The North Anatolian fault zone (NAFZ) is one of the Worldfs most important active strike-slip faults, not only because of its remarkable seismic activity between 1939 and 1967, but also its significance for the tectonics of the Eastern Mediterranean region. The NAFZ is the northern boundary of the westward moving Anatolian block and connects the compress ional regime in the Aegean Sea region. The age and total slip of the NAFZ are very important for the tectonic history of the Eastern Mediterranean, and for the tectonic history of the Eastern Mediterranean, and for the better understanding of the geometry, mechanism and evolution of the continental deformation in this region. The fault zone is about 1500 km long, extending from the Karhova trip junction in eastern Turkey to mainland Greece. It consists of mainly a single strand between the Karhova triple junction to the Mudurnu Valley, then plays into three strands in the Marmara and North Aegean regions. Its right-lateral motion is clearly expressed by an abundance of physiographic and geologic features.
Between 1939 and 1967 the fault zone had a remarkable earthquake activity during which six westward migrating large earthquakes created a 900 km lung continuous surface rupture along the fault zone from Erzincan to the western end of Mudurnu Valley, such as 1939 Erzincan (M = 7.9), 1942 Erbaa-Niksar (M = 7.3), 1943 Tosya-llgaz (M = 7.3), 1944 Bolu-Gerede (M = 7.3), 1957 Abant (M = 7.1), 1967 Mudurnu Valley (M = 7.1). The last destructive earthquake on the NAFZ is 1999 İzmit (Mw = 7.4). On August 17, 1999, a major earthquake occurred in the Marmara region, western Turkey, with a magnitude Mw = 7.4 at 40 70f N, 29 98f E and 17 km depth. This was the biggest earthquake in this area since 1967 Mudurnu Valley earthquake of Ms = 7.1. More than 15,000 people died and most of the buildings near the fault rupture were heavily damaged and collapsed. The maximum damage occurred in the towns of Izmit and Golcuk. Faults also cut Istanbul-Ankara highway and caused heavy damages on the viaducts and bridges. Surface rupture was observed for about 120km extending east from the Gulf of Izmit to near Duzce. The surface displacements varies from 1.0 to 5.0 m. Aftershocks following the Izmit earthquake were located by using data from the MARNET seismic network operated by Kandilli Observatory and Earthquake Research Institute. As of Nov. 21 we have recorded thousands of aftershocks, of which 9 are magnitude > 5 and arc 57 are magnitude > 4. Most of the aftershock activity is confined to the region bounded by 40.50f - 41.00f N and 29.00f - 31.50f E, the area from Cınarcık to Bolu. Aftershocks were concentrated: (1) within the Gulf Region from Cınarcık to Izmit:, (2) from Golcuk to Lake Sapanea: and (3) from Adapazarı to Düzce. The aftershock distribution suggests that at least 200 km of the fault was ruptured by this earthquake and the following aftershocks. The aftershock activity in the region from Yalova - Hersek to the southwest of Princess islands, along the Cınarcık basen, showed that the rupture reached to the south of Princess islands. Alignment of the aftershocks within the Marmara Sea region indicates that at least two branches have been seismically active. The November 12 Düzce earthquake occurred with a magnitude 7.2 at the depth of 11 km after nearly 2.5 months of the Izmit Earthquake. Its epicenter was on the Düzce fault extending between Gölkaya and Kaynaslı 41km long surface rupture from Golkaya to Asarsu have been mapped. Right lateral offsets reaching up to 4m in Ovapınar have been observed.
The other well-known seismically-active fault system is the EAFZ, which is an active left-lateral strike slip fault forming the boundary between the Anatolian Block to the northwest and the Arabian - African plates to the southeast. The EAFZ runs in the NE-SW direction and is approximately 650 km long and 1-30 km wide. It connects the NAFZ to the Dead Sea - Read Sea Fault System and extends its southwestern terminus to the immediate east of Cyprus. The Kozan, Savrun, Goksu and Surgu faults are considered to be the southwestern extension of the EAFZ. Major earthquakes on the EAFZ are 1905 Malatya (M = 6.8), 1971 Bingöl (M = 5.9). Bitlis Convergene zone in southeastern Turkey has also produced lots of destructive earthquakes. Historical data suggest that this area was very active during the past 2000 years. During the century 1975 Lice (M = 6.6) event occurred.
