Crustal structure beneath China from receiver function analysis

Abstract

We collected and processed a large amount of high‐quality broadband teleseismic waveform data recorded by the 48 Chinese National Digital Seismic Network stations to estimate large‐scale lateral variations of crustal thickness and Vp/Vs ratio (hence Poisson's ratio) beneath China. A statistical method was used to select mutually coherent receiver functions at each station, which yielded over 200 traces for most of the stations. With the conventional H−κ (the crustal thickness and Vp/Vs ratio) approach, there is a large trade‐off between H and κ. Consequently, multiple maxima are frequently observed in the H−κ domain. We introduced a weight function that measures the coherence between the P‐to‐S conversion and the reverberation phases at each H−κ grid to reduce the trade‐off. A 4th‐root stacking method was further applied to reduce uncorrelated noise relative to the linear stack. These modifications turned out to be very effective in reducing the H−κ trade‐off and yielded reliable estimates of crustal thickness and Vp/Vs ratio. The crust beneath eastern China is as thin as 31–33 km and the underlying Moho is relatively flat and sharp. In the western part of China, the crust is considerably thicker and shows large variations. The Moho is observed at about 51 km depth along the Tian Shan fold system and about 84 km deep beneath the central part of the Tibetan Plateau. The transition occurs at the so‐called N‐S belt between about 100° and 110°E, which is featured by unusually high seismicity and large gravity anomalies. The average Vp/Vs ratio over the mainland China crust is about 1.730 (σ = 0.249), significantly lower than the global average 1.78 (σ = 0.27) of the continental crust. This lower Vp/Vs ratio may suggest a general absence of mafic lowermost crustal layer beneath China.