Estimation of quality factor for SH waves in Qeshm Island, using aftershocks of 27 November 2005

The attenuation quality parameter (Q) is a phenomenological quantity depending on the observations and on the underlying theoretical models. Attenuation of seismic waves is expressed with inverse quality factor (Q^-1) and helps understand the physical laws governing the propagation of seismic waves in the lithosphere. Attenuation is often found to be anisotropic (directionally dependent) due to a variety of factors such as the intrinsic anisotropy of the material, the presence of aligned fluid-fractures (Batzle et al., 2005), or interbedding of thin layers with different properties (Zhu et al., 2007). The magnitude of attenuation anisotropy can be much higher than that of velocity anisotropy, and the symmetry of the attenuation coefficient can be different than that of the velocity function (Liu et al., 2007). The observed seismic-wave amplitudes usually decay exponentially with increasing travel distance after the correction for geometrical spreading, and decay rates are proportional to Q⁻¹ that characterizes the spatial attenuation for SH-wave. Qeshm Island, the largest island of the Persian Gulf, is important because of various aspects such as population, economics and some oil and gas reservoirs.
Since the most destructive part of the elastic waves, is the horizontal component of the shear waves, estimation of attenuation of the horizontal component will provide us with very useful information. Horizontal components of shear waves are also affected by the structure of the earth.
 In this study, 661 well-located aftershocks are selected and 18342 seismograms are used to the calculation. by rotation of the components, the horizontal part of the shear waves are separated and horizontal shear wave quality factor was determined by using the Coda normalization method in five frequency bands 1-2, 2-4, 4-8, 8-16, 16-32 (Hz) with a central frequency of 1.5, 3, 6, 12, 24 Hz, in the lapse time of 30 seconds. Based on the calculations, the frequency dependence relation for shear waves: Q_SH = (11 ± 1.2)f^(1.2±0.105).
The relationship between the frequency dependence for horizontal shear waves shows the attenuation in the Qeshm Island is very high and consequently the region is seismically active. Besides, a small amount of the quality factor for horizontal shear waves associated with the low velocity of shear wave propagation in the crust that may relate to the presence of gas and oil fluids and some salt dome. In the azimuthal study, the attenuation of horizontal shear waves are calculated in two directions: northeast-southwest and northwest-southeast. For low frequencies, the attenuation in the northeast-southwest direction is close to northwest-southeast direction, which seems the horizontal component of the shear waves are not affected by tectonic structures, so it seems mostly to be dependent on the material of the earth, whereas for high frequencies greater than 6 Hz, there are significant differences between two azimuthal attenuation, that can be due to some small-scale heterogeneity of the region.