Abstract

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 QP values 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.

Online Material: Plots of resistivity and caliper logs and the spectra of all 26 events.

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