GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 1.2 145 (Polonia et al. 2004; Gasperini et al. 2011). However, this estimate is much lower (less than one-half) than the geodetic rates. We suggest two possible scenarios to explain these discrepancies: 1) the Holocene plate motion has not been focused along a single fault, but it has been diffuse and distributed among different fault strands; 2) the GPS geodetic measurements reflect a recent increase in the rate of strike-slip motion, and are not representative of the long-term behavior of the fault. The consistency between estimates carried out for the same time-span and at locations spaced over ~300 km apart along the same strand of the NAF favors the second hypothesis. According to catalogs of major earthquake in the region, the recurrence interval of major events is about 200-250 years (Ambraseys and Finkel, 1991; 1995), one order of magnitude larger than time-series of GPSmeasurements. Moreover, the GPSmeasurements were performed during a period that included increased deformation rate (i.e., two major events of 1999 İzmit and Düzce earthquakes). Thus they may not be representative of the long-term regime. The Sea of Marmara is considered prone to rupture because it represents a seismic gap where 5.5 m of slip has accumulated since 1766 (Hubert-Ferrari et al. , 2000). Such estimates assume an average slip rate of 20-25 mm/yr neglecting possible creep on major seismogenic faults. Reducing slip-rates would not alter the spatial distribution of estimated historical slip, but would possibly reduce the amount of slip accumulated over a single fault branch along the NAF since the last seismic events. This has to be considered in assessing of seismic hazard, which would consequently decrease. However, we cannot exclude the possibility that other secondary fault strands played an important role in absorbing the plate motion during Holocene. Acquisition of additional data in shallow water, together with offshore/onshore correlation of seismogenic structures, would clarify this point, and help understand to what extent localized estimates are representative of the large-scale kinematics of the NAF. In that sense, slip-rate estimate carried out in the Gulf of Gemlik, along the middle strand of the NAF system is significant. This gulf lies relatively far from Istanbul and the highly populated northern Marmara coast. This is probably the reason why this area did not receive the same attention in term of seismic hazard studies. The southern strands of the NAF are considered to be absorbing a minor component of the plate motion (only 20% of the total right lateral motion). However, our 4 mm/yr slip rate estimate, in agreement with geodetic estimates, is large, representing about 50% of the 10 mm/yr estimated at the northern strand. This result suggests that attention should be given to the Gemlik segment of the NAF, which suffered that last M=7.2 event in 1419 AD (Ambraseys, 2002). Conclusions. The main result of our studies along the submerged portion of the North- Anatolian Fault system in the Sea of Marmara region is that marine geological studies of active faults have the potential of reconstructing both, large-scale geodynamic evolution of geological tectonic boundaries and the individual contribution of each fault to the overall deformation pattern and to the generation of earthquakes. We stress that in order to obtain long-term reconstructions of tectonic activity along major faults we should consider the results of regional and interdisciplinary studies, i.e. basin-wide paleoceanography, stratigraphy and large-scale tectonics. High-resolution geophysical data and stratigraphic constraints from sediment cores enabled us to compile a reliable neo-tectonic map of the submerged portion of the NAF and to estimate the horizontal slip rates at the scale of 10 ka in three key locations. This estimate, in the order of 10 mm/y for the NAF northern strand is consistently similar on both sides of the Sea of Marmara. However, these geological rates are about one-half of what expected from geodetic measurements and accepted plate-tectonic models. We also found a rate of 3.7 mm/yr along the southern strand of the NAF in the Gulf of Gemlik, which represent about 30-40% of the total (cumulative) motion Anatolia- Eurasia, and is slightly higher than those derived by geodetic models. References Armijo, R., Meyer, B., Navarro, S., King, G., Barka, A.A., (2002). Asymmetric slip partitioning in the Sea of Marmara pull-apart: a clue to propagation processes of the North Anatolian Fault? Terra Nova, 14, 80-86, doi: 10.1046/j.1365-3121.2002.00397.x