A resampling approach to test stress-field uniformity from fault data

Several methods have been proposed to constrain the stress field from fault plane orientations and slip directions within a crustal volume characterized by brittle deformation. All the methods are based on the assumption that the stress field is uniform in the volume considered. If this hypothesis is not checked in advance, however, the methodology may lead to misleading conclusions. In this work, a procedure is defined to check stress-field uniformity by a statistical analysis of the available fault data. Since, in most cases, the statistical features of the uncertainties that affect such data are not well known, a distribution-free approach is proposed. It is based on a simple search algorithm, devoted to selecting stress configurations compatible with available data, combined with a bootstrap resampling approach. The test results are more conservative than the ones so far proposed in the literature. When the test allows stress heterogeneities to be safely excluded, approximate confidence intervals for the principal stress directions can be obtained; otherwise, the level of stress heterogeneity present in the volume under study can be assessed. An application of the proposed procedure to a sample of fault data deduced from seismological data is presented.     

Microearthquake seismicity and fault-plane solutions in the southern Aegean and its geodynanic implications

Seismicity ( Ml <3.5) in the southern Aegean, located using data collected during seven weeks of recording by a temporary network of seismological stations, largely follows the Hellenic arc; the Sea of Crete is nearly aseismic, and only little activity is located south of the Hellenic trench, within the African plate. Focal mechanisms exhibit reverse faulting in the external part of the arc and normal faulting inside it. This normal faulting indicates N-S extension in the northern Aegean, the Gulf of Corinth, the Cyclades and Dodecanese Islands, but NW-SE extension in southern Peloponnese and western Crete and E-W extension in eastern Crete. This non-uniform strain pattern suggests that the Aegean region not only extends in a N-S sense, with the Hellenic arc moving south-westward relative to the Eurasian plate, but also by E-W extension of its southern margin, so that there is a net divergence of material.     

Effect of viscosity structure on fault potential and stress orientations in eastern Canada

Previous investigations of the causal relationship between postglacial rebound and earthquakes in eastern Canada have focused on the mode of failure and the observed timing of the pulse of earthquake/faulting activity following deglaciation. In this study, the observational database has been extended to include observed orientations of the contemporary stress field and the rotation of stress since deglacial times. It is shown that many of these observations can be explained by a realistic ice history and a viscoelastic earth with a uniform 10²¹ Pa s mantle.
The effects of viscosity structure on the above predictions are also examined. It is shown that, since most of the above observations are found within the ice margin, they are not very sensitive to lithospheric thickness. Also, the inclusion of a 25 or 50 km ductile layer within the lithosphere will not decouple the seismogenic upper crust. High viscosity (10²² Pa s) in the lower mantle is rejected by the stress orientation and rotation observations. A low-viscosity (6 times 10²⁰Pa s) upper mantle with 1.6 times 10²¹ Pa s in the upper part of the lower mantle and 3 times 10²¹ Pa s in the lower part of the lower mantle below 1200 km depth has been found to give predictions that are in general agreement with the observations.