Characteristics of the North Anatolian Fault at the eastern end of Marmara seismic gap based on multidisciplinary field evidence

Abstract

SUMMARY The northern branch of the North Anatolian Fault (NAF) is the primary source of the greatest natural hazard for the Marmara Megapolis and specifically the İstanbul Metropolitan area. The Sea of Marmara section of the NAF is a ∼125-km-long seismic gap bound by the M7.4 1912 Mürefte earthquake rupture in the west and M7.4 1999 İzmit earthquake rupture in the east. Terminations of these ruptures define the length of the seismic gap in the Marmara Sea and consequently the magnitude and rupture extent of the expected large magnitude earthquake. The Hersek Peninsula is the last location where the NAF can be studied on land before it enters the Sea of Marmara. In addition, it is a geologically key location for the western termination of the M7.4 İzmit surface rupture. This study integrates high-resolution onshore and offshore data from Hersek Peninsula and its vicinity in İzmit Bay to assess the seismic hazard for the Marmara region. Detailed geological and geomorphic mapping, palaeoseismic trenching, geophysical and geotechnical subsurface data and archeoseismologic investigations demonstrate that the 1999 İzmit earthquake surface rupture died off east of the Hersek Peninsula due to structural complexity of the NAF at this location and the most recent earthquake to rupture the surface at Hersek Peninsula was the historical 1509 CE event. A restraining bend and bifurcation of the NAF at Hersek Peninsula suggests that this location might be a persistent asperity forming a surface rupture segmentation point at least since Pleistocene. Surface ruptures documented in palaeoseismic trenches north of the Hersek Lagoon and an offset 6th century East Roman aqueduct; however, suggests that this section of the NAF has ruptured the surface multiple times in the past and is highly likely to rupture again during the expected Marmara earthquake. The aqueduct; which crosses the surface trace of the NAF, is offset 14 ± 1 m and yields 13.6 ± 1 mm yr–1 dextral slip rate indicating that the northern branch of the NAF accrues and releases the majority of the stress between Eurasia and Anatolian block in the Marmara region. If the ∼125-km-long seismic gap between the 1912 and 1999 ruptures fail during a single event the expected Marmara earthquake can reach M7.4.