Stress Patterns in Northern Iraq and Surrounding Regions from Formal Stress Inversion of Earthquake Focal Mechanism Solutions

摘要

The collision zone between the Arabian and Eurasian plates is one of the most seismically active regions. Northern Iraq represents the northeastern part of the Arabian plate that has a suture zone with the Turkish and Iranian plates called the Bitlis- Zagros suture zone. The orientations of the principal stress axes can be estimated by the formal stress inversion of focal mechanism solutions. The waveform moment tensor inversion method was used to derive a focal mechanism solution of 65 earthquakes with magnitudes range from 3.5 to 5.66 in the study area. From focal mechanism solutions, the direction of slip and the orientations of the moment stress axes (P, N, and T) on the causative fault surface during an earthquake were determined. The dataset of the moment stress axes have been used to infer the regional principal stress axes (δ1, δ2, and δ3) by the formal stress inversion method. Two inversion methods, which are the new right dihedron and the rotational optimization methods, were used. The results show that six stress regime categories exist in the study area. However, the most common tectonic regimes are the strike-slip faulting (43.94 %), unspecified oblique faulting (27.27 %), and thrust faulting (13.64 %) regimes. In most cases, the strike-slip movement on the fault surfaces consists of left-lateral (sinistral) movement. The normal faulting is located in one small area and is due to a local tensional stress regime that develops in areas of strike-slip displacements as pull-apart basins. The directions of the horizontal stress axes show that the compressional stress regime at the Bitlis-Zagros suture zone has two directions. One is perpendicular to the suture zone near the Iraq-Iran border and the second is parallel in places as well as perpendicular in others to the suture zone near the Iraq-Turkey border. In addition, the principal stress axes in the Sinjar area near the Iraq-Syria border have a E-W direction. These results are compatible with the tectonic setting of the Arabian-Eurasian continental collision zone and the anticlockwise rotation of the Arabian plate that is evidently responsible for the strike-slip displacements on fault surfaces.