World Stress Map Release 2005 - Stress orientations from single focal mechanisms at plate boundaries

The World Stress Map (WSM) is a global compilation of data about recent tectonic stresses from a wide range of indicators (e.g. focal mechanisms, borehole breakouts). It is a valuable tool for the solution of numerous of technological and scientific problems. The orientation of the stress field, for instance, is a primary control on subsurface fluid flow and thus WSM data can be used to improve petroleum production or the efficiency of geothermal power stations. In scientific context, information on the stress state is essential for seismic hazard assessment. The WSM database release 2005 contains more than 14,000 data sets all classified according to a unified quality ranking. Thus, the comparability of data from different types of measurement is guaranteed. The database as well as guidelines and software for plotting stress maps are available free of charge from our website www.world-stress-map.org. Users can create their own stress map including their own stress data almost instantly with the CASMO (Create A Stress Map Online) web tool. Alternatively, users can download the software CASMI (Create A Stress Map Interactively) free of charge and produce their own stress maps. In the WSM 2005 release we refined the definition of so-called Possible Plate Boundary Events (PBE) for stress data from single focal mechanisms (FMS) considering that the orientations of these earthquakes might be rather controlled by the geometry of the plate boundary than by the stress field orientation. In general, it is assumed that numerous randomly oriented faults are present in the crust, so that earthquakes occur on faults optimally oriented relative to the regional stress field. In such a setting the principal axes of the moment tensor (P, B, T) provide good approximations for the principal stress orientations (σ_1, σ2, σ3). However, plate boundaries show a different mechanical behavior. They are characterized by faults with preferred orientations and presumably include major faults with a low coefficient of friction which can be easily reactivated. The related P-B-T axes might considerably deviate from σ_1-3 of the regional stress field. We investigate whether this deviation depends on the distance between the FMS and the closest plate boundary segment. We analyze all FMS of the WSM 2005 database release and found that FMS's have a higher potential for large deviations when they meet the following criteria: (1) The tectonic regime of the FMS reflects the plate boundary kinematics. (2) The event is located within a critical distance dcrit relative to its closest plate boundary segment. (3) The deviation between the strike of the nodal plane and the strike of the plate boundary is smaller than 30(°). The critical distances dcrit depends on the type of plate boundaries. We estimate them by means of a statistical analysis with dcrit being 45 km for continental transform faults, 80 km for oceanic transform faults, 70 km for oceanic spreading ridges, and 200 km for subduction zones. The three detection criteria are met by one third of the ~9000 FMS datasets which thus were marked by a PBE flag in the WSM database. Users should be aware that these data might considerably deviate from the regional stress field.