From Lake Iznik to the Marmara Sea (NW Turkey): new insights in marine and lacustrine paleoseismology.

Abstract

The North Anatolian Fault (NAF), located in Turkey, is one of the world's most active faults and accommodates Anatolia's westward motion relative to Eurasia. Over the last century, several earthquakes (M>6.8) have migrated from east to west. It is in the Marmara region, south of Istanbul, that the subsequent rupture is expected. However, this is where the geometry of the fault becomes more complex. It divides into three branches, one of which borders Lake Iznik and the southern Marmara Sea. As there is now very little seismic activity along this portion of the NAF (MNAF), and GPS only detects small displacements (Reilinger et al., 2006), it is thought to be inactive. However, the city of Iznik, the cradle of Christianity, has preserved valuable historical evidence in contrast to its observations. Therefore, to better understand the seismic hazard in this area, it is necessary to catalogue the seismic activity and locate past ruptures.Two active faults were discovered in Lake Iznik thanks to our geophysical and coring campaigns (Gastineau et al., 2021). The study of short (<4m) sediment cores sampled on both sides of the E-W fault running close to Iznik city reveals that the previous rupture (1065 CE) coincides with a highly destructive historical earthquake recorded in the city's archaeological structures (Benjelloun et al., 2020). In addition to this localised rupture, numerous other event deposits are present in the sediments (laterally and temporally). We demonstrated that the same earthquake in 1065 CE is associated with various deposit types. One type of deposition is only observed for the 1065 CE earthquake, which takes place in the lake, unlike the others, suggesting that this type of deposition may depend on ground motion parameters besides the source-core distance.A compilation of marine and lacustrine palaeoseismological studies was carried out at the scale of the western part of the NAF. We show that the relationship between sedimentation rate and the presence of earthquake-induced slope destabilisation doesn't work in the marine environment, unlike in the lacustrine environment. We also show that Lake Iznik records earthquakes from the NNAF and the MNAF, whereas the Sea of Marmara records only NNAF earthquakes. These observations open new perspectives and demonstrate the need to consider seismology and site effects in marine and lacustrine paleoseismology.