Intrinsic and Scattering Attenuation of Coda Waves in the Sichuan-Yunnan Region

Attenuation of seismic waves is a fundamental property of the earth's media. Besides geometrical spreading, attenuation of seismic waves can be divided into intrinsic and scattering attenuation, which are affected by the absorption and scatters, respectively. Thus the crustal attenuation structure is useful in understanding regional geology and tectonics and is important for seismic hazards.

The Sichuan-Yunnan region in the southwestern China has complicated geological structures with strong seismicity. The China Seismic Experimental Site in this area provides us dense station coverage. In this study, we collect 108399 waveforms of 11517 local small earthquakes (1.5≤ML≤3.5) from January 2014 to September 2021 in the Sichuan-Yunnan region and its adjacent areas. We employ an envelope inversion technique (Eulenfeld and Wegler, 2017) for separating the intrinsic and scattering attenuation of seismic coda wave, and obtain the intrinsic and scattering attenuation structures for frequencies between 0.25 and 8.00 Hz.

The results reveal that: 1. The intrinsic and scattering attenuation decrease with the increase of frequency, and the scattering attenuation is relatively higher than the intrinsic attenuation in 0.25-1.00 Hz, while the opposite is true in 1.00-8.00 Hz. 2. The attenuation structures correlate well with the geological units. For example, the intrinsic attenuation in the Chuandian terrane is relatively stronger than those in the Sichuan Basin and the Yangtze Plate in 0.50-8.00 Hz; and for scattering attenuation, the Chuandian terrane keeps a strong attenuation in 0.50-8.00 Hz, the Yangtze Plate shows a weak attenuation in 0.50-8.00 Hz, while the Sichuan Basin's attenuation is relatively weak in 0.50-2.00 Hz, but becomes strong in 2.00-8.00 Hz. 3. Some major faults mark the attenuation variations, such as, for intrinsic attenuation, the Red River fault and Longmen Shan fault in 0.50-1.00 Hz, Anninghe Fault, Zemuhe Fault and Xiaojiang fault in 1.00-8.00 Hz; and for scattering attenuation, Longmen Shan fault in 0.50-1.00 Hz and 2.00-8.00 Hz, Red River fault and Xiaojiang fault in 1.00-8.00 Hz, and the Anninghe fault and Zemuhe fault in 2.00-8.00 Hz.