Frictional Properties Of Fault Research Articles

Earthquakes as Dynamic Fracture Phenomena

AbstractA large earthquake unlocks a fault‐zone via dynamic rupture while releasing part of the elastic energy stored during the interseismic stage. As earthquakes occur at depth, the analyses of earthquake physics rely primarily on experimental observations and conceptual models. A common view is that the earthquake instability is necessarily related to the frictional weakening that is commonly observed in shear experiments under seismic slip velocities. However, recent experiments with frictional interfaces in brittle acrylics (e.g., Svetlizky & Fineberg, 2014, https://doi.org/10.1038/nature13202) and rocks (e.g., Passelegue et al., 2020, https://doi.org/10.1038/s41467-020-18937-0) have explicitly demonstrated that no characteristic frictional strength exists. Namely, prior to nucleation, frictional interfaces can sustain a wide range of applied stresses (“overstresses”) that exceed the residual stresses, which are the stresses along the interface that remain after the sliding. Moreover, the experimentally observed singular stress‐fields and rupture dynamics are precisely those predicted by fracture mechanics (Freund, 1998). We therefore argue here that earthquake dynamics are best understood in terms of dynamic fracture mechanics and not governed by the frictional properties of faults. In this view, rupture dynamics are driven by the release of the elastic energy due to overstresses, whereas the values of the residual stresses and the energy dissipation are determined by fault frictional properties.

The spatial–temporal total friction coefficient of the fault viewed from the perspective of seismo-electromagnetic theory

Abstract. Recently, it has been shown theoretically how the lithospheric stress changes could be linked with magnetic anomalies, frequencies, spatial distribution and the magnetic-moment magnitude relation using the electrification of microfractures in the semibrittle–plastic rock regime (Venegas-Aravena et al., 2019). However, this seismo-electromagnetic theory has not been connected with the fault's properties in order to be linked with the onset of the seismic rupture process itself. In this work we provide a simple theoretical approach to two of the key parameters for seismic ruptures which are the friction coefficient and the stress drop. We use sigmoidal functions to model the stress changes in the nonelastic regime within the lithosphere. We determine the temporal changes in frictional properties of faults. We also use a long-term friction coefficient approximation that depends on the fault dip angle and four additional parameters that weigh the first and second stress derivative, the spatial distribution of the nonconstant stress changes, and the stress drop. We found that the friction coefficient is not constant in time and evolves prior to and after the earthquake occurrence regardless of the (nonzero) weight used. When we use a dip angle close to 30∘ and the contribution of the second derivative is more significant than that of the first derivative, the friction coefficient increases prior to the earthquake. During the earthquake event the friction drops. Finally, the friction coefficient increases and decreases again after the earthquake occurrence. It is important to mention that, when there is no contribution of stress changes in the semibrittle–plastic regime, no changes are expected in the friction coefficient.

Chemo‐mechanical behavior of clay‐rich fault gouges affected by CO2‐brine‐rock interactions

AbstractThe impact of long‐term CO2‐brine‐rock interactions on the frictional properties of faults is one of the main concerns when ensuring safe geological CO2 storage. Mineralogical changes may alter the frictional strength and seismogenic potential of pre‐existing faults bounding a storage complex. However, most of these reactions are too slow to be reproduced on laboratory timescales and can only be assessed using geochemical modeling. We combined modeling of CO2‐charged formation water and fault gouges (1–1000 years residence time, i.e. 10–106 pore volume flushes) with friction experiments on simulated fault gouges (T = 22–150°C; σneff = 50 MPa; Pf = 25 MPa; V = 0.2‐100 μm/s), having mineralogical compositions as predicted by the models. As an analogue for clay‐rich caprocks overlying potential CO2 storage sites in Europe, we used the Opalinus claystone. Our experiments showed that, although significant mineralogical changes occurred, they did not significantly change the frictional behavior of faults. Instead, initial fault‐gouge mineralogy imposed a stronger control on clay‐rich fault behavior than the extent of CO2‐brine‐rock interactions, even under chemical conditions allowing for significant reaction. We demonstrated that the impact of mineralogical changes due to CO2‐brine‐rock interactions on the frictional behavior and seismogenic potential of faults could be assessed using our combination of geochemical modeling and friction experiments. Note that a complete understanding requires evaluation of additional effects, such as that of shear velocity, effective normal stress, and other fault characteristics (maturity, shear strain). © 2018 The Authors. Greenhouse Gases: Science and Technology published by Society of Chemical Industry and John Wiley & Sons, Ltd.

