Eastern Turkey Seismic Experiment Report from the IRIS Newsletter

Eric Sandvol, Dogan Seber, and Muawia Barazangi, Cornell University, Ithaca, New York

Niyazi Türkelli, Cemil Gürbüz, Sadi Kuleli, Hayrullah Karabulut, Ekrem Zor, Rengin Gök, Tolga Bekler, and Esen Arpat, Kandilli Observatory and Earthquake Research Institute, Bogaziçi University, Istanbul, Turkey

Salih Bayraktutan, Atatürk University, Erzurum, Turkey

Eastern Turkey has for thousands of years been a region of convergence for European, Asian, Persian, and Arabic peoples; a region where eastern and western cultures come together. Similarly the tectonics of Turkey can be described as a place of convergence or divergence of three continental plates. The Anatolian sub-plate, the Arabian plate, and Eurasian plate form a diverse suite of tectonic boundaries. Collision of the Arabian and Eurasian plates is currently occurring in Eastern Turkey along the East Anatolian Fault Zone (EAFZ) and the Bitlis suture.

The Bitlis suture/thrust zone and the EAFZ mark a distributed, irregular, and young continental collision zone (Figure 1). This collision zone is further characterized by a 1500 meter topographic high (Anatolian plateau) with corresponding seismicity indicating that this region is still actively deforming. Tectonic escape is taking place to the west, as evidenced by the right-lateral strike-slip movement along the North Anatolian Fault Zone (NAFZ) (e.g. Reilinger et al., 1997) and left lateral movement along the EAFZ (McKenzie, 1972). The slip along the NAFZ is approximately 2 cm/yr while the slip along the EAFZ is 1 cm/yr. This indicates that the escaping Anatolian block is rotating counter-clockwise (Figure 2). This rate of westward slip does not account entirely for the strain induced by the 3.0 cm/yr convergence of the Arabian plate with respect to Eurasia (Dewey et al., 1986; Reilinger et al., 1997); therefore, this deformation must be accommodated elsewhere. The North and East Anatolian faults have been active since the Miocene and are associated with large pull-apart basins, such as the Karliova Basin located at the junction of these two fault systems. GPS results from Reilinger et al. (1997) indicate that the crust which lies to the north and east of the Karliova junction is deforming very differently from the crust to the west (Figure 2). In the east there appears to be shortening of the continental crust about 150 km north of the Bitlis Suture, while in the west escape tectonics seem to dominate. There is no consensus on precisely when collision between the Eurasian and Arabian plates began. The age estimates of collision range from 12 Ma, based on stratigraphic discontinuities in Eastern Anatolia (Sengor and Yilmaz, 1981) and the beginning of collision related volcanism (Pearce et al., 1990), to 20 Ma based on the convergence rate of the two plates (Dewey et al., 1986).

The East Anatolian plateau and Bitlis suture offer a unique and excellent opportunity to understand the early stages of continental collision and its consequences: this understanding is also essential to develop better models for the later stages of this process. Until now, the only well studied example for active continental collision has been that of India colliding with Eurasia and the subsequent uplift of the Tibetan plateau. In many respects the East Anatolian topographic uplift can be thought of as a younger version of the Tibetan plateau (e.g., Sengor, 1979; Dewey et al., 1986; Barazangi, 1989). In order to characterize the nature of this collision zone and determine the geodynamics of the collision we must resolve whether: (a) continental subduction or delamination is ongoing (Rotstein and Kafka, 1982), (b) the Arabian plate convergence is accommodated entirely by an escaping Anatolian plate (McKenzie, 1972), (c) there is an Eurasian lithospheric thickening (Dewey et al., 1986), or (d) a combination of these processes is taking place.

In order to address these important questions, we have deployed a temporary 29 station broadband PASSCAL array in eastern Turkey (Figure 3). We designed our array to greatly improve our understanding of the Bitlis/Zagros thrust zones as well as the nature of continental escape along the EAFZ and NAFZ, imaging upper mantle and crustal structure in these two fairly different tectonic environments. The average station separation is approximately 50 km for the western line of stations and 30 km for the eastern line. Our western traverse crosses a region where it has been well documented that the Anatolian block is escaping westward (e.g., Reilinger et al., 1997). Along our eastern array, recent GPS results indicate that the crust is being shortened.

This project is a collaborative, joint research between Turkish and Cornell researchers. Scientists from Kandilli Observatory and Earthquake Research Institute (KOERI), Atatürk University, and Cornell University began site selection in August 1999 and were able to locate safe and stable sites for almost all of the stations. Local government officials provided very valuable logistical support. We chose sites that allow for relatively easy station access and offer security from vandals and curious children (the latter evident in Figure 4). In many cases we installed seismometers within government establishments. At four of the stations we arranged for local residents to make sure that the station would not be disturbed. All station sites were ready for deployment in mid-September 1999.

Luckily we had excellent weather for late October and early November on the Anatolian Plateau and using two teams consisting of three to four people each (including two PASSCAL engineers) were able to deploy two stations a day. The entire array was installed in approximately 2 weeks (Figure 4). Most of our stations powered by solar panels, however, we were able to arrange continuous AC power at six of the twenty nine station sites. These six sites are located on the Anatolian Plateau and experience rough winters. In order to ensure that all the stations were operating properly, we visited each station two to three weeks after installation in November 1999. Most were working well, although we did encounter a few problems with solar panel power at the easternmost end of the array. Using Linux laptops we were able to do all quality control in the field, which is essential given that our field computer and headquarters are located 800 km away in Istanbul.

