STRESS-ANALYSIS IN THE INTRAPLATE AREA OF GAZLI, UZBEKISTAN, FROM DIFFERENT SETS OF EARTHQUAKE FOCAL MECHANISMS

Occurrence of intraplate earthquakes have been related either to react ivation of preexisting zones of weakness in a uniformly oriented regio nal stress field or to a local stress changes. More recently, buildup of pore pressure anomalies has been proposed to explain both the low r ecurrence rate and some misorientation of rupture planes in the prevai ling regional stress field. In this study, we analyze local stress fie ld orientation in the area of the three M(s) = 7.0 Gazli (western Uzbe kistan) earthquakes by using different sets of earthquakes focal mecha nisms. We show that stress tensor inversions from both seven 4.2 less than or equal to M(s) less than or equal to 7.0, forty-five 0.1 less t han or equal to ML less than or equal to 3.2 and twenty-five 1.0 less than or equal to ML less than or equal to 3.2 earthquake focal mechani sms give constant maximum horizontal stress direction in agreement wit h the one deduced from the India-Asia collision. The best model for th e local stress tensor exhibits a significant (25 degrees) discrepancy from vertical for one of the principal stresses. We note, moreover, th at the common assumption that one principal stress axis is oriented ve rtically is not strictly supported by stress states derived from earth quake focal mechanisms available in the world stress database. By eval uating each hypothesis of stress inversion methods from earthquake foc al mechanisms, it is shown that, especially in intraplate settings, th e first motion focal solution should be the most appropriate one for a formal inversion and that the magnitude of the considered event may n ot be the only criterion to assess the quality of a stress inversion. This is particularly timely because more and more focal mechanisms are moment tensor solutions instead of first motion solutions. Concerning the Gazli sequence, the slip conditions (pore pressure and frictional coefficient) are evaluated for each of the three mainshocks in terms of an independently determined local stress field. Because variations in orientations between local and regional stress field are not detect ed, and because the three mainshock fault planes seem to be relatively well oriented for a frictional activation in the local stress field, it is concluded that this seismic sequence is localized on preexisting weakness zones and is not due to local stress orientation anomalies.