Empirical Green’s functions (EGFs) between pairs of seismographs can be estimated from the
time derivative of the long-time cross-correlation of ambient seismic noise. These EGFs reveal
velocity dispersion at relatively short periods, which can be used to resolve structures in the
crust and uppermost mantle better than with traditional surface-wave tomography.We combine
Rayleigh-wave dispersion estimates from EGFs and from traditional two-station (TS) analysis
into a new approach to surface-wave array tomography with data from dense receiver arrays.
We illustrate the methodology with continuous broad-band recordings from a temporary seismographic
network on the southeastern part of the Tibetan plateau, in Sichuan and Yunnan
provinces, SW China. The EGFs are robust under temporal changes in regional seismicity and
the use of either ambient noise (approximated by records without signal from events with magnitude
mb ≥ 5 or 4) or surface wave coda produces similar results. The EGFs do not strongly
depend on the presence of large earthquakes, but they are not reciprocal for stations aligned in
the N–S direction. This directionality reflects the paucity of seismicity to the north of the array.
Using a far-field representation of the surface-wave Green’s function and an image transformation
technique, we infer from the EGFs the Rayleigh-wave phase velocity dispersion in the
period band from 10–30 s. A classical TS approach is used to determine Rayleigh-wave phase
velocity dispersion between 20–120 s. Together, they constrain phase velocity variations for
T = 10–120 s, which can be used to study the structure from the crust to the upper mantle.
Beneath SE Tibet, short and intermediate period (10–80 s) phase velocities are prominently
low, suggesting that the crust and upper mantle beneath SE Tibet is characterized by slow shear
wave propagation
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