Tomographic Pn velocity and anisotropy structure beneath the Anatolian plateau (eastern Turkey) and the surrounding regions

An edited version of this paper was published by the American Geophysical Union (AGU). Copyright 2003, AGU. See also: http://www.agu.org/pubs/crossref/2003.../2003GL017391.shtml; http://atlas.geo.cornell.edu/turkey/publications/Al-Lazki-et-al_2003.htmWe use Pn phase travel time residuals to invert for mantle lid velocity and anisotropy beneath northern Arabia eastern Anatolia continent-continent collision zone. The primary phase data were obtained from the temporary 29- station broadband PASSCAL array of the Eastern Turkey Seismic Experiment. These data were supplemented by phase data from available stations of the Turkish National Seismic Network, the Syrian National Seismic Network, the Iranian Long Period Array, and other stations around the southern Caspian Sea. In addition, we used carefully selected catalog data from the International Seismological Centre and the National Earthquake Information Center bulletins. Our results show that low (<8 km/s) to very low (<7.8 km/s) Pn velocity zones underlie the Anatolian plateau, the Caucasus, and northwestern Iran. Such low velocities are used to infer the presence of partially molten to absent mantle lid beneath these regions. In contrast, we observed a high Pn velocity zone beneath northern Arabia directly south of the Bitlis-Zagros suture indicating the presence of a stable Arabian mantle lid. This sharp velocity contrast across the suture zone suggests that Arabia is not underthrusting beneath the Anatolian plateau and that the surface suture extends down to the uppermost mantle. Pn anisotropy orientations within a single plate (e.g. Anatolia plate) show a higher degree of lateral variation compared to Pn velocity. Areas of coherent Pn anisotropy orientations are observed to continue across major fault zones such as the EAF zone

Sn attenuation in the Anatolian and Iranian plateau and surrounding regions

An edited version of this paper was published in Geophysical Research Letters by the American Geophysical Union (AGU). Copyright 2003, AGU. See also: http://www.agu.org/pubs/crossref/2003/2003GL018020.shtml; http://atlas.geo.cornell.edu/turkey/publications/Gok-et-al_2003.htmThe propagation characteristics of the regional Sn shear waves have been mapped to provide insight into the lithospheric structure of the Anatolian and Iranian plateau and the surrounding regions. Thousands of regional earthquakes within the distance range of 2?15 degrees were recorded by broadband and short period stations located in Turkey and nearby regions, especially new data recorded by 29 broadband stations in the Eastern Turkey Seismic Experiment network. The propagation efficiencies of Sn were determined visually using their amplitude and frequency content. Attenuation maps were then tomographically constructed using the observed propagation efficiencies. Our results confirm that Sn propagates efficiently in the uppermost mantle beneath the Mediterranean Sea, the Black Sea, and the Caspian Sea and along the Zagros fold and thrust belt. Sn is not observed in eastern Turkey, northwestern Iran, or central Anatolia. In contrast to previous available studies, this study considerably improved the mapped location of the boundaries between the zones of efficient and attenuated Sn. Our results are best explained by an absence of lithospheric mantle, or the presence of thin and hot lithospheric mantle beneath most of the Anatolian and Iranian plateau

Pn tomographic imaging of mantle lid velocity and anisotropy at the junction of the Arabian, Eurasian, and African plates

An edited version of this paper was published by Blackwell Publishing in Geophysical Journal International. Copyright 2004, Blackwell Publishing. See also: http://www.blackwell-synergy.com/doi/abs/10.1111/j.1365-246X.2004.02355.x; http://atlas.geo.cornell.edu/MiddleEastNorthAfrica/publications/Al-Lazki2004.htmThe interaction of the Arabian plate with the Eurasian plate has played a major role in building the young mountain belts along the Zagros-Bitlis continent-continent collision zone. Arabia's northward motion is considered to be the primary driving force behind the present-day westerly escape of the Anatolian plate along the North and East Anatolian fault zones as well as the formation of the Turkish and the Iranian plateaux. In this study we mapped Pn-wave velocity and anisotropy structures at the junction of the Arabian, Eurasian and African plates in order to elucidate the upper-mantle dynamics in this region. Pn is a wave that propagates within the mantle lid of the lithosphere and is often used to infer the rheology and fabric of the mantle lithosphere. Applying strict selection criteria, we used arrival times of 166 000 Pn phases to invert for velocity and anisotropy in the region. Using a least-squares tomographic code, these data were analyzed to solve simultaneously for both velocity and azimuthal anisotropy in the mantle lithosphere. We found that most of the continental regions in our study area are underlain by low Pn velocity structures. Broad-scale (~500 km) zones of low (<8 km s-1) Pn velocity anomalies underlie the Anatolian plate, the Anatolian plateau, the Caucasus region, northwestern Iran and northwestern Arabia, and smaller scale (~200 km), very low (<7.8 km s-1) Pn velocity zones underlie southern Syria, the Lesser Caucasus, the Isparta Angle, central Turkey and the northern Aegean Sea. The broad-scale low-velocity regions are interpreted to be hot and unstable mantle lid zones, whereas very low Pn velocity zones are interpreted to be regions of no mantle lid. The low and very low Pn velocity zones in eastern Turkey, northwestern Iran and the Caucasus region may be associated with the latest stage of intense volcanism that has been active since the Late Miocene. The low Pn velocity zones beneath the Anatolian plate, eastern Turkey and northwestern Iran may in part be a result of the subducted Tethyan oceanic lithosphere beneath Eurasia. We also found a major low-velocity zone beneath northwestern Arabia and the Dead Sea fault system. We interpret this anomaly to be a possible extension of the hot and anomalous upper mantle of the Red Sea and East Africa rift system. High Pn velocities (8.1-8.4 km s-1) are observed to underlie the Mediterranean Sea, the Black Sea, the Caspian Sea, and the central and eastern Arabian plate. Observed Pn anisotropy showed a higher degree of lateral variation than did the Pn velocity structure. Although the Pn anisotropy varies even in a given tectonic region, in eastern Anatolia very low Pn velocity and Pn anisotropy structures appear to be coherent

