Yunfeng Chen, Erdinc Saygin

Empirical Greens function (EGF) retrieval commonly relies on cross-correlating the long-term ambient seismic wavefield that is simultaneously recorded at multiple stations. Recent studies have demonstrated observationally that cross-correlating the coda of ambient noise cross-correlation functions (C3) enables reconstruction of the EGFs, regardless of the operating time of the stations. In this study, we develop a new technique to perform correlation of cross-correlation functions (C2), thus permitting the reconstruction of asynchronous EGFs. Our approach exploits the deterministic wavefield rather than the diffusive codas that may be affected by incoherent energy under non-ideal (e.g., sparse, noisy and short-duration) network configurations. We demonstrate the robustness of C2 by retrieving asynchronous EGFs between 1) nearby stations and 2) distant temporary arrays from southern Australia. The accuracy of the EGFs from C2 are examined by analyzing seismic tomography of Rayleigh wave group velocities and benchmarking them with the results from conventional ambient noise imaging. The additional ray paths from asynchronous C2 functions provide better illumination of small-scale crustal structures beneath the regional network. In the larger-scale example, involving two asynchronous arrays, the implementation of the C2 method offers new constraints to the sparsely sampled region of the southern Australian offshore. The resulting velocity model agrees well with the independent structural constraints from individual seismic array studies and sedimentary thickness measurements. This study demonstrates that C2 is a promising tool for integrating transportable arrays deployed at different times and can greatly benefit the effort of improving seismic data coverage and resolution in crustal imaging.