In order to ensure maximum data recovery of local, regional, and teleseismic waveforms, we are recording 24 bit broadband data continuously at 40 sps (samples per second). Data recorded at 40 sps should provide high enough resolution for accurately locating local events as well as analyzing regional wave propagation. Using PASSCAL's 4.1 GByte field disks and the REFTEK 72A's data compression algorithm, we plan to visit stations every 3 to 4 months. All of the stations should be accessible throughout the year since we deployed them within a kilometer of major roads.

The data recorded by this array will greatly improve our knowledge of seismicity in this region by revealing seismicity that would otherwise go undetected. The data we collect will also provide a much improved seismic velocity model with which we can more accurately locate earthquake hypocenters. A more complete catalog of seismicity in Eastern Anatolia will provide crucial information on national earthquake hazards as well as improve our understanding of the current deformational patterns. The lack of broadband instrumentation in the northern part of the Middle East makes this data set very important to understanding seismic hazard and risk along the eastern portion of the NAFZ. Furthermore, our stations will record the very frequent seismicity of the Middle East and Eastern Mediterranean. We also plan to use this extensive regional data set to improve the current regional wave propagation and Pn velocity models (Hearn and Ni, 1994; Sandvol et al., 2000) in the northern portion of the Middle East.

Preliminary receiver functions for our stations located along the Bitlis suture suggest that there may be evidence for a crustal root beneath the Anatolian Plateau. Moreover, initial analysis of shear wave splitting shows that there is no obvious change in mantle azimuthal anisotropy across the EAFZ (Figure 5); further data and analysis will reveal if this is the case for the eastern portion of the Bitlis Suture as well. In the long term, data processed from this project will help to address some fundamental questions about the extent and mechanism of this continental deformation and escape.

Given the lack of geophysical measurements in this region, determining the optimal geodynamic model cannot be treated as a simple binary hypothesis test. Thus it will be essential to integrate all of our seismological results with the large number of geologic studies that have been conducted in this region (geochemical, geochronological, GPS, etc.), creating a single database. This data integration will allow us to choose the most reliable tectonic and geodynamic model of this active continental collision zone. Cornell's Middle East and North Africa GIS databases (see web site: http://atlas.geo.cornell.edu) provide an excellent starting point and mechanism for this data synthesis and integration. We plan to create an East Anatolian GIS database and make it available via a special web site. These results will constitute the first major broadband seismological experiment in the Bitlis-Zagros thrust and mountain belt and should help enormously in our understanding of the continent- continent collisional processes.

Acknowledgements

This experiment could not have happened without the enormous help provided by the federal and local government officials in Turkey. We thank the President of Bogazici University, Prof. Ustun Erguder, for his help during the initiation of this project. We also thank Mustafa Aktar at KOERI for help and encouragement, and PASSCAL engineers Bruce Beaudoin and Tom Jackson for their hard work and helpful advice. Special thanks are due to the local residents of eastern Turkey who helped in site selection and installation of this array. This research is supported by the National Science Foundation, Geophysics Program (Grant No. EAR-9804780). Additional support is also provided for the Turkish researchers by a Bogazici University Research Fund (Grant No. 99T206).

References:

Barazangi, M., Continental collision zones: Seismotectonics and crustal structure, in Encyclopedia of Solid Earth Geophysics, edited by David James, Van Nostrand Reinhold Company, New York, 58-75, 1989.

Dewey, J. F.,Hempton, M.R., Kidd, W.S.F., and Sengor, A.M.C., Shortening of continental lithosphere; the neotectonics of eastern Anatolia, a young collision zone." edited by Coward, M. P., et al. Geological Society Special Publications; 19, 3-36, 1986.

Hearn, T. and Ni, J., Pn Velocities Beneath Continental Collision Zones: the Turkish-Iranian Plateau, Geophys. J. Int., 117, 273-283, 1994.

McKenzie, D., Active tectonics of the Mediterranean, Geophys. J. R. Astr. Soc., 30 (2), 109-185, 1972.

Pearce, J. A., Bender, J.F.,De Long, S.E.,Kidd, W.S.F.,Low, P.J.,Guner, Y.,Saroglu, F., Yilmaz, Y., Moorbath, S., and Mitchell J.G., Genesis of collision volcanism in Eastern Anatolia, Turkey, Journal of Volcanology and Geothermal Research, 44, 189-229, 1990.

Reilinger, R., McClusky, S., Oral., B., King, R., Toksoz, N., Barka, A., Kinik, I., Lenk, O., and Sanli, I., Global positioning system measurements of present-day crustal movements in the Arabia-Africa-Eurasia plate collision zone, Journ. Geophys. Res., 102, 9,983-9,9,999, 1997.

Rotstein, Y. and Kafka, A.L., Seismotectonics of the southern boundary of Anatolia, eastern Mediterranean region: subduction, collision, and arc jumping, Journ. Geophys. Res., 87, 7694-7706, 1982.

Seber, D., Vallve, M., Sandvol, E., Steer, D., and Barazangi, M., Middle East Tectonics: Applications of Geographic Information Systems (GIS), GSA Today, 7, 1-6, 1997.

Sandvol, E., Al Damegh, K., Calvert, A., Seber, D., Barazangi, M., Mohammed, R., Turkelli, N., and Gok, R., Tomographic Imaging of Lg and Sn Propagation in the Middle East, PAGEOPH, in press, 2000.

Sengor, A.M.C., Northern Anatolian fault: its age offset and tectonic significance, J. Geol. Soc. London, 136, 269-282, 1979.

Sengor A.M.C., and Yilmaz, Y., Tethyan evolution of Turkey: a plate tectonic approach, Tectonophysics, 75, 181-241, 1981.