Base Of Seismogenic Zone Research Articles

Fault mechanics at the base of the continental seismogenic zone: Insights from geochemical and mechanical analyses of a crustal-scale transpressional fault from the Argentera crystalline massif, French–Italian Alps

Faulting mechanics is examined by combining geochemical and mechanical analyses from a key exposure of a major fault zone crossing the Argentera crystalline massif (French–Italian Alps). The analyses are carried out on co-genetic and syn-tectonic quartz and chlorite extension veins and shear veins associated with late stage deformation in the fault zone. Paleothermometry based on chlorite compositions gives formation temperatures of 300 °C ± 20 °C, that is near the base of the seismogenic zone. δ18O values of quartz and δD and δ18O values of chlorite indicate that veins crystallized from a metamorphic fluid. In parallel, a mechanical analysis based on the Mohr-Coulomb theory shows that the pore fluid factor λv required to simultaneously reactivate the studied fault and to open the extension veins was close to a lithostatic value (λv ∼ 1).Comparisons with the 2003–2004 Ubaye seismic swarm, having occurred in the continuity of the studied fault zone, suggest that the base of the seismogenic zone may act as a limit separating an upper permeable reservoir saturated with meteoric waters under hydrostatic to supra-hydrostatic pressures from a lower low-permeability reservoir containing metamorphic waters under lithostatic pressure. This study suggests that overpressured fluids can be released upwards in the brittle crust by shear-enhanced permeability and can trigger earthquakes.

Temperature distribution and focal depth in the crust of the northeastern Japan

The thickness of seismogenic crust layer correlates with surface heat flow in most interplate seismic areas of the world (e.g., Sibson, 1982). Although the inverse relationship between heat flow and the base of seismogenic zone is obvious, the quantitative relationships are less certain and there should be variability of the focal depths among different tectonic settings. Comparisons of the heat flow (Yamano et al., 1997), thermal gradient (Tanaka et al., 1999) and earthquake (Japan Meteorological Agency, JMA) databases for the northeastern Japan provide detailed geologic and geophysical information about the earthquake process of island arc. Temperatures in the crust were calculated using a steady-state, one-dimensional, heat conductive transport model with heat generation as a function of heat flow and thermal gradient. The evaluated temperatures for D90, the depth above which 90% of earthquakes occur, range between 200°C and 500°C except for high heat flow and thermal gradient data. The consistency of temperature for D90 over a large depth interval supports that the temperature is the dominant factor governing the focal depth in the crust.