Multi-scale imaging of the North Anatolian Fault Zone using seismic interferometry

Seismic imaging allows us to examine the subsurface structure of fault zones. Accurate knowledge of the structure of fault zones is critical for our understanding of earthquake hazard, and the processes of strain accumulation within the crust and upper mantle. The North Anatolian Fault Zone is a ∼ 1200 km long continental strike-slip fault zone located in northern Turkey. In the 20th century, the North Anatolian Fault has accommodated a westward propagating sequence of twelve Mw > 6.5 earthquakes. The most recent of these earthquakes occurred at Izmit and Duzce in 1999, 86 km south-east of Istanbul. In this thesis I use techniques from seismic interferometry to create seismic images of the crustal and upper mantle structure along the Izmit-Adapazari section of the North Anatolian Fault, in the vicinity of the 1999 Izmit rupture. I develop methods for observing P-wave reverberations from the free surface that are contained within the ambient seismic noise field and the P-wave coda of teleseismic earthquakes. By autocorrelating the seismic records from a dense seismic array in north-western Turkey, I use these reverberations to create high resolution seismic reflection images of the crust and upper mantle beneath the North Anatolian Fault Zone. In addition, I calculate inter-station cross-correlations to observe Rayleigh and Love waves propagating between stations in the Izmit-Adapazari region. I then use Rayleigh and Love wave phase velocity measurements to perform surface wave tomography and construct an S-wave velocity model of the top 10 km of the crust in the Izmit-Adapazari region. In the reflection images, I observe a clear arrival associated with a Moho reflected P-wave (PPmP). A ~ 3 s variation in travel time of the PPmP arrival suggests that the Moho is vertically offset beneath the northern branch of the North Anatolian Fault Zone. The vertical offset in the Moho occurs over a region less than 7 km wide approximately 16 km north of the surface trace of the North Anatolian Fault. The location of the vertical offsets indicates that the North Anatolian Fault is a localised structure that dips at an angle between 60◦ and 70◦ through the entire crust and enters the upper mantle as a narrow shear zone. I also note a reduction in the amplitude of the PPmP phase beneath both the northern and southern branches of the North Anatolian Fault Zone. This amplitude reduction could result from the presence of fluids and serpentinite minerals in the upper mantle which reduce Moho reflectivity beneath the North Anatolian Fault. The surface wave tomography shows that the North Anatolian Fault Zone is a vertical zone of low S-wave velocity (2.8 – 3.0 km s−1) in the top 10 km of the crust. I also detect further low velocity anomalies (1.2 – 1.6 km s−1) associated with ~ 3 km deep pull-apart sedimentary basins along both branches of the North Anatolian Fault Zone. Both branches of the North Anatolian Fault appear to skirt the edges of the Armutlu Block, a tectonic unit of crystalline rocks that exhibits high S-wave velocity (3.2 – 3.6 km s−1). It is likely that the Armutlu Block has a strong rheology, and localises strain along the faults at its northern and southern edges. I also measure the azimuthal anisotropy of the phase velocity observations, which displays an average magnitude of ~ 2.5% with a fast direction of 70◦ from north. The 70◦ fast direction aligns parallel with the direction of maximum extension in the Izmit-Adapazari region, and indicates that deformation-aligned mineral fabrics may dominate the anisotropy signal in the top 10 km of the crust.