Combining Three Component Beamforming and Wave Hindcast Model Data to Investigate Source Regions of Ambient Seismic Noise: a New Zealand Case Study
Abstract
Recent studies employing ambient seismic noise as an energy source have extended our knowledge of crustal and upper mantle structures and of time-variable geological processes. It has further been demonstrated that the combination of earthquake and ambient seismic noise signals can potentially overcome restrictions inherent to traditional event-based seismology alone. However, uncertainties in the source location and source mechanism of ambient seismic noise constitute a major obstacle in fully utilizing the results of seismic noise investigations. Several authors have addressed this problem but focused on the source regions of vertical-component noise recordings only. In this study, we conduct plane-wave beamforming using vertical- and horizontal-component seismic noise recordings from a dense deployment of 61 broadband instruments on New Zealand's western North Island to investigate the source regions and source mechanisms of both Rayleigh and Love waves contained in the ambient seismic noise wavefield. We compare beamformer observations with locations of potential sources inferred from bathymetry and an ocean wave hindcast model of the New Zealand region employing the theoretical framework for microseism excitation of Longuet-Higgins (Phil. Trans. R. Soc. Lon., 1950] and Bromirski and Duennebier (J. Geophys. Res., 2002). Our results indicate that the source regions of both the primary and secondary microseisms are mostly located close to New Zealand's coastline and few significant deep water sources can be identified. The strongest source region appears to be the south-western tip of New Zealand's South Island where large ocean swells coincide with a narrow continental shelf. This observation is underlined by the good correlation between near-coastal ocean significant wave height and seismic significant wave height. However, quantification of this relationship in terms of Longuet-Higgins' [1950] theory requires a more detailed treatment of coastal effects on ocean waves than is currently common in wave hindcast models. We also observe that Rayleigh and Love waves are generated in similar areas but with differing intensities. When averaged over several months, our beamformer results for vertical- and horizontal-component noise recordings show broadly similar and homogeneous azimuthal distributions. This is an important result for ongoing studies focused on estimating Green's functions from ambient noise cross correlations in New Zealand.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.S23A2214B
- Keywords:
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- 4259 OCEANOGRAPHY: GENERAL / Ocean acoustics;
- 4277 OCEANOGRAPHY: GENERAL / Time series experiments;
- 7255 SEISMOLOGY / Surface waves and free oscillations;
- 7299 SEISMOLOGY / General or miscellaneous