Crustal Structure of the Northern Zagros Zone from Seismic Observations

The current study is concerned with estimating the local and regional crustal structure of north and northeastern Iraq, including the northern extension of the Zagros collision zone. The goal of our work is to derive local and regional seismic velocity structures using receiver function- and surface wave dispersion analyses and to use these velocity models to obtain accurate hypocenter locations and event focal mechanisms. Global seismic network coverage in this region is poor and extrapolated velocity models found in the literature lack sufficient accuracy to permit events to be located with significant precision. Ten three- component broadband stations composing the North Iraq Seismographic Network (NISN) were installed in late 2005. At present, over 650 GB of seismic waveform data have been analyzed. Our analysis of waveform data indicates clear propagation paths from the south or west across the Arabian shield as well as from the north into NISN. Phases including Pn, Pg, Sn, Lg, as well as LR are clearly observed on these seismograms. In contrast, blockage or attenuation of S-wave energy is observed for propagation paths across the Zagros- Bitlis zone from the east, as well as along the axis of the Zagros from the south-east. Supporting these findings are the results of surface wave analysis. Waves generated by events located to the south of NISN propagating across the Arabian shield produce dispersion curves with energy distributed over a broad frequency band including the development of higher modes. In contrast, waves from events to the south-east which propagate along the axis of the Zagros mountains generate dispersion curves that diverge over a broad frequency range, indicating multi-pathing caused by the complex structure of the Zagros zone. These results are corroborated by receiver function analyses which indicate a dipping Moho beneath the Zagros mountains. While the Moho below the foothills is estimated at 40-50 km depth, it dips to 45-55 km depth below the northern extension of the Zagros zone. Furthermore, lower-than-average shear wave velocities were found for north-east Iraq when compared to other crustal regions of the Earth. Common among the receiver functions is the presence of a significant velocity discontinuity at a depth of 15 km and 20 km for the stations below the foothills and Zagros mountains, respectively. The increase in velocity across this discontinuity lead to the observation of mid-crustal refracted body waves throughout NISN.