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SUMMARY The estimation of shear wave velocity and attenuation in near-surface geology is of primary importance in engineering seismology. In fact, such knowledge is essential for site response studies when preparing improved seismic hazard scenarios. In this study, we propose a linear inversion of the spectra of a deconvolved wavefield collected by a borehole array in Istanbul, Turkey. The spectra are calculated using as a reference the recordings collected by a sensor at the surface. This allows us to minimize the effect of the deconvolution filter on the peaks' amplitudes. The feasibility of the proposed inversion scheme and the parametrization of the velocity and attenuation models were tested and assessed using synthetic data. The real data inversion is carried out using observation from three weak motion events. Several starting models are used for each event, changing the values of the quality factor Qs, whereas the starting Vs profiles are fixed to what was obtained by the analysis of the deconvolved wavefield in the time domain. Our results showed that fairly well-constrained Vs models can be obtained while the variability of the Qs profile, both between different inversions of the same data set, and between the inversions of different data sets, might be large. This indicates that Qs variations observed in models derived by strong motion data inversions should be considered with care. However, the high agreement of the estimated soil profiles in the uppermost layer, both when using different events and different starting models, suggests that any variation in the S-wave velocity that might be estimated when using strong motion data should be reliable. [ABSTRACT FROM AUTHOR]

The estimation of shear wave velocity and attenuation in near-surface geology is of primary importance in engineering seismology. In fact, their knowledge is essential for site response studies when preparing improved seismic hazard scenarios. In this study, we propose two approaches for estimating the average shear wave quality factor Qs by using recordings of a vertical array of accelerometers. The methods are mainly based on the deconvolution of the wavefield recorded in a borehole with that recorded at the surface. The first method requires the Fourier transform of the deconvolved wavefield to be fitted with a theoretical transfer function valid for the vertical or nearly vertical (in the case at hand up to 30° incidence angle) propagation of S waves. The second method is based on the spectral fitting of the Fourier transform of only the acausal part of the deconvolved wavefield with a theoretical transfer function. Both methods can be applied without any prior knowledge of the subsoil structure (since they are based on empirical data analysis) and do not require a precise knowledge of the azimuthal orientation of the sensors in the boreholes (which is seldom available). First, we describe the theoretical framework of the proposed methodologies for Qs estimation, which are based on the assumption that the structure in the borehole is weakly heterogeneous in the vertical direction (i.e. no large impedance contrast exists between the borehole sensor and the surface). Second, by using synthetic accelerograms, we verify that in a realistic subsoil structure, the assumption of vertical homogeneity can hold and we investigate the robustness and the suitability of the proposed methods. Finally, only the method that was shown to provide the more stable results, based on fitting the borehole-to-surface spectral ratio with a theoretical function, is applied to earthquakes signals recorded by a vertical array of accelerometers installed in Ataköy (western Istanbul). Results show that using borehole data provides a fair and robust estimate of an average Qs (of about 30, 46 and 99 for the 0–50, 0–70, 0–140 m depth ranges, respectively) that can be used for numerical simulations of ground motion. [ABSTRACT FROM AUTHOR]

We investigate the crustal structure beneath the western part of the North Anatolian fault zone (NAFZ), an area where at least five damaging earthquakes occurred during the twentieth century. This study is based on local earthquake tomography using the data from aftershocks of the Izmit event (17 August 1999, M 7.4) recorded by stations of permanent and temporary networks. We derive the distribution of VP, VS, and the VP/VS ratio based on the iterative inversion for both VP - VS and VP - VP/VS using the LOTOS code. Innovatively, in this study we perform an inversion for frequency-dependent S-wave attenuation (1/QS). The reliability of the results is assessed through synthetic tests. The distributions of the resulting seismic parameters (VP, VS, VP/VS, and QS) highlight important geodynamical features in the study area. The low-velocity and high-attenuation patterns mostly correlate with the fracturing zones of the NAFZ. Low velocities are also observed beneath the main sedimentary basins (e.g., Adapazarl, Dtizce, and Kuzuluk). High-velocity and low-attenuation patterns correlate with blocks presumed to be rigid (Kocaeli, Armutlu, and Almacik blocks). The rupture traces of the largest earthquakes in this area pass generally in the transition areas between high and low velocities, while moderate and weak seismicity is mostly concentrated in low-velocity areas. Based on these results we propose and discuss the role that the Almacik block could have played in producing the largest earthquakes in the study area in the twentieth century. [ABSTRACT FROM AUTHOR]

