Abstract. The analysis of S wave polarization from a large number of aftershocks of the 1992 Erzincan earthquake has revealed a clear S wave splitting, which we interpret in terms of seismic anisotropy in and around the Erzincan basin and which concerns at least the first few kilometers of the crust. In the sedimentary basin, our interpretation of the origin of the anisotropy is the alignment of cracks due to the stress field (extensive dilatancy anisotropy (EDA) model) within the deep sediments and possibly below, in the mean direction N335 ø. This is rotated by 20 ø from the compressire stress direction N355 ø deduced from other aftershock studies. This difference rna, y either be explained by uncertainties in the stress direction or by a stress distortion related to the activity of some nearby active faults. In the outcropping basement, to the south, the fast S direction is N315 ø, significantly different from the present regional stress direction, which implies that the anisotropy is controlled either by a possible, though yet unreported, rock foliation or by cracks related to a local, strongly distorted stress field. Following the preferred hypothesis of stress rotations and of the EDA model, we use a two-dimensional model to calculate the direction of the elastic stress resulting from the combined activity of the adjacent segments on the North Anatolian fault east to the basin (NAE) and within the basin (NAB) with a similar stress release. The resulting trajectories of the maximal compressire stress in the shallow crust are mainly controlled by the NAE segment, owing to i•s much greater length, and correctly fit the inferred crack directions in the bedrock sites as well as in the basin sites, provided that the stress release on the segments is of the order of the regional deviatoric stress amplitude stored in the sha. llow crust (1 km in depth). In this model, the stress and crack orientation inferred from shear wave anisotropy can be explained by so•ne slip deficit on the NAB segment with respect to the NAE segment, which is independently suggested by the historical earthquakes in the area. Studies of the crack-induced anisotropy in the shallow crust can thus bring important information on the spatial and possibly temporal variations of crustal strain and stress near active faults, in particular, near the limits of the fault segments marked by jogs, steps, or bends, and might constrain the relative potential of adjacent segments for future earthquake ruptures.

The focusing process for time imaging is improved drastically when high-density parameter fields are used. Large offsets, steep dips and finally the anisotropy of the subsurface revise the bases of time processing. Today, two parameters are required: velocity (V) and anellipticity (η). Picking V and η using two-pass techniques cannot be a long-term solution. The estimation of both parameters is very sensitive to the mute function separating near to far offsets. Picking both parameters simultaneously using dense bispectral analysis overcomes this situation.We are proposing in this paper an original parameterization of the non-hyperbolic moveout, which increases the sensitivity of the analysis and allows static moveout corrections, necessary for automatic dense pickings.An intelligent QC sorting of the raw V and η fields, based on lateral coherency of the semblance and the Dixinversion ability of local V and η functions, prepares skeleton fields for simultaneous geostatistical filtering of both parameters.

Over recent years, many authors have proposed to compensate the absorption loss effects inside of the imaging process through the use of an attenuation model. This is necessary in the presence of strong attenuation anomalies. Q tomography has been developed for estimating this attenuation model but is generally limited to estimating attenuation in predefined anomaly areas. In this paper, we show how shallow gas pockets are revealed automatically by using a high-resolution volumetric Q tomography on the complex offshore Brunei dataset. A key component of our approach is the estimation of effective attenuation in pre-stack migrated domain through accurate picking of the frequency peak. Estimated Q-model is then used to compensate for absorption in the imaging process.

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We present a robust residual gather flattening technique based on trace correlations, which time-aligns coherent events across offsets or angles. We show how gathers to which we apply this method following migration and residual moveout corrections yield more coherent stacks, cleaner AVO results as well as more reliable velocity estimates.