Viscous Accretionary Prisms: Viscoelastic Relaxation of the Makran Accretionary Prism Following the 2013 Baluchistan, Pakistan Earthquake

AbstractGeodetic observations of postseismic transients following earthquakes commonly help to inform the rheology of the lower crust and mantle, frictional properties of faults, and the kinematics of deformation across the earthquake cycle. Here we use interferometric synthetic aperture radar time series observations of postseismic deformation following the 2013 Mw 7.7 Baluchistan, Pakistan earthquake to investigate the viscoelastic rheology of the Makran accretionary prism. Using observations that began 15 months after the earthquake and a suite of potential fault geometries, we show that the ongoing deformation transient in the vicinity of the earthquake cannot be explained by afterslip alone. Instead, the gross spatial and temporal characteristics of the transient can be explained by viscoelastic relaxation of a shallow (>6 km) weak zone bounded by the Baluchistan earthquake and the underthrusting Arabian oceanic plate. Our first‐order results show that the viscoelastic lower accretionary prism has a thickness of 8–12 km and exhibits power law (n = 3.5) behavior with viscosities of 1017–1018 Pa·s. These estimated viscosities may be higher if afterslip occurred during the initial period of postseismic deformation not constrained by our observations. The power law rheology suggests that creep processes common at lower crustal and mantle temperatures may be active at comparatively lower temperatures in the Makran. We hypothesize that the weak nature of the Makran accretionary prism is driven by high pore fluid pressures introduced by underplated sediments and/or hydrocarbon development, and we show a mechanically weak accretionary prism may lead to overestimates of plate coupling when linear elasticity is assumed.

Stress field sensitivity analysis in a sedimentary sequence of the Alpine foreland, northern Switzerland

Abstract. The stress field at depth is a relevant parameter for the design of subsurface constructions and reservoir management. Yet the distortion of the regional stress field due to local-scale features such as sedimentary and tectonic structures or topography is often poorly constrained. We conduct a stress sensitivity analysis using 3-D numerical geomechanical modelling with an elasto-plastic material law to explore the impact of such site-specific features on the stress field in a sedimentary sequence of the Swiss Alpine foreland. The model's dimensions are 14 × 14 × 3 km3 and it contains 10 units with different mechanical properties, intersected by two regional fault zones. An initial stress state is established involving a semi-empirical relationship between the ratio of horizontal to vertical stress and the overconsolidation ratio of argillaceous sediments. The model results indicate that local topography can affect the stress field significantly to depths greater than the relief contrasts at the surface, especially in conjunction with horizontal tectonic loading. The complexity and frictional properties of faults are also relevant. The greatest variability of the stress field arises across the different sedimentary units. Stress magnitudes and stress anisotropy are much larger in stiffer formations such as massive limestones than in softer argillaceous formations. The stiffer formations essentially carry the load of the far-field forces and are therefore more sensitive to changes of the boundary conditions. This general characteristic of stress distribution in the stiff and soft formations is broadly maintained also with progressive loading towards the plastic limit. The stress field in argillaceous sediments within a stack of formations with strongly contrasting mechanical properties like in the Alpine foreland appears to be relatively insensitive to changes in the tectonic boundary conditions and is largely controlled by the maximum stiffness contrast with respect to the load-bearing formations.