Earthquake scarps associated with recent historical events have been found on the floor of the Sea of Marmara, along the North Anatolian Fault (NAF). The MARMARASCARPS cruise using an unmanned submersible (ROV) provides direct observations to study the fine-scale morphology and geology of those scarps, their distribution, and geometry.

The observations are consistent with the diversity of fault mechanisms and the fault segmentation within the north Marmara extensional step-over, between the strike-slip Ganos and Izmit faults. Smaller strike-slip segments and pull-apart basins alternate within the main step-over, commonly combining strike-slip and extension. Rapid sedimentation rates of 1-3 mm/yr appear to compete with normal faulting components of up to 6 mm/yr at the pull-apart margins. In spite of the fast sedimentation rates the submarine scarps are preserved and accumulate relief. Sets of youthful earthquake scarps extend offshore from the Ganos and Izmit faults on land into the Sea of Marmara.

Our observations suggest that they correspond to the submarine ruptures of the 1999 Izmit (Mw 7.4) and the 1912 Ganos (Ms 7.4) earthquakes. While the 1999 rupture ends at the immediate eastern entrance of the extensional Cinarcik Basin, the 1912 rupture appears to have crossed the Ganos restraining bend into the Sea of Marmara floor for 60 km with a right-lateral slip of 5 m, ending in the Central Basin step-over. From the Gulf of Saros to Marmara the total 1912 rupture length is probably about 140 km, not 50 km as previously thought. The direct observations of submarine scarps in Marmara are critical to defining barriers that have arrested past earthquakes as well as defining a possible segmentation of the contemporary state of loading. Incorporating the submarine scarp evidence modifies substantially our understanding of the current state of loading along the NAF next to Istanbul. Coulomb stress modeling shows a zone of maximum loading with at least 4-5 m of slip deficit encompassing the strike-slip segment 70 km long between the Cinarcik and Central Basins. That segment alone would be capable of generating a large-magnitude earthquake (Mw 7.2). Other segments in Marmara appear less loaded.

Figure 1. The Sea of Marmara pull-apart and the submarine breaks found with ROV Victor 6000 (map redrawn from Armijo et al. [2002]). The NAF splays into a main northern branch (N) and a southern branch (S) that bypasses the pull-apart and accommodates much less motion. The North Marmara Basin is located by the step-over between the strike-slip Ganos and Izmit faults. The boxed area including the North Marmara Basin is enlarged in Figure 2, where names of smaller basins and step-overs are defined. Recent earthquake breaks and fault plane solutions of large earthquakes are outlined (fault plane solution for the 18 September 1963 event from Taymaz et al. [1991]; those for the 1999 events are from the USGS catalog). Submarine breaks found during the MARMARASCARPS cruise appear associated with recent historical earthquake ruptures (outlined with colors and labeled by date of occurrence): (1) the eastern extension of the 1912 (Ms 7.4) Ganos earthquake; (2) the western end of the 1999 (Mw 7.4) Izmit earthquake; (3) the 1963 (Ms 6.4) Yalova earthquake; (4) the 1894 (M 7) Cinarcik earthquake. However, no young break was found between the Central Basin and the Cinarcik Basin (names labeled in Figure 2). Geologic markers (dashed blue) are those used for restoring slip in Marmara as discussed by Armijo et al. [1999]. The inset shows the tectonic framework. NAF, North Anatolian Fault; EAF, East Anatolian fault; DSF, Dead Sea Fault; NAT, North Aegean Trough. GPS velocity vectors (in red, referenced to a fixed Eurasia) from McClusky et al. [2000].

Acknowledgments

We wish to dedicate this work to the memory of our friend Aykut Barka, who was always present in our minds during the course of this extraordinary adventure. The MARMARASCARPS cruise was performed within the framework of the collaborative program on the seismic risk in the Istanbul and Sea of Marmara region coordinated by the Turkish TUBITAK and the French INSU-CNRS, with support from the French Ministry of Foreign Affairs (MAE). Crucial for this cruise was the enthusiastic and always very positive help provided by Philippe Vidal from INSU. Yves Gallet is also acknowledged for affirmative help to the program. Naci Görür from TUBITAK must be thanked for coordinating the effort in Turkey. Rear Admiral of the Turkish Navy Nazim Çubukçu from SHOD has kindly provided all the necessary support for navigating safely in the Sea of Marmara. S. Nahirci and E. Hacioglu contributed to that purpose aboard Atalante. We wish to thank warmly captain Alix and all the crew of Atalante, as well as the team of pilots of the ROV Victor 6000, led efficiently by A. Christophe. The IFREMER was specifically associated in this venture for this first very extensive survey using experimentally the multibeam microbathymetry facility mounted on the ROV Victor 6000. Vincent Rigaud from DNIS/IFREMER has taken the responsibility of this technical issue. Very special thanks go to the team led by Jan Opderbecke, who successfully installed and courageously operated the microbathymetry facility on board: Anne-Gaëlle Vincent, Henri Martinossi, and Toussaint Edmond. Geoff King and Steve Wesnousky are acknowledged for stimulating constructive comments. Jordan Muller and an anonymous reviewer made thorough reviews. Celal Sengör is also thanked. This is Institut de Physique du Globe de Paris (IPGP) paper 2044. INSU paper 380.