T017-0005
Initial Ambient Noise Tomography above a region of Slow-Slip, Hikurangi Margin, New Zealand

Wednesday, 9 December 2020
Poster
Katrina M Jacobs1, Bill Fry1, Rebecca E Bell2, Stuart A Henrys1, Ake Fagereng3, Lisa McNeill4, Joanna V Morgan2, Mike Warner2, Thomas Luckie5, David A Okaya5, Victoria Lane6 and Debra Daly6, (1)GNS Science, Lower Hutt, New Zealand, (2)Imperial College London, London, United Kingdom, (3)Cardiff University, Cardiff, CF24, United Kingdom, (4)University of Southampton, Southampton, United Kingdom, (5)University of Southern California, Los Angeles, CA, United States, (6)University of Leicester, Leicester, LE1, United Kingdom
Abstract:
The NZ3D-FWI project deployed a dense grid of broadband and short period instruments above a part of the Hikurangi subduction interface, North Island, New Zealand, that experiences slow slip events every 18 -24 months. The array was deployed to record air-gun shots from the R/V Marcus Langseth, which acquired a 3D multichannel seismic volume offshore in January 2018. Following the active source experiment, the broadband array of 49 CMG-6TD broadband seismometers from the NERC Geophysical Equipment Facility (SEIS-UK) were left in place for an additional nine months for further passive studies.

Here we present initial results of ambient-noise tomography using this dense broadband array. The CMG-6TDs have a frequency range of 30 s – 100 Hz and were deployed with ~2 km spacing over a 20x10 km grid. We also incorporate surrounding permanent GeoNet network seismometers to expand the range of useable wavelengths and increase the spatial extent of the study. The Gisborne region studied here also overlaps with the Seismogenesis at Hikurangi Integrated Research Experiment (SHIRE) which carried out a wide-angle, onshore-offshore, reflection-refraction active seismic survey together with a four-month-long passive deployment.

To analyze the surface wave velocities, we first create daily cross-correlations using the MSNoise package. For each station-pair, daily cross-correlations are stacked for all available data during the 10-month deployment. Surface wave dispersion curves are then picked for all stacked station-pair cross-correlations. Initial surface wave moveouts suggest very slow seismic velocity material (Rayleigh Wave Velocity < 1 km/s), in line with other recent results.

We aim to determine the distribution of surface wave group velocities in the top 10-15 km across the Gisborne Region in the 1-30 s period band. We will then invert the resulting Rayleigh wave group velocity maps for a 3D crustal structure. These results will give a detailed image of the shallow structure in this region and will also help inform other ongoing imaging efforts.

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