We present a 2‐D tomographic seismic velocity image in eastern Marmara region along a N‐S trending 120‐km long seismic refraction profile, which traverses the northern branches of North Anatolian Fault (NAF) and tectonically active Çınarcık Basin, in the Sea of Marmara. The 2‐D velocity model, which is constrained down to upper crustal depth of 7 km, shows significant heterogeneities in the upper crust displayed by well‐constrained velocity anomalies. The seismicity observed following the 17 August, 1999 İzmit earthquake concentrates in three distinct zones and takes place near the inferred low velocity zones and below high velocity anomalies obtained from tomographic inversion. This correlation is interpreted as a result of intensely sheared zones of deformation imposed by strike‐slip motion of the northern branches of the North Anatolian Fault.

Because the Rayleigh waves generally have the largest amplitude of all waves generated by a vertical surface impact and because the near-surface shear-wave velocity primarily determines the Rayleigh wave velocity, the Rayleigh waves may be used to image shallow shear-wave structures. The Rayleigh wave group velocity can be measured from records of surface waves that have traversed a study area, typically, with a surface source on one side and an array of geophones along the opposite side. The multiple-filter technique is used for group velocity determination. First, each trace is narrow-band filtered and the instantaneous amplitude of its complex trace is computed. Second, the time of the peak in the instantaneous amplitude of the complex trace consistent with the mean group travel time is identified. Third, all the observed group travel times are corrected for group delays introduced by instrument response, geophone ground coupling and the source time function. Finally, the effect of noise can be minimized by constraining the observed group arrival times to be consistent among arrivals that follow similar paths. By obtaining data from different source and receiver locations, the resulting group travel times can be used in a tomographic inversion to image the distribution of group velocity within the study area. The tomographic inversion is repeated for many frequencies giving a dispersion curve for each point in the study area. Then, vertical shearwave velocity structure at any point can be interpreted from its dispersion curve. In this study we obtained data covering a square area that was [Formula: see text] on a side. Usable group velocities were obtained for frequencies from [Formula: see text] using a sledgehammer source. The resulting tomographic image and velocity anomaly was sufficient to delineate a suspected [Formula: see text] deep burial trench.

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The tomegraphic inversion of group velocities of Rg waves is a viable technique for mapping lateral variations in the lithology of sedimentary basins. In order to determine a velocity structure, we first use the moving-window maximum entropy spectral analysis to measure the average Rg wave group travel times. Second, the group velocities within blocks (subregions)covering the area sampled by the Rg wave propagation paths are determined by tomegraphic inversion. Third, the vertical shear wave velocity profile within each block is determined from the inversion of group velocities for that block. Finally, a structural cross section is obtained by connecting blocks located along a profile. Rg waves recorded from mining explosions in Alabama were used in a test of the technique by modeling the Paleozoic sedimentary rocks in the Black Warrior Basin of northern Alabama. The high resolution of moving-window maximum entropy spectral analysis allows estimation of the Rg wave group velocities with a precision of better than 3 %. The usable frequency range of 0.5 to 1.5 Hz allows determination of structures to depths of about 2.4 km with layers of 0.2 km thickness. Blocks of widths of 20 km or less could be resolved with appropriate data. Shear wave velocities in the Black Warrior Basin range from 1.6 to 4.0 km/s. The layering in the velocity structure corresponds to major geologic units. Within units, an increase in velocity to the northeast corresponds to observed facies changes. INTRODUCTION The Rg waves observed on records of near-surface earthquakes and blasts are predominantly fundamental-mode Rayleigh waves. As such, their group velocities are controlled mainly by variations in the velocity structure of the upper few kilometers of the crust along their propagation path. Rg waves on short-period seismograms are most prominent at epicentral distances of 25 to 200 km along propagation paths characterized by smooth terrain and by low-velocity surface sediments or weathered rock. In this paper, we show that a detailed analysis of Rg wave dispersion can be a useful tool for the study of shallow structures. In particular, the range of distances of propagation and depth of penetration make Rg waves appropriate for the study of facies changes in sedimen• basins. Rg waves with periods of 0.5 to 2.0 s have been used to demonstrate regional variations of Rg wave dispersion in southern New England [Kajka and Dollin, 1985] and southeastern Maine [Kajka and Reiter, 1987]. Similarly, Woods et al. [1989] observed differences in the Rg wave dispersion properties of the Ozark Uplift and the Illinois Basin. Differences in dispersion were attributed to variations, from 0.3 to 1.0 kin, in the thickness of a single surface layer.These interpretations of Rg wave dispersion were based on clusters of propagation paths, each cluster consisting of paths principally in one structural region. In these studies, a tomegraphic inversion technique was not applied. In contrast. tomegraphic inversion techniques have been used extensively to study the la...

Time-frequency analysis techniques, including the classical use of zero crossings to measure period, have been widely used in seismology for the estimation of surface wave group velocities. Group velocity estimation by the short-time Fourier transform and the multiple filter techniques are equivalent. Although these techniques are used most often, their resolution is limited. The resolution is controlled by the window length in the short time Fourier transform and the filter band width in the multiple filter technique. The moving-window autoregressive spectral estimation provides the highest resolution with the shortest possible window length by predicting the properties of the signal outside the analysis window; however, high resolution is obtained at the expense of uncertainty in the amplitude. Recently, the Wigner distribution has been introduced as a tool for mapping dispersed surface waves into the time-frequency domain. Resolution of the Wigner distribution is comparable to that of the moving-window autoregressive spectral estimation. When the spectral density at a given time contains two or more dominant frequencies, their interference causes the Wigner distribution to introduce spurious spectral peaks complicating the interpretation. The Choi-Williams distribution, in which these interference effects are minimized, can be used for such dispersed signals. However, the implementation is computationally complex and the distribution offers only a medium resolution.