Scattering and Absorption Imaging of the North Anatolian Fault Zone, northern Turkey

The North Anatolian Fault (NAF) is a right-lateral, strike-slip fault in the northern part of the Anatolian peninsula. It is estimated to have a length of up to 1500 km, extending westwards between the Karliova Triple Junction, where it nucleates, to the Aegean Sea. In the west and close to the Sea of Marmara, the NAF splays into northern (NNAF) and southern (SNAF) strands. The splay of the western part of the NAF separates the area into three primary terranes: the Istanbul Zone (north of the northern strand), the Armutlu-Almacik Block (between the two strands of the fault) and the Sakarya Zone (south of the southern strand).

There have been a series of high-magnitude earthquakes along the NAF since the 1930s, migrating from east to west. In order to investigate the western part of the North Anatolian Fault Zone (NAFZ), which is the most seismically active at the moment, the Dense Array for North Anatolia (DANA) temporary seismic network was deployed for 18 months between 2012 and 2013. A set of local earthquakes, recorded by DANA, were utilised to study the 2D scattering and coda attenuation structure in the western NAFZ, between 1 and 18 Hz. P-wave arrival times were manually picked and the events were re-located using the Non-Linear Location software. Peak-delay travel times were calculated as a measure of forward scattering, and the exponential decay of the coda wave envelopes was used to invert for the absorption structure using multiple scattering sensitivity kernels.

The obtained models are 2D averages of the first 10-15 km of the crust, where the majority of the seismic activity is located and they have been compared to recent geophysical studies in the same area. The scattering structure, between 1 and 6 Hz, highlights the three main tectonic units in the area. The absorption structure is generally more heterogeneous than the scattering structure, with the overall absorption decreasing as the frequency increases. The lithological variations and heterogeneity between and within the three terranes of the area, arising from the complex tectonic history of the region, are believed to be the main reasons for the scattering and absorption observations made. The high absorption zones observed along the two branches of the fault, and especially the southern branch, is a very important finding, as the signature of the southern branch in geophysical studies is often unclear.