The Characteristics of High-Frequency Attenuation of Shear Waves in the Longmen Shan and Adjacent Regions

A database of 1597 accelerograms from more than 500 events (3.3 < M-s < 8.0) of the 2008 Wenchuan and 2013 Lushan earthquakes, recorded on 44 stations in the range of 0-300 km, is used to study the characteristics of kappa, which is the key parameter controlling the shape of Fourier acceleration spectrum at high frequency in stochastic simulation. The results indicate tha kappa is independent of magnitude but clearly dependent on near-surface geology, site condition, and epicenter distance. The obtained dependences of kappa on distance in the Songpan-Garze orogen (SGO) and western Sichuan basin (WSB) are 0.0001427 and 0.0001031 skm, respectively. The average kappa(0) of the horizontal component (kappa(h)(0)) of the WSB and SGO ranges from 0.0144 to 0.0469 s and from 0.0321 to 0.0539 s, respectively. The dependence of kappa(h)(0) on site condition shows opposite tendencies in the WSB and SGO. In the SGO, it suggests that V-S30 cannot fully represent the features of near-surface geology, which is believed to be the main contributor of kappa(h)(0). Furthermore, we observed that the spatial distributions of kappa(0) of the horizontal and vertical components are closely related to elevation and velocity images at 0-10 km depth, respectively. The correlations of kappa(h)(0) and elevation in the Longmen Shan, southeastern Alps and northern Dinarides region, and ANZA seismic network are studied. The results show good correlation and indicate that elevation may be used as a complementary parameter to develop a regional kappa(h)(0) model in addition to the kappa(h)(0)-V-S30 correlation.