Consequent boundary example5/2/2023 ![]() Taking account of effective stress, the condition for reshear overcoming the frictional shear strength of the fault, τ f, is then approximated by a criterion of Coulomb form: Further justification comes from a global dip histogram for near-pure reverse-slip ruptures showing a dip range, 0 \, \sigma_ \right) \) (Hubbert and Rubey 1959). Célérier 2008 Zoback 1992), and it will generally be assumed that ‘Andersonian’ horizontal or vertical stress trajectories prevail in the discussion that follows on fault formation and reactivation. A range of evidence supports the predominance of ‘Andersonian’ stress trajectories through much of the Earth’s seismogenic crust (e.g. While topography may impose short wavelength stress heterogeneity in the near-surface, this diminishes with depth so that formation or reactivation of faults and fractures generally occurs under three basic stress regimes where σ v= σ 1 (normal fault regime), σ v= σ 2 (wrench or strike–slip fault regime), or σ v= σ 3 (thrust fault regime). Anderson ( 1905) argued that Earth’s free surface, incapable of sustaining shear stress, imposes an important mechanical boundary condition requiring one of the principal compressive stresses ( σ 1> σ 2> σ 3) to be vertical and the other two to lie in a horizontal plane. In deforming quartzo-feldspathic crust away from areas of active subduction, seismic activity is largely restricted to the top 15 ± 5 km of the crust with the base of this seismogenic zone (b.s.z.) apparently bounded by isotherms defining the onset of crystal plasticity in quartz (c. Systematic, rigorous evaluation is needed to test how widespread these associations are in different tectonic settings, and to see whether they exhibit time-dependent behaviour before and after major earthquake ruptures. In some instances, these low-velocity zones also exhibit high electrical conductivity. There is a tendency for large crustal earthquakes to be associated with extensive ( L ~ 100–200 km) low-velocity zones in the lower seismogenic crust, with more local Vp/Vs anomalies ( L ~ 10–30 km) associated with rupture nucleation sites. A range of seismological observations in compressional–transpressional settings are compatible with this hypothesis. The association of rupture nucleation sites with local concentrations of fluid overpressure is consistent with selective invasion of overpressured fluid into the roots of major fault zones and with observed non-uniform spacing of major hydrothermal vein systems along exhumed brittle–ductile shear zones. ![]() ![]() Postfailure, enhanced fracture along fault rupture zones promotes fluid discharge through the aftershock period, increasing fault frictional strength before hydrothermal sealing occurs and overpressures begin to reaccumulate. Localized fluid overpressuring nucleates ruptures at particular sites, but ruptures on large existing faults may extend well beyond the regions of intense overpressure. In these circumstances, ‘fault-valve’ action from ascending overpressured fluids is likely to be widespread with fault failure dual- driven by a combination of rising fluid pressure in the lower seismogenic zone lowering fault frictional strength, as well as rising shear stress. This is especially the case for compressional–transpressional tectonic regimes which, beside leading to crustal thickening and dewatering through prograde metamorphism, are also better at containing overpressure and are ‘load-strengthening’ (mean stress rising with increasing shear stress), the most extreme examples being associated with areas undergoing active compressional inversion where existing faults are poorly oriented for reactivation. A combination of geological evidence (in the form of hydrothermal vein systems in exhumed fault systems) and geophysical information around active faults supports the localized invasion of near-lithostatically overpressured aqueous fluids into lower portions of the crustal seismogenic zone which commonly extends to depths between 10 and 20 km. ![]()
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