Secret Archives of the Basilica of Nicea provided by the Sediments of Iznik Lake – BASILIZNIK-SECRETS

Lake geophysics : Bathymetry, seismic reflection
Lake sediment coring, sedimentological and geochemical analyses of cores and dating of sediments.
Terrestrial geophysics; GPR on land fault segments, seismic noise analysis to identify potential site effects, soil penetration testing to identify soil properties.
Trenching on faults, earthquake dating.
Sediment analysis for tsunami research.
DNA on lake cores to see the evolution of biodiversity

Discovery of a new active fault segment in the Iznik Lake, using bathymetry and seismic reflexion.
Identification of the last earthquake in 1065 CE that ruptured the Iznik fault. It may have induced the desturction of the Basilica of Nicea. Estimation of the vertical co-seismic displacement of 42 cm during this earthquake.
Identification of subaqueous slip in the lake related to this and previous earthquakes.
Characterization of the 2000-year seismicity catalog felt in the Iznik Lake and identification of a period of increased flood frequency in the Iznik region from lake sediment cores.
Characterization of the segmentation of the MNAF on land
Identification of at least four very large earthquakes on these segments over the past 4000 years of magnitude > 7.3
Precision on the period of operation of the basilica.
Characterization of lake level changes over time from preliminary seismic data

Based on the results obtained, we have completely reorganized our long core campaign which should be done in November 2021. We will do 2 long cores on both sides of the Iznik fault (which offers a greater potential of exploitation than a core in the deep basin) and a long core in the north near the shoreline to identify and date the variations of the lake level.
Once we have these long cores we will hire the two post docs planned in the project to study them (1/sedimentology/geochemistry; 2/DNA).
On land we hope to be able to make trenches during our next mission in November 2021.
We will then have to acquire data on potential site effects. These effects can be of two types, either related to the propagation of waves in a medium of variable impedance (multi-strate soil effect or topographic effect), or related to specific properties of a poorly consolidated soil such as liquefaction. To identify the large scale site effects (valley scale) related to wave propagation we will make seismic noise measurements during the next mission in Nov 2021. We will estimate the potential amplification of the waves near Iznik. The measurements of liquefaction of the ground are postponed to a next mission

Ces résulats ont donné lieu à trois publications
Gastineau, R. ; de Sigoyer J., Sabatier P., Fabbri S.C., Anselmetti F.S., Develle AL., Sahin M., Gündüz S., Niessen F. and Gebhardt A. C., (2020) Active subaquatic fault segments in Lake Iznik along the middle strand of the North Anatolian Fault, NW Turkey. Tectonics DOI: 10.1002/essoar.10504104.
R. Gastineau, P. Sabatier, S.C. Fabbri, F.S. Anselmetti , P. Roeser, N. Findling , F. Arnaud, M. Sahin, S. Gündüz , S.O. Franz , J. de Sigoyer Extensive lateral variations of earthquake versus flood-triggered event deposits in tectonic Lake Iznik, NW Turkey. Sedimentology accepted.
Benjelloun, Y., de Sigoyer, J., Garambois S., Carcaillet J., and Klinger Y. Segmentation and Holocene Behavior of the Middle Strand of the North Anatolian Fault (NW Turkey) Tectonics accepted.

The famous city of Nicaea (now Iznik) was established on the shore of the Lake Iznik (Turkey), 5 km north of the active Middle segment of the North Anatolian Fault (MNAF). In May 325 AD, the city has hosted the first Council of Christians, the founding event of Christian religions. The location of this council remained unknown until Prof. M. Sahin discovered a basilica in the Lake Iznik in 2014 (under 1.5 m of water and 50 m from shore). The basilica was destroyed and submerged after an unknown catastrophic event, probably an earthquake, as suggested by the damage observed on the city’s archaeological remains. Six large earthquakes (Mw>7) were documented in the past 2500 years according to the texts and archaeo-sismotectonic studies carried out in the Iznik region. Seismic hazard remains poorly assessed on the MNAF, as no earthquake was documented over the last 150 years. The horizontal slip rate on the MNAF (5 mm/yr) is four time lower than the one on the northern segment NNAF, which explain its lower seismicity level, and a long recurrence time (>500 yr) between earthquakes. Two super-shear earthquakes (with a break propagation faster than the speed of sound) were described along the NNAF during the Izmit and Düzce earthquakes, 1999. This type of earthquake is very destructive, often associated with soil liquefaction. It is important to know if such earthquake could have destroyed in Nicea basilica and to better characterize the MNAF seismic behaviour on a long-time scale.
In this project, we want to clarify the cause and timing of the basilica’s destruction (WP1). Archaeological survey will specify the stratigraphy of buildings and the timing of basilica lifetime. Cores of sediments (2 m) in and around the basilica can document the seismic history on the site. Dates below and above the basilica ground will frame the age of its construction and destruction. The mechanical properties of the soil on the shore and trenches may highlight potential liquefaction markers that may have led to destabilization of the ground and creep of the basilica into the lake. It is all the more important to assess the vulnerability of this site as it will become an underwater museum.
To improve our knowledge on the seismicity of this zone (WP2), we will map the faults in the Lake Iznik and Çerkesli (20 km eastward) during bathymetric and seismic surveys, looking for straight lineament where super-shear earthquake could occur. The mass-wasting deposits in the deep part of the basin are evidences of past earthquakes or extreme precipitations. We will map them in 3D, and core (25m) them to obtain a long calendar of past seismicity. Short cores (2 m) collected from different areas of the two lakes will confirm their seismic origin and assess their spatial extension. Earthquakes and extreme precipitations events will be distinguished using sedimentological and geochemical analyses on the turbiditic sediments cores. On land, trenches will digg on the MNAF to attribute some segment rupture to earthquake dates. These data will be uses to improve the probabilistic seismic hazard assessment of the MNAF.
Changes in climate, biodiversity and human practices around the lake (WP3), can also be highlighted through soil-erosion proxies and the analysis of ancient DNA (aDNA) metabarcoding in the lacustrine sediments. Such analyses will help to understand the causes of lake and ground level variations as well as the multiple feedbacks between tectonic, climatic and anthropic factors.
This pluridisciplinary project will help to elucidate the secrets of the lost basilica, cradle of the Christianity, to better know the seismic cycle on the MNAF and on the NAF system and to understand how tectonics, climate and anthropization interact and can change the fate of human communities.