Underwater Signatures of the Kocaeli Earthquake of 17 August 1999 in Turkey

(17 Agustos 1999 Kocaeli Depreminin Deniz Tabani ve Guncel Cokeller Uzerindeki Etkileri)

Bedri Alpar

University of Istanbul, Institute of Marine Sciences and Management, Vefa, 34470, Istanbul, Turkey

Turkish J. Marine Sciences 5(3): 111-130 (1999)

Hosted Page Edited and reformatted by George Pararas-Carayannis (with author's permission )

Abstract

The fault rupture of the Kocaeli Earthquake (Mw 7.4, at 03:02 on August 17th, 1999) was well defined on land. In the present study, single-channel, high resolution seismic reflection data were used to characterize the underwater fault signatures of the Kocaeli Earthquake in the Izmit Bay and at the eastern Marmara Sea. Some personal opinions are also given about the speculations for a possible (<30 years) and devastating (>magnitude 8) earthquake in the Marmara Sea.

Key words: Marmara, Izmit Bay, earthquake, seismic exploration

Introduction

On August 17th, 1999 at 03:02 am, an earthquake with magnitude Mw 7.4 occurred on the North Anatolian Fault Zone (NAFZ) - one of the best known seismic, right lateral, strike-slip, faults in the World. Its epicenter was 11 km SE of Izmit, in the western part of Turkey . This event has been given different names by different researchers, such as the Kocaeli, Izmit, Golcuk or Marmara Earthquake. The quake's predominant focal mechanism showed a dextral transcurrent movement.

The Kocaeli Earthquake was the last one of a series of westward migrating earthquakes on the NAFZ, beginning with the 1939 Erzincan Earthquake in the eastern segment. Historical records show that similar migrations took place in the past. Field observations indicate that some parts of the fault rupture were under the sea. Along the Sapanca-Golcuk fault segment, many buildings along the coast slid down. Sea water inundated extensively the shore. Divers reported many underwater cracks along the fault line.

The Marmara Sea (278 x 80 km) forms a complex structure with a shelf break at a depth of about 110 m. Recent bathymetric data shows three central basins (1152 - 1276 m deep) separating the northern (2-13 km wide) and southern (32 km wide) shelves. Easternmost, Izmit Bay which occupies E-W position (53 x 2-10 km) forms a narrow depressional marine realm consisting of three main basins with two straits. The deepest part of Izmit Bay is 204 m.

The resultant suture after the Oligocene-collision of the Rhodop-Pontid Block with the Sakarya Zone is now largely replaced by the North Anatolian Fault Zone (NAFZ). The NAFZ is the most prominent active fault zone (30-80 km in length) in Turkey and has been the source of numerous large earthquakes throughout history. The NAFZ is a relatively simple, narrow, and right-lateral transform, across most of Turkey. It became active in the late Miocene-Pliocene period (Sengor and Yilmaz, 1991; Perincek, 1991; Barka, 1992; Okay and Tansel, 1992; Okay and Gorur, 1995). The NAFZ splays into three strands to the west at about 30.5E (Sengor et al., 1985). The northern strand passes through Izmit Bay, traverses the Marmara Sea and reaches the Gulf of Saros.

Marmara Sea Region

Several researchers believe that the Marmara Sea region is a graben structure (i.e. Crampin and Evans, 1986). To explain the structure of the northern strand of the NAFZ in the Marmara Sea, Barka and Kadinsky-Cade (1988), and Wong et al. (1995), have developed similar models, that consider right stepping en echelon, strike-slip, segments giving rise to small open basins and blocks - as pull-apart structures.
The Marmara Basin is segmented into a series of rhomboidal or wedge-shaped NE-SW oriented small sub-basins. The presence of these deep marine sub-basins in the modern Marmara Sea is mainly related to the dextral movement of the NAF along its northern strand.

Both strike-slip and pure normal faulting earthquakes occur in the region. These indicate that normal faulting plays an important role in the recent tectonic evolution of the Marmara Sea. The interaction between the NAF and the present N-S extensional tectonic regime of the Aegean, developed this complex basement which consists of various paleotectonic units (Ketin, 1948; Saltik, 1974; Brinkman, 1976; Sengor, 1979; Hancock and Barka, 1981; Sumengen et al., 1987; Senturk et al., 1987; Adatepe, 1988; Alpar, 1988; Siyako, et al., 1989; Akgun and Ergun, 1995; Sakinc et al., 1995; Ergun et al., 1995; Ergun and Ozel, 1995; Yaltirak, 1996; Barka, 1997).

