We observed Rayleigh and Love wave dispersion
characteristics between 5 s and 20 s using yearly-stacked cross-correlation
functions (CCFs) of ambient seismic noise data, from the dense seismic
broadband network consisting of 17 stations in the Ulaanbaatar region,
Mongolia. The extracted empirical Green's functions of Rayleigh and Love waves
between 136 pairs of stations showed substantially large signal-to-noise ratios
(> 17) in the target periodic range, indicating clear trends of wave
propagation up to ~270 km. We removed P-wave contamination, which exists as
precursors, from the stacked CCFs using vertical-to-radial (ZR) and
radial-to-vertical (RZ) components and succeeded in obtaining dominant
Rayleigh-wave signals. The estimated Rayleigh and Love-wave phase velocities
using the matched-filter frequency-time analysis showed little variation over
the period between 6 s and 10 s, showing small subsurface heterogeneity in the
Ulaanbaatar region. The estimated Rayleigh wave phase velocity map for 8 s
indicated small (~0.05–0.1 km/s) velocity contrasts between the western and
eastern parts, and this suggests the possibility of local-scale seismic
velocity heterogeneity in the study area. The resulting velocity maps
correspond well with the estimations derived with a sparse seismic array. We
also found a feature which could extend our Rayleigh wave phase velocity
dispersion to a much longer period, and shorter spacing stations, (which are
typically excluded in seismic interferometry) could be included using the SPAC
method. The combined use of seismic interferometry and SPAC method will make it
possible to conduct a much more precise surface-wave tomographic study in the
Ulaanbaatar region.
Keywords:
Ambient noise
cross-correlation, Ulaanbaatar region, Rayleigh wave, Seismic Interferometry.