Regional wave propagation in Turkey and surrounding regions

This paper was published by the American Geophysical Union (AGU). Copyright 2000, AGU. See also: http://www.agu.org/journals/gl/gl0003/1999GL008375/pdf/1999GL008375.pdf; http://atlas.geo.cornell.edu/turkey/publications/Gok-et-al_2000.htmDigital and analog seismic waveform data collected by 34 stations in and around Turkey provided excellent ray coverage for a detailed attenuation study of regional shear waves (Sn and Lg). Over 2000 seismograms within a distance range of 15 degrees were visually inspected and the quality of Sn and Lg phases categorized into three different classes: efficient, inefficient, or not present. Our results show that Sn and Lg propagation is mostly inefficient in western Turkey and the Aegean Sea. Sn is efficient in parts of southwestern Turkey, the western Pontides, and western Greece. Sn is not observed in eastern Turkey and along the Aegean volcanic arc. Lg propagates efficiently in the Arabian plate including paths that cross the Dead Sea fault zone and in northwestern Turkey. Lg does not propagation in northeast Anatolia, across the Lesser Caucasus, and north of the Hellenic arc (Sea of Crete). These results are a major improvement on prior attenuation studies in this region and provide new constraints for proposed tectonic models

Seismic event location using the Eastern Turkey Seismic Network: Analysis of the Agri Dam explosion

Copyright 2004, Seismological Society of America. See also: http://www.seismosoc.org/publications/bssa-toc.html; http://atlas.geo.cornell.edu/turkey/publications/Gurbuz-et-al_2004.htmA 12 ton controlled source explosion took place in eastern Turkey on June 5, 2001 and was recorded by 18 stations of the Eastern Turkey Seismic Experiment (ETSE) PASSCAL broadband network. This is a unique recording obtained for the first time in this region. Due to the blasting type and extremely high Lg and Sn attenuation in eastern Turkey, the blast is only observed out to a distance of about 300 km. We have used travel time data from this explosion to obtain average crustal structure and site correction terms for the stations. The explosion was located using two new regional velocity models and the IASP91 velocity model to test the location capabilities of the ETSE network. We found that for surface focus events, the ETSE network is able to locate events to within 1 - 2 km of the true epicenter

Seismogenic zones in eastern Turkey

An edited version of this paper was published by the American Geophysical Union (AGU). Copyright 2003, AGU. See also: http://www.agu.org/pubs/crossref/2003.../2003GL018023.shtml; http://atlas.geo.cornell.edu/turkey/publications/Turkelli-et-al_2003.htmA 29-station temporary broadband PASSCAL network was operated from late October 1999 to August 2001 in eastern Turkey in order to decipher the geodynamics of one of the youngest continent-continent collision zones in the world. This paper focuses on the hypocentral distribution of local earthquakes located during the operation of the network and provides new insights into the active faulting in the Anatolian plateau. A total of 1165 earthquakes were located and classified into four different categories based on the reliability of the locations as established by the data coverage. The accuracy of the locations ranked in the best two categories is estimated to be less than approximately 5 km. The results show that seismic activity in Eastern Turkey is higher than previously documented and there were no subcrustal earthquakes beneath the Arabian- Eurasian collision zone or beneath the Anatolian plateau during our deployment. This result suggests no or very little underthrusting of the Arabian plate beneath Eurasia. Our results also suggest that the North Anatolian Fault zone extends farther toward the southeast, well beyond the Karliova triple junction, and that a number of unmapped active, seismogenic faults exist in the region. We also observed a possible difference in the seismogenic thickness of the East Anatolian fault zone (EAFZ) and the North Anatolian fault zone (NAFZ)

Erratum to Deep Earthquakes beneath the Northern Caucasus: Evidence of Active or Recent Subduction in Western Asia

In Mellors et al. (2012), Table 2 is missing in the print journal and was initially missing in the online edition of BSSA. The table is presented here. The online edition was corrected 29 March 2012. The production team regrets the error

New constraints on micro-seismicity and stress state in the western part of the North Anatolian Fault Zone: Observations from a dense seismic array

With the aim of extensively investigating the crustal structure beneath the western segment of the North Anatolian Fault Zone where it splays into northern and southern branches, a temporary seismic network (dense array for North Anatolia-DANA) consisting of 70 stations was deployed in early May 2012 and operated for 18 months in the Sakarya region during the FaultLab experiment. Out of 2437 events contaminated by explosions, we extracted 1371 well located earthquakes. The enhanced station coverage having a nominal station spacing of 7 km, lead to a minimum magnitude calculation of 0.1. Horizontal and vertical location uncertainties within the array do not exceed 0.8 km and 0.9 km, respectively. We observe considerable seismic activity along both branches of the fault where the depth of the seismogenic zone was mostly confined to 15 km. Using our current earthquake catalog we obtained a b-value of 1. We also mapped the b-value variation with depth and observed a gradual decrease. Furthermore, we determined the source parameters of 41 earthquakes with magnitudes greater than 1.8 using P-wave first motion polarity method. Regional Moment Tensor Inversion method was also applied to earthquakes with magnitudes greater than 3.0. Focal mechanism solutions confirm that Sakarya and its vicinity is stressed by a compressional regime showing a primarily oblique–slip motion character. Stress tensor analysis indicates that the maximum principal stress is aligned in WNW–ESE direction and the tensional axis is aligned in NNE–SSW direction