Atilla Uluğ

THE SEA OF MARMARA: A PLATE BOUNDARY SEA IN AN ESCAPE TECTONIC REGIME

Abstract The Sea of Marmara is dissected by two major fault systems. The first consists of two east-west-striking, transtensional boundary faults and a number of secondary faults subparallel to them. The second is made up of NE-SW-trending, subvertical strike-slips and their conjugates oriented NW-SE that offset the first system. These fault systems segment the deep Marmara Sea into 5 blocks that are either rhomboidal, lazy-Z or wedge-shaped. Three of these blocks may be interpreted as pull-apart basins characterized by transtension, while the other two (sill areas) which separate the basins are transpressional push-up structures. The blocks are subjected to rapid, episodic subsidence, but they also undergo pervasive vertical motions and possible rotations relative to one another. The sedimentary column above the acoustic basement mapped within the Sea of Marmara can be divided into 3 seismic units: a folded and truncated pre-transform unit and two syn-transform units. The lower, thick syn-transform unit exhibits 3 distinct seismic facies: a well-stratified basin-fill facies cut by subvertical growth faults, a push-up facies with a contorted or chaotic internal configuration and a slump facies. The observed neotectonic and sedimentary regimes result from compressional movement between Eurasia and Africa, which has led to major dextral transform movements along the North Anatolian Fault Zone (“escape” tectonics), and from the fact that this fault zone splinters into two overlapping, right-stepping, oblique master faults at the eastern and western border of the Sea of Marmara respectively.

Quaternary sedimentary history of Adana, Cilicia and Iskenderun basins: northeast Mediterranean Sea

Detailed single-channel airgun and 3.5 kHz profiles from the northeastern Mediterranean Sea show that the continental shelf is formed by superimposed deltaic successions (depositional sequences), separated by major erosional unconformities. Each depositional sequence is composed of a sigmoid prograding package overlain by an oblique prograding package, representing respectively delta progradation during high and low sea levels of interglacial and subsequent glacial stages. During the glacio-eustatic lowstands of sea level, deltas prograded seaward. The present-day shelf break denotes the topset to forest transition at maximum progradation during the last glacial period. During post-glacial transgressions, the deltas initially lost their dynamic equilibrium with the environment and rapidly retreated landward, leading to the deposition of a thin veneer of sediments originating from reworking of formerly coastal sediments. At the highstand position the deltas were re-established in the ancestral Adana Bay and foreset progradation started. Chronology suggests that the Cilicia and Iskenderum Basins are subsiding at rates of 0.38 m 1000 yr−1 and 0.33 m 1000 yr−1, respectively. Seismic reflection profiling shows that delta architecture in the Adana, Cilicia and Iskenderun Basins is mainly controlled by glacio-eustatic sea-level fluctuations and continuous basin subsidence.

Evaluation of seismic hazard of the Gökova bay in terms of seismotectonics

While discovering the seismicity of our country, knowing the array of earthquake occurrence which reflects the characteristic tectonic features of each region makes vital contributions to the earthquakes that have occurred and to the pursuit of the processes which might occur in the future.When considering the region’s seismic activity, the presence of active faults that create earthquake within the bay is obvious. Many active fault parts in the Gulf of Gokova region continues their seismic activity with the opening effect that is generally prevailing in Western Anatolia. The region has generally been continuing its seismic activity under the control of normal faults. Considering the marine studies that are made and marine continuity of the faults which are on land in addition to the seismological and tectonic studies, the determination of seismic hazard in the Gulf of Gokova and its surroundings is also important in terms of introducing the earthquake scenarios with minimized errors.

Relationship Between Quasi Transfer Spectrums and Dynamic Amplification Factor

Quasi Transfer Spectrums (QTS) and Dynamic soil amplification factor defining which ratio earthquake acceleration will reach the soil surface by changing is one of the most important factors in seismic risk studies. When computing the value of DAF at a point without a strong motion station, peak horizontal acceleration values at the bedrock and soil transfer function are needed. PGA value at the bedrock can be obtained by using either real seismic records or the earthquake scenario. However, the soil transfer function can be computed observationally and theoretically. Observational soil transfer function is defined by microtremor horizontal/vertical spectral ratio. In case of theoretical computation, the density belonging to the soil layers between the bedrock and the soil surface is used together with the change of P-S wave rates with the depth and the damping factor. In this study, the dynamic amplification factor has been computed for 57 points by using observational QTS obtained by microtremor horizontal/ vertical spectral ratio as well as the earthquake scenario. Also, theoretical soil transfer function at 1 point was obtained through spatial autocorrelation method study and determined to be compatible with observational result.

The Evaluation of Shallow Seismic Studies in Izmit Bay and the Relationship with the North Anatolian Fault

Abstract Turkey was greatly affected by the earthquake disaster of Izmit, a 7.4–moment magnitude earthquake that occurred in the southeast Marmara region on August 17, 1999. A shallow seismic survey using a 0.65-lt air-gun was carried out in September 1999. The results of this investigation were compared with the conclusions obtained from seismic studies conducted along the same profiles in 1994 (using a 0.65-lt air-gun). The main objective of this research was to investigate structural and tectonic differences. Izmit Bay is seismically under the control of the North Anatolian Fault. This regime developed within the former pull-apart structures. Seismic surveys reveal the presence of intensive gas formation in the Izmit Basin. The gas formation was overlaid by sediments in some areas of the basin and migrated from active fault fractures through the sea bottom. The penetration depth is rather shallow due to the acoustic masking effect of the gas formation. It is believed that the fractures caused by the 1999 Izmit earthquake will be reactivated along the existing tracks of the active fractures of the previous periods. It is accepted that the North Anatolian Fault was a single, deep, dextral strike-slip fault. This fault cuts the former pull-apart basins, which are the residuals of the former Thrace Basin. Occasionally, the Izmit segment of the North Anatolian Fault constitutes flower structures using some of the faults of the former pull-apart structures. It is suggested that this fracture, which was constituted with these alterations, continues toward the town of Ulaşli.

Transition of the North Anatolian Fault Zone (NAFZ) in the Sea of Marmara

The North Anatolian Fault Zone (NAFZ), with a total length of ca. 1500 km, is one of the worlds important active strike-slip fault. It is a seismically active right-lateral transform, which takes up the relative motion between the Anatolian and Eurasian plates. This fault zone became first active in the Late Miocene to Pliocene and its lateral displacement is about 20 mm/year, and it consists along most of its length essentially of a single fault; however it splits first into two branches near Bolu and then into three branches near Adapazan-Izmit region. In order to determine the situation of the NAFZ in the Sea of Marmara some geophysical studies were carried out. Over 1670 km long of airgun seismic and magnetic profiles were run in the Sea of Marmara in 1989, and over 300 km long of seismic profiles in the Bay of Izmit in 1994 with using R/V K. Piri Reis of Dokuz Eylul University (Izmir). As a result of the seismic studies the Sea of Marmara is dissected by two major fault systems. One of the first system consists of E-W striking, transtensional boundary faults and a number of secondary fault system subparallel to them. The second system showing NE-SW trending subvertical strike-slips, and their conjugates oriented NW-SE direction. According to our data all these fault systems segment the deep part of the Sea of Marmara into 5 tectonic blocks. Three of these 5 blocks are interpreted as pull-apart basins characterized by transtension. They are separated by trans-pressional push-up structures. However, the new data obtained from recent studies in the Sea of Marmara are interpreted that the northern branch of the NAF is going mostly as a single dextral strike slip fault system across the Sea of Marmara.