Conférence GM & ED Gaïa :
Secrets of the sunken basilica on the activity of Middle branch of the North Anatolian Fault

by Julia de SIGOYER

ISTerre, Grenoble

at 11 am amphi 23.01 campus Triolet – Université de Montpellier

Participer à la conférence Zoom (ID de réunion : 93757873200)

The famous city of Nicaea (where the first Christian Council was held) was established on the shore of Lake Iznik (Turkey), 5 km north of the middle strand of the North Anatolian Fault (MNAF). In 2014, a basilica was discovered in Lake Iznik; it was destroyed and submerged following an unknown catastrophic event. During this conference, we will follow the scientific investigations of an international team of 15 researchers from different disciplines (archaeology, tectonics, sedimentology, geophysics) in order to understand the seismic behaviour of the MNAF and see how it relates to the history of this basilica.

The MNAF is the slowest branch of the NAF, with a slip rate of < 5 mm/year. To reveal the seismic activity of this slow fault, we have developed new methods for investigating the timing and intensity of earthquakes, based on the stratigraphy of archaeological buildings, the dating of repairs and their modelling. The timing of seismicity was acquired independently by bathymetric, seismic and limnological studies of Lake Iznik and by geomorphological and palaeosimological studies of the fault segments on land.

As a result of these studies, two new fault segments were discovered in Lake Iznik, linked to the MNAF. The study of long sediment cores has made it possible to establish a chronicle of regional and local earthquakes over more than 6,000 years, with a recurrence of 1,000 years on the Iznik fault. The last earthquake to rupture the Iznik fault was in 1065 AD, which led to the destruction of the basilica. We have identified a seismic gap on the Iznik fault since 1065, which represents a major risk for the city of Iznik and the surrounding region.

Julia de SIGOYER is Professor in geodynamic at ISTerre, Université Grenoble Alpes. I have been studing the major continental fault zones from a scale of hundred million years to a year since 30 years. The first half of my career focused on long-term deformation of fossil subduction zones and intracontinental faults. Since 10 yrs, I work at short scale of time; developing archaeoseismological tools to get independent calendar of seismicity that is compared with the one from the study of lake and other palaeoseismological data to determine the segmentation, age and magnitude of past earthquakes.