Deep crustal structure
of the
North Anatolian Fault Zone
and
the Earthquake Cycle



  


Summary


Deformation of the Earth's upper crust is localised onto narrow fault zones where movement usually occurs by sudden slip associated with earthquakes. The deformation of the lower crust and the upper mantle is probably more broadly distributed and is typically thought to occur by ductile creep. The transition from a possibly broad shear zone in the lower crust, which creeps continuously, to a narrow structure in the upper crust, which can move infrequently and catastrophically in an earthquake, is an essential but poorly understood aspect of the lithosphere. To understand the earthquake cycle on major fault systems we need detailed measurements of fault geometry and surface displacements. Geodetic observations are now able to accurately determine the slip distribution that occurs during an earthquake, and they begin to constrain the post-seismic deformation of the medium around the slipped fault. Little understanding exists however of the long fault-loading cycle, which is moderated in particular by the ductile shear zone which is thought to extend downward and outward beneath the fault. We propose a multi-disciplinary project to solve this problem, focussing on the North Anatolian Fault Zone (NAFZ) in Turkey. We plan a novel seismic experiment that will image lower-crustal structure beneath the North Anatolian Fault, analysis of geodetic measurements of surface displacement, and petrofabric analysis of exhumed fault surfaces that have exposed rocks representative of the mid to lower crust beneath the NAFZ. These data will be used to constrain geodynamic models of the earthquake cycle using 3D visco-elastic continuum mechanics simulations. Computational experiments simulating the visco-elastic deformation of a faulted block driven by boundary forces will be constrained by geodetic and finite strain observations (seismic images and petrofabric analysis) in order to constrain the variation of creep viscosity within and around the fault zone. We aim in this project to develop a deeper understanding of the earthquake cycle for this major fault system, and to contribute to better constraints on the seismic hazard associated with it. The resulting synthesis of data and model may guide future investigations for fault zones worldwide.


Objectives

The North Anatolian Fault Zone (NAFZ) is the major continental strike-slip fault system located in northern Anatolia. This fault slips with an average speed of 20-30mm/yr and is the main focus of deformation on the northern edge of the Anatolian region as it moves west toward the Hellenic subduction zone. The fault accommodated 12 large earthquakes (M=6.7 and above) since 1939 and the dominantly westward movement of strong seismicity along the fault poses a large hazard of a future strong earthquake in the Istanbul region. The main aim of this project is to develop a crustal scale model of the NAFZ that is consistent with cumulative strain history indicated by seismic images of fault zone structure and geological measurements of petrophysical properties, and the short-term strain history constrained by geodetic data. The research will encompass measurements from three major disciplines within the earth sciences: (1) Seismic data analysis, (2) Satellite geodesy, (3) Geological mapping and fabric analysis. These results will be synthesized in the context of a geodynamical model for the visco-elastic deformation of the fault zone over the complete earthquake cycle. Specific objectives are: 1) To collect a seismic dataset which enables us to image the structure of the NAFZ throughout the crust and to measure the variation of seismic properties and anisotropy in the vicinity of the fault, particularly the so-called "damage zone". 2) To use new InSAR data along with existing geodetic databases to describe surface deformation at different stages of the earthquake cycle 3) To use structural and petrological information from exhumed fault zones close to the NAFZ to constrain the constitutive law parameters in the geodynamic model. 4) To develop a numerical modelling procedure for the time-dependent deformation (viscous and elastic) of the crust, enabling surface displacements and internal strain fields to be predicted for arbitrary 3D variation of constitutive law parameters. 5) To use the seismic images and geological data to inform models of the parameter variation at depth in the shear zone, and to use available geodetic data to constrain modelled earthquake cycle strain rates. 6) To explain the apparent contradiction between crustal viscosity values estimated from geodetic data for post-seismic and inter-seismic deformation rates at strike-slip fault zones world-wide.. If we are to use geophysical observations to assess medium-term (month to decade) seismic hazard, then an accurate understanding of the structure and properties of the fault zone is essential. The inter-disciplinary work proposed here will make significant progress towards this goal, and towards the understanding of continental deformation.



Seismic Stations and
                    Geodetic Information


Project Team


Dr Sebastian Rost (Leeds)                   Prof. Niyazi Turkelli (KOERI Istanbul)           Prof. Levent Gulen (Sakarya Univ.)

Dr Tim Wright (Leeds)                            Ugur Teoman (KOERI Istanbul)                          Dr Murat Utkucu (Sakarya Univ.)

Prof Greg Houseman (Leeds)            Metin Kahraman (KOERI Istanbul)

Dr Dave Cornwell (Leeds)

Dr Geoff Lloyd (Leeds)

Dr Richard Phillips (Leeds)

Dr Tadashi Yamasaki (Leeds)





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