Extracting surface wave dispersion curves from asynchronous seismic stations in NE Tibet: A two-station C2 method

Ambient noise tomography (ANT) based on empirical Green’s functions (EGFs) retrieved from cross-correlation functions (CCFs) of ambient noise is widely used to construct shear-wave velocity structures. EGFs from ambient noise can be treated as virtual seismograms with one station working as a virtual source and the other station working as a receiver. We propose a method named two-station C² method(Rao et al., 2021), using one single station as a virtual source to obtain surface waves between a pair of asynchronous stations. This method can significantly improve ray path coverages and enhance the resolution in ANT for areas between asynchronous seismic arrays.

In our method, we select three stations, called a station triplet, which share the same great-circle path. We take one long-term station as a virtual source rather than using a number of stations as sources in the C³ method(Stehly et al., 2008; Ma and Beroza, 2012; Spica et al., 2016; Zhang et al., 2019). We use data from the USArray to demonstrate the feasibility of our method in retrieving surface waves from asynchronous stations.

Due to the harsh environment and inaccessibility of most of parts of the plateau, it is nearly impossible to deploy a large-scale synchronous seismic array across Tibet. In the past few decades, several isolated arrays have been deployed in Tibet at different periods of time. ANT has been applied to Tibet to generate phase velocity maps using these seismic arrays (e.g., Yang et al.,2012;Xie et al.,2013; Shen et al., 2016). However, due to the fact that these seismic arrays were not deployed synchronously, inter-array paths between asynchronous arrays cannot be obtained from the traditional C¹ method, resulting in low resolution in the gaps of these seismic arrays.

We applied our method to the two seismic arrays (Z4 and X4) deployed in NE Tibet. The Z4 array was deployed from July 2007 to July 2008 and X4 from September 2008 to September 2009. For these two arrays, if we follow the C¹ method, we can get at most 153 paths for Z4 array and 300 paths for X4 array. But no crossing-array paths can be obtained. Fortunately, there is a permanent Chinese National Seismic Network (Zheng et al., 2010) deployed across China. We can take the permanent stations from the Chinese National Seismic Network as source stations and obtain C² functions following our method. Here, to illustrate the application of our C² method for these two arrays, we select 153 permanent stations from the Chinese National Network as virtual sources. And, using these stations and our C² method for these two arrays, we can retrieve 413 C² functions with the source stations located within 5 degrees of the great-circle paths of receiver station pairs. The path coverage is improved by over 91%. Combining C¹ and C² paths, we can much better image the structures between these two arrays.