The Marmara sub-basins are separated by structurally controlled saddles which rise about 600 m above their surroundings (Barka and Kadinsky-Cade, 1988; Smith et. al., 1995). The sub-basins are internally cut by numerous steeply dipping faults (Wong et al., 1995). The westernmost of the deep basins is the fault-bend Tekirdag Basin, which was filled by Pliocene-Quaternary, syn-transform sediments of over 2500 meters in thickness (Okay et al., 1998, 1999).

Such a morphology is the result of splaying of the NAF at its western end in Northwest Anatolia, where a N-S extension is found (Dewey and Sengor, 1979; Gorur et al., 1995). The deformation, caused by this N-S extension across the northern Marmara Sea, is taken up by a series of splays from the Cinarcik Fault which enters into the sea in the east. These splays are the features bounding the basins (Kuscu et al., 1999).

Izmit Bay

The basins of the Izmit Bay were filled with mainly continental siliclastic material resulting from fluvial and littoral processes. The average sedimentation rate was calculated as 20 cm / 1000 year with a maximum of 150 cm / 1000 year for the deepest parts (Ergin and Yoruk, 1990). Deltaic fans developed in front of the rivers where the currents and waves are weak (Alpar and Guneysu, 1999). The Plio-Quaternary deposits are 25-30 m thick and overlay the basement (Ozhan et al., 1985). This deposition is considered to take place under the influence of the NAF.

The tectonic basins in the Izmit Bay were created by the E-W compressional and N-S tensional forces which resulted as a response to the kinematical block displacements at active zones (Barka and Kadinsky-Cade 1988; Kurtulus, 1990; Barka, 1992; Wong, 1995). Recent faults affect the actual basin-fill deposits in the Izmit Bay, and form a negative flower structure (Sakinc and Bargu, 1989; Seymen, 1995) at the seabed. Barka and Kuscu (1996) describes the best model representing the structural geology of the Izmit Bay as a pull-apart model. According to their model, strike-slip fault segments, which are laterally (echelon) descending towards right, create the three small basins; western, central (Karamursel) and eastern (Izmit) basins. The grabens are considered to lie at overlapping sections of the faults which display an en-echelon pattern (Koral and Oncel, 1995; Senoz, 1998).

Consequently, the highly industrialized Marmara Region is under high earthquake risk. The main objectives of this paper are to describe the bottom and subbottom signatures of the Kocaeli Earthquake and to establish the near surface tectonic settings of the study areas, based on the analysis of high resolution subbottom, seismic, profiling data.

Material and Method

Following the Kocaeli Earthquake, single-channel, high-resolution, digital seismic reflection profiles, were acquired (Figure 1) using 1.25 kJ multi-electrode , spark array and a 11-element, 10-m-long , surface-towed hydrophone streamer. The seismic source is less than one meter in length, with 30 discharging electrodes (6 kV and 30 mF), spaced about 5 cm apart. Sampling interval was º ms, record window length was 250 ms (twtt) and shot interval was 2 sec (about 4.1 m). These parameters provided details on sedimentary deposits up to 150 m below the seabed. Positioning was carried out by using an integrated GPS with an accuracy of ± 20 m.

Conclusions

The fault rupture was well defined by many researchers on land to be from Golyaka to the Hersek Point. INCLUDE PICTURE \d "Bull2.gif" The rupture (Sapanca-Golcuk segment) enters the sea at the easternmost part of the Izmit Bay. The seismic profiles, recorded at the Basiskele area, show the ongoing tectonic deformation in the sea, with a well defined seismic pattern (Figure 2). Gas plumes can be seen in the water column coming from weak and discontinuous seismic reflections which may represent different depositional environments. When the NAF became active in the late Miocene, the Izmit Bay was representing a fluvio-lacustrine environment (Seymen, 1995; Meric, 1995). The faults in Izmit Bay cut different depositional environments (brackish-deltaic, continental, marine and shallow marine), developed under the effects of regional climatic fluctuations and active tectonism from late Pliocene to late Holocene. Due to the reduced recent, current velocities in the lower layer, the eastern and partly central sub-basins of Izmit Bay, are depositional areas for fine grained material which forms cap rock for the underlying, gas-charged, sediments.

