Seismic-wave propagation in shallow layers at the GONAF-tuzla site, Istanbul, Turkey

Using the first dataset available from the downhole Geophysical Observatory of the North Anatolian Fault, we investigated near-surface seismic-wave propagation on the Tuzla Peninsula, Istanbul, Turkey. We selected a dataset of 26 seismograms recorded at Tuzla at sensor depths of 0, 71, 144, 215, and 288 m. To determine near-surface velocities and attenuation structures, the waveforms from all sensors were pairwise deconvolved and stacked. This produced low-noise empirical Green’s functions for each borehole depth interval. From the Green’s functions, we identified reflections from the free surface and a low-velocity layer between ?90 and ?140 m depth. The presence of a low-velocity zone was also confirmed by a sonic log run in the borehole. This structure, plus high near-surface P- and S-wave velocities of ?3600–4100 and ?1800 m=s, lead to complex interference effects between upgoing and downgoing waves. As a result, the determination of quality factors (Q) with standard spectral ratio techniques was not possible. Instead, we forward modeled the Green’s functions in the time domain to determine effective Q values and to refine our velocity estimates. The effective QPvalues for the depth intervals of 0–71, 0–144, 0–215, and 0–288 m were found to be 19, 35, 39, and 42, respectively. For the S waves, we obtained an effective QS of 20 in the depth interval of 0–288 m. Considering the assumptions made in our modeling approach, it is evident that these effective quality factors are biased by impedance contrasts between our observation points. Our results show that, even after correcting for a free-surface factor of 2, the motion at the surface was found to be 1.7 times greater than that at 71 m depth. Our efforts also illustrate some of the difficulties of dealing with site effects in a strongly heterogeneous subsurface. © 2016, Seismological Society of America. All rights reserved.Deutsche Forschungsgemeinschaft, DFG: Bo1877/7; Helmholtz Association; Helmholtz-Zentrum Potsdam - Deutsches GeoForschungsZentrum GFZ, GFZThe Geophysical Observatory at the North Anatolian Fault (GONAF) project is funded by the Helmholtz Centre Potsdam GFZ German Centre for Geosciences; the Disaster and Emergency Management Authority (AFAD), Ankara, Turkey; the International Continental Drilling Program (ICDP); and the Helmholtz Association, Germany. We acknowledge funding within the Helmholtz Young Investigators Group "From Microseismicity to Large Earthquakes" and from the German Research Foundation (DFG) within the project Bo1877/7. The GONAF instrumentation was assembled by the technical staff of the Institute of Earth Science and Engineering (IESE). We thank Georg Dresen and Murat Nurlu for their strong and invaluable support in the GONAF project. Fatih Bulut was onsite party chief and, together with Recai Kartal, managed to improve local recording conditions at the Tuzla site, which is highly appreciated. We thank the ICDP Operational Support Group, in particular Jochem Kueck, for conducting the logging measurements and B?t?n Geothermal Ltd. for performing the drilling of the Tuzla well. We also would like to thank Esen Arpat from the Technical University of Istanbul for the Tuzla borehole-cutting analysis and his valuable contribution to understanding the geology at Tuzla