Fundamental and higher-mode Rayleigh wave characteristics of ambient seismic noise in New Zealand

In order to use ambient seismic noise for mapping Earth's structure, it is important to understand the spatio-temporal characteristics of the noise field. This study uses data collected during four austral winter months of 2002 to investigate New Zealand's ambient seismic noise field in the double-ocean-wave-frequency range (0.1-0.3 Hz). It is shown via beamforming analysis that there are two distinct dispersive waves in the data. These waves can be separated. Their estimated phase velocities (2.5-2 and 4-3 km/s in the frequency range 0.14-0.25 Hz) match well with fundamental and higher-mode Rayleigh dispersion curves. Studies of double-wave-frequency microseisms elsewhere generally show the Rayleigh noise fields to be dominated by fundamental mode waves. The reason why higher-mode signals are observed here may reflect a combination of long ocean wave periods, large waveheights, the direct deep water approach to narrow continental margins, and the proximity of the seismograph array to the source regions. Citation: Brooks, L. A., J. Townend, P. Gerstoft, S. Bannister, and L. Carter (2009), Fundamental and higher-mode Rayleigh wave characteristics of ambient seismic noise in New Zealand, Geophys. Res. Lett., 36, L23303, doi:10.1029/2009GL040434.