Estimation of surface wave Green's functions from correlation of direct waves, coda waves, and ambient noise in SE Tibet

Empirical Green's functions (EGFs) between receivers can be obtained from seismic interferometry through cross-correlation of pairs of ground motion records. Full reconstruction of the Green's function requires diffuse wavefields or a uniform distribution of (noise) sources. In practice, EGFs differ from actual Green's functions because wavefields are not diffuse and the source distribution not uniform. This difference, which may depend on medium heterogeneity, complicates (stochastic) medium characterization as well as imaging and tomographic velocity analysis with EGFs. We investigate how source distribution and scale lengths of medium heterogeneity influence surface wave Green's function reconstruction in the period band of primary microseisms (T = 10-20 s). With data from a broad-band seismograph array in SE Tibet we analyze the symmetry and travel-time properties of surface wave EGFs from correlation of data in different windows: ambient noise, direct surface waves, and surface wave coda. The EGFs from these different windows show similar dispersion characteristics, which demonstrates that the Green's function can be recovered from direct wavefields (e.g., ambient noise or earthquakes) or from wavefields scattered by heterogeneity on a regional scale. Directional bias and signal-to-noise ratio of EGFs can be understood better with (plane wave) beamforming of the energy contributing to EGF construction. Beamforming also demonstrates that seasonal variations in cross-correlation functions correlate with changes in ocean activity.