It is well-known that the subaqueous sedimentary units in Izmit Bay are charged with gas and that the Holocene, post-trangressional, marine deposits, act as a cap layer over the gas-charged sediments. The gas-charged sediments are generally placed in the central parts of the basins, especially where the gulf becomes wider (personal communication with Mehmet Simsek). The gas could have been produced by bacterial fermentation reactions during the early diagenesis. These reactions produce large amounts of methane and carbon dioxide, as a result of microbial degradation of organic matter in the sediments. Following the Kocaeli Earthquake, many gas seepages have been observed on the seabed. The most prominent of them was about one km NW of Topcular (Figure 3), where significant gas bubbles were observed on the sea surface. During the research cruise and survey, the lateral distance between the blocks was measured to be as much as 5 m wide on the echo-sounder. The gas plumes are sometimes very thick and diverted from their vertical paths by the currents (Figure 4).

Tsunamis have been reported in the Marmara Sea and the surrounding area from antiquity to the present time (Soysal, 1985; Altinok and Ersoy, 1998). Large waves generated by an earthquake or a submarine landslide in the area can stike and inundate nearby coastal areas in a matter of minutes. During the Kocaeli Earthquake, a considerably damaging tsunami hit both the northern and southern sides of the bay in about one minute. Even the tsunami was not very large, subsidence and coastal landslides, have left a substantial portion of the towns of Golcuk, Degirmendere and Karamursel inundated by the sea (Altinok et al., 1999, this volume). Precise observation of tsunami runups is rather important in order to discriminate the most possible generating causes; tectonics, submarine landslides or other kind of subsidence events.

Submarine landslides, which often accompany large earthquakes, can disturb the overlying water column as sediment and rock slump downslope and are redistributed across the sea floor. Landslides disturb the sea surface by tanferring the momentum of the falling debris. However, slump-generated tsunamis dissipate quickly and rarely affect coastlines distant from the source area. Therefore, the Kocaeli Tsunami could not have been been simply a slump-generated tsunami. This is confirmed by our seismic data as well. There is not any apparent submarine landslides on the seismic sections, except a small slump close to the deepest part (see Fig. 2a in Altinok et al., 1999). So, one can say that the dextral segments extending in the sea floor should have contributed to the tsunami generation. When the Kocaeli Earthquake occurred beneath the sea and large areas of the sea floor subsided, then the water above the deformed area was displaced from its equilibrium position and the waves were formed from the displaced water mass. On the other hand, the subsidence events along the southern coasts (Kavakli, Degirmendere and Karamursel) are probably the result of the tectonic deformation along the shores (see also Altinok et al., 1999).

It is well-known that stress loading is largest at the two ends of the ruptured fault. According to the aftershock distribution, the western end of the fault is offshore of Esenkoy. There are some seabed ruptures on the seismic sections in that area (Figure 5). The fiber optic cable of Hesfibel between the Yalova and Yassiada has been broken and burnt 8 km offshore Yalova (personal communication with Cem Degirmenciler). These findings may explain why no ground ruptures were observed on land along the Yalova fault segment. These seabed ruptures may possibly be connected with the en echelon faults between the northern coasts of Imrali and Marmara islands, defining the northern edge of the southern shelf.

The Pendik Fault, with a strike slip character between the Tuzla Peninsula and the islands of Sedef and Balikci (Figure 6), is possibly caused by a historical big earthquake which occurred in the deep basins of the Marmara Sea. The Pendik Fault is possibly responsible for the cut of the telephone cable lying between Kartal and Canakkale, which was determined by Eginitis (1894), the director of the Observatory of Athens, who was invited by Sultan Abdulhamit (II) following the 1894 earthquake (Gundogdu, 1991).

