Comprehensive analysis on present tectonic stress field in Urumqi region

Urumqi city is located in central part of northern piedmont of Tianshan mountain. The mean direction of principal compressive stress in central section of northern Tianshan deduced from the focal mechanism solutions is around N10deg;E, with a clear feature of gradual change from west to east. In the region of Urumqi city, the direction derived from focal mechanism solutions of moderate-strong earthquakes is N15deg;mdash;20deg;E.The principal stress direction found from fault striation inversion is N17deg;Wmdash;N2deg;E. The stress direction deduced from the above two data sets are basically in the variation range of the principal compressive stress in the central section of northern Tianshan. However, the stress direction inferred from composite fault-plane solutions of small earthquakes within 25 km of the Urumqi seismic station is N40deg;mdash;46deg;E, which shows a deviation of 20deg;mdash;30deg;from that of regional principal compressive stress. Nevertheless, the direction of maximum horizontal compressive stress measured in the boreholes in Urumqi city is around N55deg;E, which displays a further deviation of 10deg;mdash;15deg;, and its deviation from that of regional stress field is up to 30deg;mdash;40deg;. These phenomena indicate that a tension stress in the shallow layers in front part of the compressive fault-fold region has been superposed to the regional NNE-SSW trending compressive tectonic stress and makes the direction of local principal compressive stress turn to NEE-SWW. This is one of the causes why the normal faults, such as the Jiujiawan fault groups, were developed in Urumqi city.