Stress in the Descending Relic Slab beneath the Vrancea Region, Romania

Abstract

We examine the effects of viscous flow, phase transition, and dehydration on the stress field of a relic slab to explain the intermediate-depth seismic activity in the Vrancea region. A 2-D finite-element model of a slab gravitationally sinking in the mantle predicts (1) downward extension in the slab as inferred from the stress axes of earthquakes, (2) the maximum stress occurring in the depth range of 70 km to 160 km, and (3) a very narrow area of the maximum stress. The depth distribution of the annual average seismic energy released in earthquakes has a shape similar to that of the depth distribution of the stress in the slab. Estimations of the cumulative annual seismic moment observed and associated with the volume change due to the basalt-eclogite phase changes in the oceanic slab indicate that a pure phase-transition model cannot solely explain the intermediate-depth earthquakes in the region. We consider that one of the realistic mechanisms for triggering these events in the Vrancea slab can be the dehydration of rocks which makes fluid-assisted faulting possible.

Characterization of Seismogenic Zones of Romania

“…They are close to the similar distribution presented by TRIFU (1990). The seismic moment rate (M : 0 = 1.2× 10 19 Nm/year) is close to the estimation of ISMAIL-ZADEH et al (1996), who demonstrate that such a high value could not be explained by a pure phase-transition in the Vrancea slab.…”

“…The compression field seems to be ''consumed'' at subcrustal depth by a very confined fluid-assisted faulting process (GREEN and HOUSTON, 1995;ISMAIL-ZADEH et al, 1996). If continent -continent collisional forces continue to act in the Vrancea area (CONSTANTINESCU and ENESCU, 1984;FUCHS et al, 1979), it is difficult to correlate them with the available seismicity and focal mechanism data in the crust.…”

“…Even if we admit the hypothesis of a decoupling of the slab from the overlying crust (FUCHS et al, 1979), how the deformation in the subcrustal range reflects in that in the crustal range remains an open question. A possible way to explain this is a model of dehydration-induced faulting as a plausible source for the Vrancea intermediate-depth earthquakes (GREEN, 1994;ISMAIL-ZADEH et al, 1996).…”

“…A possible explanation of this drastic difference is the existence of a fluid-assisted faulting mechanism at intermediate depth (GREEN, 1994;ISMAIL-ZADEH et al, 1996).…”

Geohazard research, modeling, and assessment for disaster risk reduction

“…The combination of these forces produces tectonic stresses high enough to cause earthquakes. Other processes contributing to stress generation in the descending lithosphere and its release in earthquakes can be plastic instability at high temperature [Griggs and Baker, 1969], faulting due to metamorphic phase transitions [Green and Burnley, 1989], dehydration-induced embrittlement [Ismail-Zadeh et al, 2000], and some others. Earthquakes occur as a sudden release of stresses.…”