The fault maps prepared by the MTA's or TPAO's scientists represent the mid-depth ranges (up to 5 km). From conventional seismic data, synthetic / antithetic faults, and flower structures within the sedimentary infill , are evident. In this depth range, the NAF zone splays into three (or four) strands. Barka and Kuscu (1996) suggested that there was a higher earthquake risk along the northern strand, based on the fact that the GPS slip rate was higher along the northern strand (from Izmit Bay to Saros Gulf) than that along the middle strand. Considering that the GPS observations establish a 24±4 mm/a deep, slip rate on the North Anatolian fault (Straub and Kahle, 1994, 1996, 1997), and also the right lateral displacement offset of the Kocaeli Earthquake to be 4.4 m, it can be calculated that the energy introduced by the Kocaeli Earthquake can be compensated in 180±40 years.


However, 6-7 km below the surface, the fault plane (not a zone anymore) is believed to be a single plain. This is not a new idea of the author! This fault plane (the North Anatolian Fault) occupies a certain position under the rhomboidal, deep, marine sub-basins in the modern Marmara Sea, and also under the deepest parts of the Saros Gulf, close to the northern margin, but not along the northern coasts of the Gelibolu Peninsula (Fig 10 in Yaltirak et al., in press). Therefore the Ganos Fault is one of the shallower branches of the North Anatolian Fault. The steep slopes along the northern margin of the Marmara Sea (Fig. 3 in Aksu et al., 1999) are the geomorphic evidences of this sight. In fact, there is literally no shelf area in front of the Ganos Mountain where very steep slopes continue down to >1000 m water depth. The Moho discontinuity in the Marmara region, which gets deeper from north to south ,as defined by Ozer et al., (1996) using azimuthal anomalies computed from the particle motion diagrams, may play an important role in this configuration.

The NAF or the segments of the NAFZ will evidently be triggered off in future. However, the author hardly believes that a single earthquake (Le Pichon, 1999) will rupture the fault from east (Hersek or Esenkoy) to west (Gazikoy, where the seismic energy discharged on August 9th, 1912; Ambraseys and Finkel, 1987) all along the Marmara Sea. It seems to be rather difficult, because there should be another old fault system cutting the Marmara Sea in a NW-SE direction (Yaltirak et al., 1998; Yaltirak et al., in press). This old fault system, which can be partly seen from the conventional oil seismic studies in the Ergene Basin (Thrace), underlies the present N-S extensional tectonic regime and possibly aged as 60 my B.P.

The old fault system, which should be chopped up internally by numerous faults, might form a natural barrier in the Marmara Sea and might prevent the NAF from breaking the Marmara segment at one time. This assumption can be tested with a piece of paper thorned in the middle. One can not tear such a piece of paper at an oblique angle from one end to other. Consequently, it may be considered that the rupture speed (about 3 km/s) can not overcome the remnants of the old NW-SE fault (or a half-graben ?) system, lying deep of the Marmara sub-basins.

On the other hand, such kind of sticking processes of the NAF to the old NW-SE fault underneath the Marmara deeps, may cause some seismic activities at the coastal sections of the old NW-SE fault (especially along the Marmara coasts of the Thrace - such as Ambarli and Yesilkoy). Offshore, part of the Halkali (or Ayamama) River (which is placed in a valley caused by the old fault or graben system), is another place where gas plumes were observed on the seismic sections following the Kocaeli Earthquake (Figure 7). The author believes that such gas plumes were caused by stress transfer of the Kocaeli Earthquake on the old NW-SE fault. Additional offshore studies will refine details of the underwater, geological effects and seismological parameters of this major earthquake.

Acknowledgements

This work would not be carried out without the financial support of Prof. Dr. Ertugrul Dogan who made everything out of nothing. I wish to express my sincere thanks to Assoc. Prof. Dr. Yildiz Altiok for her proposals based on the unusual geomorphic feature of the Balikci island; to Assoc. Prof. Dr. Berkan Ecevitoglu for his technical hints on data acquisition; to Dr. Oguz Gundogdu for his pessimism about coming seismic hazards which caused us to increase the number of seismic lines; to Mr. Sahin Akkargan for his on land work expressing dignity; to Col. Nazim Cubukcu for his historical data support; to Mr. Mehmet Simsek for his comments about the gas charged sediments in the Izmit Bay before the Kocaeli Earthquake; to Mr. Sabri Bal for his operational and technical support; and to the officers and crew of the research vessel RV Arar for their guidance, comments and assistance in data acquisition.

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Received 22.11.1999
Accepted 18.12.1999

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