Comprehensive analysis of the local seismic response in the complex Büyükçekmece landslide area (Turkey) by engineering-geological and numerical modelling

Multi-risk management requires a strong comprehension of possible effects induced by natural hazardous events. In this regard, landslides triggering due to earthquakes results from complex interactions between seis- mic waves and slopes. Multidisciplinary approaches can significantly contribute to better understand such inter- actions. The large Büyükçekmece landslide (about 1500 m wide and 1830 m long) located in Turkey (Avcilar peninsula), about 15 km northward from the North Anatolian Fault Zone (NAFZ), was selected as case-study in the framework of the European project “MARSite – Marmara Supersite: new directions in seismic hazard assessment through focused Earth observation in the Marmara Supersite”. The Avcilar area was recently affected by the 17th August 1999 Mw 7.4 Kocaeli and by the 12th November Mw 7.2 Düzce earthquakes. The Büyükçekmece landslide involves upper Oligocene to lower Miocene deposits, consisting of silty clays, tuffs and sands. No earthquake-in- duced re-activations are testified so far but the landslide area was interested by a very intense urbanization dur- ing the last decade. A detailed engineering-geological model for the local seismic response of the Büyükçekmece landslide slope was constructed based on geophysical measurements, data from a multisensor in-hole monitor- ing system and stress-strain numerical modelling. Several tens of earthquakes were recorded from October 2014 to May 2015 in the landslide site by considering in-hole and surface data. The reliability of the local seismic re- sponse obtained by numerical modelling respect to the empirically derived one was checked in terms of both site-to-reference spectral ratios and transfer function between surface and downhole sites inside the landslide mass. The 2D numerical amplification functions confirm that the local seismic response is a consequence of the complex geological setting of the landslide slope while no relevant amplification effects can be referred to topo- graphic features. Based on these results, the interaction between seismic waves and the Büyükçekmece landslide slope cannot be neglected to evaluate the possibility of future landslide re-activations.