Jiafu Hu

S-wave Q structure of the crust and upper mantle beneath Yunnan from surface waves

A phase-matched filtering technique is applied to extract fundamental mode signals from Rayleigh waves recorded at 62 digital seismic stations in the Yunnan and Sichuan regions. We use the fundamental mode of vibrations at two stations that are located on the same great circle as the focus to calculate an inter-station attenuation coefficient of the Rayleigh wave with periods between 0. 0 and 80.64 s, and invert for the inter-station S-wave Q-factor (Qβ) at depths of 0–200 km. The results indicate that Qβ in Yunnan is 20–140, presenting a low Qβ background with apparent lateral variation. Taking the Honghe Fault as the boundary, Qβ of the crust is only 20 on the west side, extending to a depth of 120 km. The distribution of Qβ is consistent with large-scale Cenozoic volcanic and intrusive rocks in western Yunnan, implying that the crust and mantle are in the thermally active state. In the eastern Yunnan Block, east of the Xiaojiang Fault, Qβ in the upper 120 km is 140 in the south but only 20 in the north. Additionally, around the Dukou-Chuxiong in the mid-Yunnan Block, Qβ in the lithosphere is relatively high at 60–100, corresponding to a stiff crust. This is because the suture between the Indian and Eurasian plates reversed the tension in the rifting stage into the compression of orogenesis, leading to the closure of a gap in the crust. After some time, interstitial fluids gradually disappeared, resulting in a high velocity layer in the crust and low heat flow on the surface. The Yunnan region consists of an obvious block of elevated Qβ, distributed within the low background, consistent with the distribution of heat-flow values on the surface. The Honghe and Xiaojiang faults are tectonic boundaries in addition to being boundaries between regions of high and low crustal Qβ. The low Qβ is probably the result of crustal rupture and disturbance caused by strong earthquakes and the upwelling of hot substances along the deep fault zones.