Crustal Structure of North Iraq from Receiver Function Analyses

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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 30º to 90º 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.

The objective of the current paper is to estimate the seismic velocity structure beneath the stations of the North Iraq Seismographic Network (NISN) based on RFs inversion techniques. This effort will subsequently be supplemented by the determination of crustal structure from surface wave dispersion analyses. The two methods are complementary and provide valuable constraints on estimating seismic velocity structures. Evaluation of the resulting models will be performed through relocation of recorded events, synthetic waveform analysis, and correlation with available geophysical and geological information.

In November 2005 and in cooperation with the Sulaimaniyah, Erbil, Baghdad, and Mosul seismological observatories (SSO, ESO, BSO and MSO, respectively) the first eight stations of NISN were deployed, followed by stations BHD in Baghdad and MSL in Mosul in April 2006. Figure 1 shows a generalized diagram of the seismotectonic framework of the Arabian plate. NISN stations (white triangles) are located along the northeast boundary of the plate where the Zagros thrust zone and the Bitlis suture zone converge. This region is characterized by a high level of seismic activity, as evidenced by the large number of local and regional events recorded by NISN (Ghalib et al., 2006) and by the teleseismically located earthquakes reported by the United Stated Geological Survey (USGS) bulletins.

A close up of the geographic location of NISN is presented in Figure 2. The NISN stations, located parallel to the strike of the Zagros fold belt, are divided in two groups; one along the foothills and the other at higher elevation in the Zagros mountains. Since December 2005, Array IT has collected over 760 GB of data from NISN including local, regional and teleseismic events. The data consist of high-quality, continuous, three-component broadband recordings collected at a rate of 100 sps. Although the seismicity rate of the Zagros fold and thrust zone alone is relatively high, it may have gone mostly undocumented in the past. One explanation is that many of the unreported events are small and may not have been detectable by the regional stations in Turkey and Iran. Second, degradation of Iraq Seismological Network (ISN) capability has significantly impacted monitoring events in this region.

Also, noteworthy to mention is that many of the events reported by the USGS and ISC seem to cluster around the local stations in Iran and Turkey that reported their parameters, leaving a major section of the Zagros, its foothills and fold belt to be covered by NISN in an effective and comprehensive The average seismicity rate for local and regional earthquakes recorded by NISN is approximately 150 events per month, resulting in an unprecedented dataset available for research under this effort.

RFs are reflection time-series computed from three-component seismograms that represent the relative response of seismic waves to the earth structure near a receiver (Ammon, 2006). The RF methodology has been extensively used by seismologists. While the overall method is straightforward to define, the computation of reliable receiver functions can be problematic due to the non-uniqueness of RF inversions and has been the subject of many studies, including Burdick and Langston (1977), Langston (1977), Ammon (1991), Cassidy (1992), Ligorría and Ammon (1999), Park and Levin (2000), and Helffrich (2006). The current approach is based on the techniques by Zhu and Kanamori (2000), Julià et al. (2000), and Ammon (2006) to determine RFs and invert for crustal seismic velocity models.

Figure 1: Map of the Arabian Peninsula and surrounding regions.

Figure 2: Geographic location of the North Iraq Seismic Network (NISN).