Crustal Structure of North Iraq from Receiver Function Analyses

摘要

A primary objective of this project is to estimate the local and regional seismic velocity structures of north and northeastern Iraq, including the northern extension of the Zagros collision zone. Furthermore, earthquake source mechanisms will be investigated once a velocity model is derived for this region. Thus far, global seismic network coverage is poor throughout the region 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, waveforms from 290 teleseismic events, from November 30, 2005 through March 31, 2007, have been processed for P-wave receiver functions (RFs). Based on the USGS Preliminary Determination of Epicenters (PDE) bulletins, the epicentral distances of these teleseismic events to NISN stations range from 30 deg to 90 deg while their magnitudes equal or exceed 5.5. Results obtained to date indicate a lower-than-average shear wave velocity when compared to other crustal regions of the Earth. Additionally, Moho depths appear slightly shallower below the stations located along the foothills compared to the stations at higher elevation in the Zagros mountains. The Moho below the foothills is estimated at 40-50 km depth, while it dips down to a depth of 45-55 km below the northern extension of the Zagros zone. 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. Evaluation of the resulting models will be performed through relocation of recorded events, synthetic waveform analysis, and correlation with available geophysical and geological information.