An Observation Related to Directional Attenuation of SKS Waves Propagating in Anisotropic Media

zhao, liang ; xue, mei;

Azimuthal anisotropy of attenuation is a physical phenomenon related to the directional change of attenuation. This study examines the frequency properties and directional attenuation of SKS waves. The directional frequency-dependent characteristics of SKS waves are investigated in the frequency band of 0.02–0.5 Hz using data from 53 permanent seismic stations located throughout the northern Yangtze Craton, the southern North China Craton and adjacent areas. In addition to normal splitting behavior, the analysis reveals that many SKS splitting measurements exhibit a lemniscate shape, reflecting frequency differences along fast and slow polarization directions. Frequency analysis shows that spectral ratios between fast/slow components of the lemniscate-type splitting results fluctuate strongly in a higher frequency band of 0.2–0.5 Hz, and fluctuate less within the main frequency band of 0.02–0.2 Hz. For each station, the ratio of the peak amplitude of the fast/slow components can be represented as a
cotangential function of event backazimuth multiplying with a constant = 0.42 ± 0.10. This transformation shows that the regional average angles consistently fall within the relatively narrow range of −46.5 ± 3◦ with respect to the north, suggesting that a regional tectonic controlling factor dictates the relatively uniform directional attenuation of SKS waves within the frequency band of 0.02–0.2 Hz. Further analysis is performed by projecting the SKS waves
onto the components along and perpendicular to the regional average angles. The calculation also shows that, in the 0.02–0.2 Hz band, the relationship between amplitude ratio and event backazimuth matches a cotangential functions with the same best matching angles and constant a < 1. Synthetic calculations demonstrate that although different filters influence the splitting parameters, attenuation anisotropy cannot be explained by elastic anisotropic media, including
multilayer anisotropy and anisotropy with a tilting symmetrical axis. This observed behavior of the SKS wave may arise from the combined effects of frequency-dependent attenuation anisotropy and small-scale heterogeneities in the crust and the upper mantle.