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.