The Cyprean Arc encompasses a wide and complex zone of oceanic subduction along which the African Plate subducts under the Turkish (Anatolian) Block. The region accounts for both shallow and deep-focus earthquake activity. Numerous earthquakes in the region have focal depths exceeding 100km. The EAFZ and the Hellenic and Cyprean arcs represent the boundary between the African and Anatolian plates in the Eastern Mediterranean region (Mc Kenzie ; Ben- Avraham et al. )
Western Turkey and its surrounding area
is also a seismically very active part of the country. The Turkish plate moves
westward from Karitova junction. Interaction of this motion and the subduction
of the Mediterranean lithosphere beneath the Turkish plate cause a N-S extension
and E-W shortening in western Turkey. As the region is under extension in NNE-SSW
direction. Aegean grahcn system consists of several grabcns
and horsts bounded by oblique E-W trending normal faults. This direction
coincides with directions of T axis obtained from focal plane solutions of
some well-known earthquakes that occurred in this
century. From north to west, these grabens are called below:
(a) Edremit Bay, (b) Bakırçay-Simav, (c) Gediz- Küçük Menderes, (d) Büyük Menderes (e) Gökova Bay. Seismic activity of this area is mainly associated with these systems. Fault plane Solutions of destructive earthquakes show dominantly normal faulting such as 1969 Alasehir (M = 6.9), Gediz (M = 7.3) events etc.
Seismicity in the Marmara region is the result of tectonic movements along two possible westward extensions of the NAFZ beyond the Mudurnu Valley where the influence of the Aegean extensional tectonic regime has been recognized (Barka ). At this point it divides into two strands. The northern strand called lzmit-Sapanca fault extends from Sapanca Lake through the northern part of the Armutlu Peninsula Toward inside the Marmara Sea. Here, it makes some steps forming troughs like a kind of pull-apart basin; it appears again on the land near Murefte, continuing along the Saros Bay and then enters the Aegean Sea. The southern branch called Iznik-Mekece fault runs from Geyve through Mekece and passing south of Iznik Lake to Gemlik Bay. It goes into the Marmara Sea, appearing near the Bandırma Bay and cutting the Kapıdağ Peninsula continues in the Biga Peninsula and then enters the Agean Sea. The number of earthquakes identified in this region for the historical period is around 600. Thirty-eight of them are estimated to be relatively large shocks of magnitude Ms > 7.0 (Ambraseys ). For the instrumental period (after 900), earthquake activity in the Marmara Sea region shows typical swarm-type activities.
Ambraseys N. N. and C. Finkel, 1991. Long-Term Seismicity of Istanbul and of the Marmara Sea Region Engineering Seismology and Earthquake Engineering ort 91/8, imperial College of Science and technology.
Barka A.A., Toksöz N.M., Gülen L. and Kadınsky-Cade K.: The structure, seismicity and earthquake potential of the eastern part of the North Anatolian fault one. Spec. Hacettepe Univ. Ankara, Turkey, 14, 337-352, 1988.
McKenzie, D.P., 1972. Active Tectonics of the Mediterranean Region. Nature, 226,239-243.
Ben-Avraham, Z., Kempler, D., and Ginzburg, A., 1988. Plate convergence in the Cyprean arc Tectonophysics, 146,231-240
Data source (Country report)
1. Name: Kivanc KEKOVALI
2. Organization: Geophysical Engineer, Kandilli Observatory and Earthquake Reaserch Institute, Bogazici University
3. Course: 2000 Gs
4. Title: SEISMIC OBSERVATION IN TURKEY.