Characterising the New Zealand ambient seismic noise field: an oceanographicinterpretation of western North Island spectra and beamforming results

Abstract

Although seismometers are generally designed to record episodic events such as earthquakes and volcanic eruptions, most of the time they are actually recording ambient seismic noise. At frequencies lower than about one hertz, Rayleigh waves produced by nonlinear ocean wave processes constitute much of the ambient noise field. The seismological community has recently taken advantage of this and there is now much interest in using ambient seismic noise for purposes such as measuring surface wave velocities and thereby inferring the velocity structure of the crust and upper mantle. However, in order to account for spatio-temporal or azimuthal biases in the noise field underpinning surface wave velocities, the characteristics of the noise field itself must first be understood. This study investigates the spatial and temporal characteristics of the New Zealand ambient seismic noise field. Rayleigh waves at the ocean wave frequency are typically generated by direct ocean wave-induced pressure fluctuations at the sea floor, and Rayleigh waves at double this frequency are generated by opposing wave fields. New Zealand’s geographic isolation exposes it to a particularly energetic ocean wave climate. Examples of signals generated in the oceans surrounding New Zealand are shown here in spectrograms of seismometer data, and are related to observed wind and ocean wave characteristics. Data collected on a seismometer array of up to 58 instruments in the Taranaki region (western North Island) were beamformed in order to estimate the relative source directions of incoming seismic noise. Signals with phase velocities corresponding to those expected for both fundamental and higher-order Rayleigh waves are observed. Regions of ambient seismic noise generation highlighted by the beamformer output are shown to agree well with modelled shallow-water (50m isobath) ocean wave heights from around the New Zealand coast.