Offshore Rayleigh Group Velocity Observations of the South Island, New Zealand, from Ambient Noise Data
KAUST Grant NumberOCRF-2014-CRG3- 2300
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AbstractWe present azimuthally anisotropic Rayleigh group velocity models from 8 - 35 s both offshore and onshore of the South Island of New Zealand. We use MOANA (Marine Observations of Anisotropy Near Aotearoa) broadband ocean seismic data in combination with on land data from the New Zealand National Seismography Network (NZNSN) to investigate the seismic structure of the flanks of the Australian-Pacific plate boundary. At 8 s, we observe low offshore group velocities best explained by the influence of the water layer and thick water-laden sediments. At long periods (20-30 s), group velocities are lower on the South Island relative to its offshore flanks, due to thickened crust beneath the island, with the lowest velocities primarily beneath the Southern Alps. Group velocity azimuthal anisotropy fast directions near the Alpine Fault align with the direction of relative plate motion between the Australian and Pacific plates. In the southern portion of the island, fast directions rotate anticlockwise, likely in response to a decrease in dextral shearing away from the plate boundary. Azimuthal anisotropy fast directions align with absolute plate motion offshore on the Pacific plate. Based on the depth sensitivity of our observations, we suggest diffuse deformation occurs throughout the crust. Our observations match trends in previous Pn anisotropy and SKS shear wave splitting observations, and therefore suggest a consistent pattern of distributed deformation throughout the lithosphere.
CitationYeck WL, Sheehan AF, Stachnik JC, Lin F-C (2017) Offshore Rayleigh group velocity observations of the South Island, New Zealand, from ambient noise data. Geophysical Journal International 209: 827–841. Available: http://dx.doi.org/10.1093/gji/ggx054.
SponsorsWe thank Captain and Crew of the R/V Thomas G. Thompson (cruise TN229), the R/V Roger Revelle (cruise RR1002), and the SIO OBSIP facility for expert assistance at sea. The OBS instruments used in this field program were provided by the U.S. National Ocean Bottom Seismic Instrumentation Pool (http://www.obsip.org). MOANA seismic data are archived at the IRIS Data Management Center. The collection of ocean bottom seismic data was funded by the National Science Foundation under grants EAR-0409564, EAR-0409609, and EAR-0409835. New Zealand onshore seismic data made available by the GeoNet project, sponsored by EQC, GNS Science, and LINZ. J. Stachnik acknowledges support from the CIRES postdoctoral fellowship program.. F.-C. Lin was supported by NSF grant CyberSEES- 1442665 and the King Abdullah University of Science and Technology (KAUST) under award OCRF-2014-CRG3- 2300. We thank Martha Savage, Dan Zietlow, Justin Ball, Craig Jones, Bill Fry, and Peter Molnar for valuable comments and discussion, and John Collins for leading the MOANA OBS experiment. We thank two anonymous reviewers and Editor Mike Ritzwoller for their constructive comments.
PublisherOxford University Press (OUP)