Deep crustal structure of the North Anatolian Fault Zone and the earthquake cycle

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The Earth's surface is broken into numerous tectonic plates, which are continually moving. The movement of the plates relative to each other is the source for most earthquake activity on Earth, which is typically focussed into narrow fault zones where the plates collide, pull apart, or slide past each other. Within the fault zones the deformation in the upper 10-15 km of the Earth's crust is localised onto narrow fault planes. Earthquakes occur when the stresses on the fault planes caused by plate motions overcome frictional resistance, and these represent significant hazard for communities living in fault zones - in the first decade of the 21st century alone, earthquakes killed 700,000 people. In strike-slip fault zones, where plates slide past each other, earthquakes typically only break the upper crust. We know that the lower crust (deeper than 10-15 km) must be deforming continuously, because we can measure how the ground surface deforms between earthquakes. But because rock samples or other direct measurements cannot easily be obtained from these depths, we have a poor understanding of how the lower crust behaves and influences the loading of stresses in the upper crust to cause major earthquakes.We propose an inter-disciplinary project with the aim of understanding the earthquake loading cycle (how stresses build through plate motions and are released in earthquakes) along a major European fault, the North Anatolian Fault Zone (NAFZ) in Turkey. The NAFZ is a strike-slip fault comparable in length and slip rate to the San Andreas Fault in California. It crosses a densely populated region of northern Turkey and constitutes a major seismic hazard - over 1000 km of the fault ruptured during 12 large earthquakes in the 20th century. The western end of the NAFZ ruptured in two major earthquakes in 1999 at Izmit on 17 August and Düzce, 87 days later, killing more than 30,000 people. A seismic gap remains south of Istanbul, an urban centre of more than 10 million people, where there is ~60% chance of significant shaking within the next few decades (Parsons et al. 2000).We aim to measure the properties of the fault in the lower crust to set constraints on the earthquake loading cycle along the NAFZ. The project involves (i) a novel high-resolution seismic experiment aimed at resolving the fault zone structure at depth, (ii) geological analysis of an exhumed fault zone representative of the mid to lower crust under the fault, and (iii) analysis of satellite measurements of surface displacement. The results from these studies will be used to build computational models of the earthquake loading cycle. In this project we aim to explain how the movements of the tectonic plates interact with the fault zone and how this is affected by the lower crustal structure. This will ultimately contribute to better assessment of the seismic hazard associated with large fault zone. The resulting synthesis of the geophysical and geological data together with geodynamical modelling will guide future investigations for other major strike-slip fault zones.