Multistep modelling of teleseismic receiver functions combined with constraints from seismic tomography: crustal structure beneath southeast China

Abstract

SUMMARY With a growing number of modern broad-band seismographic stations in Asia, the conditions have improved to allow higher resolution structural studies on regional scales. Here, we perform a receiver-based study of the lithosphere of southeast China using waveform records ofexcellentqualityfrom14ChineseNationalDigitalSeismicNetworkandfourGlobalSeismic Network stations. Calculating the theoretical receiver functions (RFs) that match the observed RFs from teleseismic waveforms is an established technique for retrieving information about crustalanduppermantlestructurebeneathaseismicreceiver.RFs,however,arepredominantly sensitivetothegradientsinthelithosphericelasticparameters,anditisimpossibletodetermine a non-unique distribution of seismic parameters such as absolute shear wave speeds as a function of depth unless other geophysical data are combined with RFs. Thus, we combine RFswithindependentinformationfromshearandcompressionalwavespeedsaboveandbelow the Mohoroviy´ discontinuity, available from the existing tomographic studies. We introduce a statistical approach for automatically selecting only mutually coherent RFs from a large set of observed waveforms. Furthermore, an interactive forward modelling software is introduced and applied to observed RFs to define a prior, physically acceptable range of elastic parameters in the lithosphere. This is followed by a grid-search for a simple crustal structure. An initial model for a linearized, iterative inversion is constructed from multiple constraints, including results from the grid-search for shear wave speed, the Moho depth versus vp/vs ratio domain search and tomography. The thickness of the crust constrained by our multistep approach appears to be more variable in comparison with tomographic studies, with the crust thinning significantly towards the east. We observe low values of vp/vs ratios across the entire region, which indicates the presence of a very silicic crust. We do not observe any correlation between the crustal thickness or age of the crust with vp/vs ratios, which argues against a notion that there is a simple relationship between mineralogical composition and crustal thickness and age on a global scale.