Crustal Structure Beneath China Inferred from Receiver Function and Apparent SS wave Splitting

Global crustal models have traditionally been compiled mainly based on active-source refraction and reflection profiles. However, such profiles are usually not evenly distributed. In addition, S-wave velocities are poorly constrained due to the lack of shear wave energy in active sources. Ideally, information on both the P- and S-wave velocities (or P-wave velocity and the Poissons ratio) is preferred in order to infer the crustal composition. In this study, we demonstrate how earthquake data can improve the global coverage of crustal models. Two types of data, receiver function and apparent SS waveform splitting data have been compiled to obtain the desired information. As a feasibility test, we have chosen China as our starting point. We have analyzed receiver function data at more than 270 stations, deployed largely by previous PASSCAL experiments. Very large variations in crustal structure are observed in the study region. The crustal thickness varies from ~25 km to about 90 km and shows a good correlation with geologic terranes, as observed in many previous studies. The crust beneath the extended crust of eastern China and the North China Craton is about 35 km thick. Large P to S conversions are observed at stations in these regions, which suggests a sharp Moho-discontinuity with a large velocity and density contrast. This is suggestive of the lack of a high-velocity mafic layer in the lowermost crust. In contrast, the crust beneath the deformed regions tends to be thick with a diffuse Moho boundary. At large epicentral distance the transverse component of the SS wave always appears to be leading the radial component by a few to tens of seconds. This apparent splitting is produced by several arrivals that include the precursory reflection/conversion at the Moho, followed by several later Moho reverberations. From synthetic data we confirmed that the reflection/conversion series, when filtered to the low-frequency band, produce a large apparent SS splitting. We also found that the apparent splitting time is proportional to the crustal thickness. Our preliminary analysis of several locations in China with the SS data yield very consistent crustal thickness with the receiver function results. The teleseismic SS dataset, which has a very good global coverage, thus could be very useful in developing global crustal models. Crustal Structure beneath China Inferred from Receiver Function and Apparent SS wave Splitting.