Present-day Tectonic Stress Regimes in Northern Iraq and Surrounding Regions from Focal Mechanism Data

The collision zone between the Arabian and the 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 named Bitlis-Zagros suture zone. The present-day tectonic stress regimes can be estimated from focal mechanism data. The waveform moment tensor inversion method was used to derive the focal mechanism solution of 65 earthquakes with magnitudes of ≥3.5. Green's functions for the local and regional seismograms were generated in order to perform the moment tensor inversion using complete seismograms. From focal mechanism solutions, the direction of slip and the orientations of the moment tensor axes (P, N, and T) on the fault surface during an earthquake were determined. The tectonic stress regime of each focal mechanism was estimated based on Zoback (1992). The results show that all six categories of the stress regimes, which are normal faulting (NF), normal faulting with strike-slip component (NS), strike-slip faulting (SS), thrust faulting with strike-slip component (TS), thrust faulting (TF), and unknown or oblique faulting (UF), exist. However, the most common tectonic regimes in the study area are the SS (43.94%), UF (27.27%), and TF (13.64%). In most cases, the strike-slip movement on the fault surfaces consists of left-lateral movement. The NF regime, which is located in one small area near Diyala city at the Iraq-Iran border, might be a local tensional stress regime. The directions of the compressional stress axes show that the stress regime at the Zagros suture zone has two directions. One is perpendicular to the suture near the Iraq-Iran border, and the second is parallel to the suture near the Iraq-Turkey border. In addition, the compressional stress axes at the city of Sinjar, near the Iraq-Syria border, have E-W direction. These results are compatible with the tectonic setting of the Arabain-Eurasian continental collision zone and the anticlockwise rotation of the Arabian plate that might be responsible for the strike-slip displacement on the fault surfaces.