Present-day stress field vs faults: some examples in Italy

Abstract

The World Stress Map is a standard global compilation of information on the present-day stress field of the Earth's crust including: earthquake focal mechanisms, well bore breakouts and drilling-induced fractures, in-situ stress measurements (overcoring, hydraulic fracturing, borehole slotter) and young geologic data (from fault-slip analysis and volcanic vent alignments). This compilation and several single papers have clearly proved the existence of a first-order stress field controlled by plate boundary forces, and a second-order stress field (regional) controlled by major intraplate stress sources, such as topography, density, strength contrasts and major fault systems. In areas where high data density is present, a third order stress field (local) can also be accounted, linked to the presence of minor geological features (i.e. active faults, local inclusions, detachment horizons or density contrasts). The local stress orientations in some cases overrule the first- and second-order stress pattern: major discontinuities within a rock mass disturb the stress field causing localized increases in differential stress and an associated change in the orientation of the stress trajectory. Stress field at local scale can be recognized by detailed analysis of stress measurements, low magnitude earthquake focal solutions and minor tectonics features. In particular, the breakout analysis points out that stress perturbation occurs in a well - with depth - when open fractures, or recently slipped faults or mechanically weak zones are crossed by, or close to, the borehole. The free surfaces of an open fracture in a rock body deflect stress trajectories in the closer surrounding area, so the smallest principal stress approaches the free surface at right angle. The stress field rotates in proximity to an active fault due to small slip increments on the fault, possibly induced by the tectonic movement or due to an increase of pore pressure on the fault when drilling through it. Stress rotations identified near active (or young) faults can be used to make assumptions about the strength of the fault zone compared with the surrounding rock mass. In fact shear zones usually show physical properties different from the nearby undamaged rock: these features can be recorded by downhole logs and then compared to stress data. On the contrary, if any stress perturbation occurs close to a fault, this latter can be considered a sealed fault with characteristics similar to the host rock body. Here, we present some examples relative to small-scale stress rotations related to the presence of active faults gathered from borehole breakout and focal mechanism analysis in some areas of Italy.