Shear wave anisotropy in the Erzincan Basin and its relationship with crustal strain

Abstract

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.

Anisotropy Study of the February 4th 2008 Swarm in Nw Peloponnesus (Greece)

“…It is worth noticing that the observation of stress aligned seismic shear-wave splitting in almost all in situ rocks, in various sedimentary and crystalline geological regimes, below some critical depth, usually between about 500 m and 1 km is nearly universal (Kaneshima, 1990;Crampin and Lovell, 1991;Coutant, 1996;Gamar and Bernard, 1997;Papadimitriou et al, 1999;Crampin and Chastin, 2001;Hao et al, 2008). Shear-waves split into two approximately orthogonal fixed-polarizations with different velocities, which is characteristic of propagation in media with some form of elastic anisotropy.…”

Study on S wave splitting in Dayao earthquake sequence with M=6.2 and M=6.1 in Yunnan in 2003

“…As constrained by the data and medium difference, the value of delay time between fast and slow wave appears to be scattered (Crampin et al, 2004), therefore, the relation between its value and depth is difficult to be determined (Gamar and Bernard, 1997;Savage et al, 1989;Zhang and Schwartz, 1994). Many results derived from overseas authors show that the delay time does not change with focal depth and epicentral distance, and their relation coefficients are small (Cochran et al, 2003;Ben-Zion, 2004, 2005).…”

Study on shear wave splitting in the aftershock region of the Yao’an earthquake in 2000

“…The delay times between fast-waves and slow-waves scatter for the limit of data and medium difference (Crampin et al, 2004); hereby it is difficult to ascertain its relation with their own focal depths (Gamar and Bernard, 1997;Savage et al, 1989;Zhang and Schwartz, 1994). We hope to reveal some relation between delay time and focal depth using records of station Mayou and Balazha with large number and high-quality waveforms.…”