation of three margins, Sumatra, Chile, and Cascadia, that are presently at different stages of their great earthquake cycles. Model results indicate that the earthquake cycle deformation of different margins is governed by a common physical process. The afterslip of the fault must be at work immediately after the earthquake. The model of the 2004 Sumatra earthquake constrains the characteristic time of the afterslip to be 1.25 yr. With the incorporation of the transient rheology, the model well explains the near-field and far-field postseismic deformation within a few years after the 2004 Sumatra event. The steady-state viscosity of the continental upper mantle is determined to be 10^19 Pa S, two orders of magnitude smaller than that of the global value obtained through global postglacial rebound models. / Graduate

This dissertation consists of two parts and investigates the crustal deformation associated with great subduction zone earthquake at two different spatial scales. At the small scale, I investigate the stress transfer along the megathrust during great earthquakes and its effects on the forearc wedge. At the large scale, I investigate the viscoelastic crustal deformation of the forearc and the back arc associated with great earthquakes. Part I: In a subduction zone, the frontal region of the forearc can be morphologically divided into the outer wedge and the inner wedge. The outer wedge which features much active plastic deformation has a surface slope angle generally larger than that of the inner wedge which hosts stable geological formations. The megathrust can be represented by a three-segment model, the updip zone (velocity-strengthening), seismogenic zone (velocity-weakening), and downdip zone (velocity-strengthening). Our dynamic Coulomb wedge theory postulates that the outer wedge overlies the updip zone, and the inner wedge overlies the seismogenic zone. During an earthquake, strengthening of the updip zone may result in compressive failure in the outer wedge. The inner wedge undergoes elastic deformation. I have examined the geometry and mechanical processes of outer wedges of twenty-three subduction zones. The surface slope of these wedges is generally too high to be explained by the classical critical taper theory but can be explained by the dynamic Coulomb wedge theory. Part II: A giant earthquake produces coseismic seaward motion of the upper plate and induces shear stresses in the upper mantle. After the earthquake, the fault is re-locked, causing the upper plate to move slowly landward. However, parts of the fault will undergo continuous aseismic afterslip for a short duration, causing areas surrounding the rupture zone to move seaward. At the same time, the viscoelastic relaxation of the earthquake-induced stresses in the upper mantle causes prolonged seaward motion of areas farther landward including the forearc and the back arc. The postseismic and interseismic crustal deformation depends on the interplay of these three primary processes. I have used three-dimensional viscoelastic finite element models to study the contemporary crustal deformation of three margins, Sumatra, Chile, and Cascadia, that are presently at different stages of their great earthquake cycles. Model results indicate that the earthquake cycle deformation of different margins is governed by a common physical process. The afterslip of the fault must be at work immediately after the earthquake. The model of the 2004 Sumatra earthquake constrains the characteristic time of the afterslip to be 1.25 yr. With the incorporation of the transient rheology, the model well explains the near-field and far-field postseismic deformation within a few years after the 2004 Sumatra event. The steady-state viscosity of the continental upper mantle is determined to be 10^19 Pa S, two orders of magnitude smaller than that of the global value obtained through global postglacial rebound models. / Graduate