Attenuation of High-Frequency P and S Waves in the Crust of Central and Western Tien Shan
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Pure and Applied Geophysics
Attenuation of High-Frequency P and S Waves in the Crust of Central and Western Tien Shan XIAOLONG MA1 Abstract—In order to better understand the attenuation characteristics in the crust of the central and western Tien Shan orogenic belt, we investigate both Qp and Qs values by applying the extended coda-normalization method. We estimate the frequency-dependent attenuation of both P and S waves in the frequency band of 1.0–20.0 Hz using data from local networks. The average frequency relations of Qp and Qs have been derived as Qp = (61 ± 9) 9 f(1.21±0.08) and Qs = (77 ± 6) 9 f(1.11±0.04) by fitting a power-law frequency dependence model for the study region. The low Q0 and high g values indicate that the central and western Tien Shan is a tectonically and seismically active region. We also find lateral variations of both Qp,s and Qs/Qp values, reflecting complex tectonic structures in the study area. In general, the relatively high-attenuation areas corresponding to tectonically active regions are found beneath mountainous ranges in the central Tien Shan whereas the relatively low-attenuation areas are associated with stable regions, such as Fergana Basin and Issyk-Kul Lake. Besides, regions in the central Tien Shan with Qs/Qp [ 1 are likely partially saturated with fluids or rich in scattering heterogeneities, whereas regions in the western Tien Shan display Qs/Qp \ 1, possibly suggesting almost complete fluid saturation. Keywords: Seismic attenuation, central and western Tien Shan, coda normalization method, Qp and Qs, Qp/Qs.
1. Introduction As one of the largest intra-continental orogenic belts in the world, the Tien Shan is still actively developing today. Thus it provides a natural laboratory for studying the ongoing continental collision and evolution mechanism of the intra-continental orogenic belt (Lu¨ and Lei 2018). The Tien Shan orogenic belt is located at the southwestern margin of the Central Asian Orogenic Belt (CAOB) and is bounded by the Kazakh Shield, Yili Basin to the
1 State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China. E-mail: [email protected]
and ZONGYING HUANG1 north and the Tarim Basin to the south (Fig. 1). It stretches approximately 2500 km in the east–west direction and extends about 300–500 km longitudinally (Sychev et al. 2018), exhibiting rather complex tectonic environments (Windley et al. 1990). Geological evidence has demonstrated that the Tien Shan was originally formed by the closure of the PaleoAsian Ocean in the late Paleozoic (Gao et al. 1998; Burtman 2015) and reactivated in the Oligocene due to the collision between Indian and Eurasian plates (Molnar and Tapponnier 1975; Yin et al. 1998). Currently, a north–south crustal shortening of * 20 mm/year is occurring in the Tien Shan, the rate of which is nearly one half of the Indo-Eurasian convergence rate (Abdrakhmatov et al. 1996). Furthermore, the directions of crustal shortening are generally in parallel with
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