Other
Scientific paper
Mar 1989
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989e%26psl..92..247m&link_type=abstract
Earth and Planetary Science Letters, Volume 92, Issue 2, p. 247-264.
Other
17
Scientific paper
Usual analysis of fault kinematics in terms of stress supposes that the slip occurs in the direction of the shear-stress resolved on the fault plane due to an homogeneous state of stress. However, as a block moves, this yields perturbations of the static stress conditions. The resulting displacement may produce interactions of blocks and may induce complex fault motions such as those demonstrated by the aftershock sequences of the Thessaloniki and Campania-Lucania earthquakes. Each of which shows a main group of focal mechanisms whose normal fault motions are modelled using a stress deviator (TM) in agreement both with the motion of the main seismic fault and with the regional stress pattern deduced from Quarternary and Recent faults. It is therefore considered as representative of the mean state of stress acting in the epicentral area. But two other families of fault motions are also demonstrated. One shows reverse motions explained by a compressional stress deviator (TC), the other, involving normal motions, is explained by a tensional stress deviator (TE). These deviators are not representative of the mean (regional) state of stress. The compressional σ1 axis of (TC) and the tensional σ3 axis of (TE) are roughly orthogonal to the σ3 axis of the mean state of stress (TM). These (TC) and (TE) deviators may be deduced from the mean state of stress (TM) by adding, respectively, (TD1) and (TD2) deviators to this latter. This means that on each fault plane having a reverse τc or a normal τe motion, this motion may be deduced from the normal kinematics τm it should have occurred in the mean regional state of stress (TM) by adding local pushes τd1 and τd2 respectively. The deviator (TD2) being nearly axisymmetric around a vertical σ3 axis, the local reverse pushes τd1 are nearly dip-slip and (TD2) having a high-angle dipping σ2 axis, the local pushes τd2 are essentially strike-slip. Approximated solutions of these (TD1) and (TD2) deviators have been calculated by taking (TM) + (TD1) = (TC) and (TM) + (TD2) = (TE). The mean state of stress (TM) being divided into two parts, (Tv) and (Ts), which respectively produce the dip-slip and essentially strike-slip components of the stress resolved on the fault plane, a first approximation of the data shows that (TD1) = -kv(Tv) and (TD2) = -ks(Ts) with kv and ks equal to 2. This approximation supposes that (TM), (TC) and (TE) are coaxial and that (TD1) and (TD2) are respectively axisymmetric around the vertical τ3 axis and planar in the horizontal plane. Actually, the data give kv ~ 2, and ks in the order of 4-5. It is suggested that the local reverse and strike-slip pushes are induced by the reaction of the rocky body surrounding small blocks moving as a consequence of a motion on a major fault particularly as there are incompatibilities of motion of adjacent small blocks. However, data are not sufficient to demonstrate the mechanical and geometric conditions which control the observed different behaviors of the small seismic faults. Numerical simulations or experiments could probably precise these conditions. Whatever, under certain conditions, the stable solution τm is replaced by kinematics instabilities τe and τc. However, it is usually observed that faults studied in the field, in a simple way, often yield a single stress deviator. This is probably due to the nature of the measured striations which are often growth fibers and oblique stylolithes resulting from creep fault motions. In this case, incompatibilities of block motions are probably accommodated by a pressure-dissolution phenomena. Therefore, it is suggested that the bifurcation parameter which controls the occurrence of the characteristic instabilities possibly depends on a ratio of the deformation rate versus the pressure-dissolution rate of the material. A question remains unanswered: are these instabilities scale invariant or is there a changeover in the behavior between minor and major faults?
Carey-Gailhardis Evelyne
Louis Mercier Jacques
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