The source parameters of 523 aftershocks (0.5 < ML < 5.9) of the 1999 Kocaeli earthquake are determined by performing a two-step spectral fitting procedure. The source spectrum, corrected for both site and propagation effects, is described in terms of a standard x-square model multiplied by an exponential term of frequency. The latter term is introduced to estimate the high-frequency (f > 12 Hz) fall-off of the acceleration source spectra by computing the j parameter. The seismic moments obtained range between 1.05 x 1014 and 2.41 x 1017 N m, whereas the Brune stress drops are between 0.002 and 40 MPa. The κ value varies between 0.00 and 0.08 sec, indicating a decay of the acceleration level at the higher frequency part of the spectrum greater than that assumed by the ω-2 model. Both the stress drop and the j parameter show the tendency of increasing with aftershock magnitude. No evidence of self-similarity breakdown is observed between the source radius and M0. Finally, both the seismic moment and the moment magnitude are compared with the local magnitude to derive new moment--magnitude relationships for the area. [ABSTRACT FROM AUTHOR]

We present an update of the local magnitude scale previously calibrated for northwestern Turkey by Baumbach et al. (2003). The path coverage in the westernmost part of the analyzed area has been increased, as well as the number of amplitudes for distance greater than 110 km. Furthermore, a set of recordings from accelerometric stations operated by the Kandilli Observatory and Earthquake Research Institute (KOERI) has been merged with the recordings by the Sapanca-Bolu and German Task Force seismological networks. In all, 4047 recordings from 528 earthquakes recorded by 31 seismometers and 23 accelerometers are considered to calibrate the local magnitude scale over a hypocentral distance range from 10 to 190 km. By analyzing the unit covariance matrix and the resolution matrix, we show how the source-to-station geometries of the seismic and strong-motion networks affect the uncertainties of the computed station corrections, attenuation coefficients, and magnitudes. The assumptions made concerning the reference station correction, and the change in the amplification for the Wood-Anderson torsion seismograph from 2800 to 2080 (Uhrhammer and Collins, 1990) introduced an offset of about 0.34 in the magnitudes with respect to Baumbach et al. (2003), with the updated local magnitude scale ranges from 0.50 to 5.91. The distribution of the residuals with distance confirms that the extension of both the magnitude and distance ranges and the improved path coverage have preserved the high quality that characterized the data set analyzed by Baumbach et al. (2003). [ABSTRACT FROM AUTHOR]

On 17 August 1999, the Mw 7.4 Izmit earthquake occurred in northwestern Turkey. A temporary seismic network was installed to improve the geometry of a previously installed network. In this study, 262 aftershocks of the Izmit earthquake with magnitude ME ranging between 0.4 and 4.5 were analyzed using digital recordings from 17 stations of these networks. S-wave and P-wave spectral records, corrected for path attenuation function, were inverted by means of the generalized inversion technique (GIT). The GIT site responses were compared with those calculated by horizontal-to-vertical (H/V) ratios applied to both earthquake data and to pre-event noise. In most cases, the H/V of the earthquake data provided site responses with shapes consistent with those obtained using GIT. However, the level of amplification was occasionally found to be different. In general, these discrepancies can be explained either by the amplification affecting the vertical component of the ground motion that we detected by GIT or by the waves having a propagation in almost the vertical direction. Using the H/V ratio applied to noise, for most cases, we obtained not only the peaks for the fundamental resonance frequencies, but site responses with shapes similar to those obtained by GIT. The detected level of amplifications using the H/V ratio showed a tendency not to exceed those calculated by the inversion. Finally, amplifications up to a factor 5 are found for stations placed over complex topography. For these sites, amplifications are not always correctly identified by the H/V technique, using either earthquakes or noise. [ABSTRACT FROM AUTHOR]