GNGTS 2019 - Atti del 38° Convegno Nazionale

144 GNGTS 2019 S essione 1.2 to the east, and the extensional regime affecting most of the Aegean Sea to the west and south. The Sea of Marmara includes three ~1250 m deep basins that accommodate the splaying of the NAF system into several strands (Fig. 1). A remarkably good record of historical earthquakes in the region documents the intense historical seismicity of the NAF system (Ambraseys and Finkel, 1995). A sequence of eight M>7 earthquakes progressively ruptured this boundary from east to west during the last century. The most recent and westernmost events in this sequence, the M=7.4 İzmit and M=7.1 Düzce mainshocks of 1999, were particularly destructive, since they ruptured about 160 km of this fault system, including the entire submarine portion in the Gulf of İzmit, in the eastern Sea of Marmara (Gasperini et al 2010). However, little strain has been apparently released since the mid 1700’s by earthquakes along the 150 km of the transform through the Sea of Marmara. The segment of the NAF connecting the Gulf of Saros and the western Sea of Marmara (Fig. 1) ruptured in 1912. The remaining submarine portion of the NAF system therefore constitutes a seismic gap, where the accumulated elastic strain is at least as large as that released by slip in the 1999 sequence (Reilinger et al. , 2000; Hubert- Ferrari et al. , 2000). Different earthquake scenarios can be described on how the strain gap in the Marmara Sea between the 1912 and 1999 ruptures will be filled, with different implications for the hazard facing the circum-Marmara region; a good understanding of the seismogenic behaviour of the submarine strands of the NAF system is thus critical. Methods. Estimates of seismic hazards around the Sea of Marmara require a multiscale and multidisciplinary approaches, which should address a number of topics. They include: identification of the active faults; association of these faults with specific historical earthquakes; dating their most recent ruptures at the same scale as typical paleoseismic studies on land; estimating slip-rates over specific faults strands, and compare with typical earthquake and recurrence using historical catalogs; deciphering the effect of major events in the sedimentary sequence. We have combined high-resolution multibeam and side-scan sonar maps with single- and multichannel seismic reflection profiles to study seismogenic structures offshore, while ground-truthing geophysical observations with carefully positioned core samples. Our approach was to resolve the shallow geometry and kinematics of the fault system in the Sea of Marmara in the frame of a complex geodynamic evolution, and to study geological effects of individual and multiple earthquakes. To this purpose, we attempted to detect the effects of the fault motion along the rupture plane, and secondary features developed by shaking, such as landslides in the vicinity of the fault. As study areas, we selected the relatively shallow water embayments along the NAF system, the gulfs of İzmit and Gemlik, in Marmara, and the Gulf of Saros in the NE Aegean (Fig. 1). All these areas include relatively narrow strike-slip deformation zones connecting sectors affected by more diffuse deformation patterns, where transtensive and transpressive deformations replace strike-slip tectonics. Results and discussion. Figure 1 displays a neotectonic map of the NAF below the Sea of Marmara obtained by analysis of morphobathymetric images and seismic reflection profiles collected during almost 20 years in the Sea of Marmara after the 1999 Izmit earthquake. It shows how the two N and S strands are segmented at different spatial scales. This setting have profound implications in the nucleation of earthquakes along each segments in the highly populated Istanbul metropolitan area. However, recognition of geometry and nature of active faults is only the first step towards reliable seismic risk assessment. Topographic-sedimentary structures (canyons, channels, scarps) have important implications for earthquake geology. Contour-parallel scarps may represent paleo-shorefaces and could thus represent horizontal paleo-reference levels (i.e. piercing lines) whose offset along the fault strands in the different regions of the Marmara basin may be used to estimate the NAF horizontal and vertical slip rates for the Holocene. The present day slip rate of the NAF has been estimated at 24 mm/y from geodetic measurements (McClusky et al. , 2000). Geological strike-slip estimates of about 10 mm/y on both sides of the Marmara overstep (i.e., in the Gulf of İzmit in the east and the Gulf of Saros in the west) are mutually consistent