STATISTICAL TRENDS IN THE INTRAPLATE STRESS-FIELD

The World Stress Map (WSM) database contains thousands of intraplate stress indicators, with the potential to provide important constraint for arguments about the relationship between tectonic stresses and both the kinematics and dynamics of plate motion. Previous studies, which relied almost exclusively on visual inspection of the data, established the existence of broad regions of uniform maximum horizontal compressive stress orientation (S-Hmax) and stress regimes on a regional scale. In the present study, we present a statistical analysis of the WSM stress indicators with the aim of quantifying trends in both the S-Hmax orientations and stress regimes. The analysis was carried out within 5 degrees x 5 degrees bins which provide a resolution of several hundred kilometers. Only the 4537 high-quality WSM indicators with rating of A to C were used in the analysis. We present results for two types of analysis on the information contained within the bins. First, we evaluate the spatial distribution of the average stress regime (normal, strike-slip, or thrust). Second, we apply the Rayleigh test,a standard statistical method in the analysis of directional data, to the distribution of S-Hmax orientations to test the null hypothesis that the orientations are random. An important aspect of our study is the quantification of the conclusions drawn from visual inspection of the World Stress Map. Our results indicate that broad regions of uniform S-Hmax orientations exist in most continental regions at high confidence levels (90% and 95%) and are less robust in the slowest moving continental plates. We also quantify the predominance of strike-slip and compressional stress regimes in continental regions. Importantly, our analysis provides information about trends in the S-Hmax orientations in regions where large amounts of scatter in the directional data prevented conclusions being drawn from visual inspection of the data, for example, in western North America and continental Australia. Furthermore, we find a strong correlation between average S-Hmax orientations and both the ridge push torque and the absolute plate velocity azimuths. Our observation that a greater number of S-Hmax orientations correlate with the ridge push torque directions is further evidence that the intraplate stress field is strongly influenced by the ridge push force.