EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Using P- to S- wave conversions from controlled sources to determine the shear-wave velocity structure along Hikurangi Margin Forearc, New Zealand

Pasan Herath1, Tim Stern1, Martha Savage1, Dan Bassett2, Stuart Henrys2, Dan Barker2, Harm Van Avendonk3, Nathan Bangs3, Adrian Arnulf3, Ryuta Arai4, Shuichi Kodaira4, and Kimihiro Mochizuki5
Pasan Herath et al.
  • 1School of Geography, Environment and Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand.
  • 2GNS Science, 1 Fairway Drive, Avalon, Lower Hutt 5011, New Zealand.
  • 3Institute for Geophysics, University of Texas, Austin, Texas, USA.
  • 4Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokohama, Japan.
  • 5Earthquake Research Institute, University of Tokyo, Tokyo, Japan.

The Hikurangi subduction margin offshore of the east coast of New Zealand displays along-strike variations in subduction-thrust slip behavior. Geodetic observations show that the subduction-thrust of the southern segment of the margin is locked on the 30-100 year scale and the northern segment displays periodic slow-slip on the 1-2 year scale. It is hypothesised that spatial variations in pore-pressure may play a role in this contrasting phenomenon. Higher pore-pressures would result in lower effective stresses, which promote slow-slip of the subduction-thrust. In addition, the presence of a sedimentary wedge with very low shear wave-speeds in the northern Hikurangi margin has been proposed to fit the ultra-long duration of ground motions observed following the 2016 Kaikoura earthquake. Compressional (P-) wave velocities (Vp) of the subsurface provide useful information about the lithological composition. Combined with shear (S-) wave velocities (Vs), the Vp/Vs ratio which is directly related to Poisson’s ratio can be obtained. This is a diagnostic property of a rock’s consolidation and porosity. Typical Vp/Vs ratio of consolidated and crystalline rocks range from 1.6 to 1.9 and that of unconsolidated sediments can range from 2.0 to 4.0.

We use the controlled sources of R/V Marcus G Langseth recorded by a profile of 49 multi-component ocean bottom seismometers (OBS) along the Hikurangi margin forearc for the Seismogenesis at Hikurangi Integrated Research Experiment (SHIRE) to derive the Vs structure and estimate the Vp/Vs ratio. The orientations of the horizontal components of each OBS are found by a hodogram analysis and by an eigenvalue-decomposition of the covariance matrix. Using the orientations, the horizontal components of each OBS are rotated into radial and transverse components. P to S converted phases are identified on the radial and transverse components considering their linear moveout, polarisation angle, and ellipticity. We confirm incoming S-waves to OBSs by comparing them with their hydrophone components. We identify both PPS (up-going P-wave after reflection or refraction converts to an S-wave at an interface) and PSS (down-going P-wave from the controlled source converts to an S-wave at an interface) type conversions. The identified conversion interfaces are the sediment-basement interface and the top of the subducting crust. The travel-time delay of a PPS type conversion relative to its P-wave arrival is indicative of Vs above the converting interface. The linear-moveout of PSS type conversions are indicative of Vs along the raypath after the conversion. Preliminary results from the southern Hikurangi margin suggest Vp/Vs ratios of ~1.70 for the basement rocks above the subducting crust and ~1.90 for the sediments overlying the basement rocks. These values indicate that the basement rocks are consolidated and less porous than the overlying sediments.

We expect to estimate the Vp/Vs ratios in the northern Hikurangi margin to assess the role played by pore-pressure in the along-strike variation in subduction-thrust slip behavior. We also expect to ascertain the presence and estimate the thickness of the low-velocity sediment wedge in the northern Hikurangi margin.

How to cite: Herath, P., Stern, T., Savage, M., Bassett, D., Henrys, S., Barker, D., Van Avendonk, H., Bangs, N., Arnulf, A., Arai, R., Kodaira, S., and Mochizuki, K.: Using P- to S- wave conversions from controlled sources to determine the shear-wave velocity structure along Hikurangi Margin Forearc, New Zealand, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11940,, 2020

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Presentation version 2 – uploaded on 04 May 2020
Added a map with OBS locations on slide no. 13
  • CC1: Comment on EGU2020-11940, Richard Davy, 04 May 2020

    Hello Pasan,

    You have an adequate impedance contrast to generate shear-waves at the basement-sediment boundary on the upgoing ray-path. Why is this same impedance contrast then not suffecient to generate shear-waves on the downgoing ray-path? Is this a function of going from faster velocities to slower velocities on the upgoing ray-path, as opposed to the opposite on the downgoing ray-path?



    • AC1: Reply to CC1, Pasan Herath, 05 May 2020

      Hi Richard,

      Thanks very much for your question. It is very interesting.

      You are right that it is the same impedance contrast (magnitude-wise) at the sediment-basement interface for both downgoing and upgoing rays.

      There are two things at play here.

      1. Incident angle

      For a downgoing raypath, the incident angle on this boundary is near vertical. However, for an upgoing raypath, the incident angle is much higher, particularly in the case of the P- to S- conversion of a refracted P-wave in the basement. From Zoeppritz equations, the magnitude of the converted wave is observed to increase with increasing angles of incidence.

      1. Location of conversion

      For a downgoing raypath, the location or the point of conversion is far away from the OBS, close to where the airgun source is located (the distance from the would be very close to the source-receiver offset). At this point, the impedance contrast may not be sufficient for effective P- to S- conversion. However, for an upgoing P-wave, the point of conversion is immediately below the OBS (within about 5 km). The impedance contrast at this point of conversion may be sufficient for the conversion to occur.

      From these two points, I think there is a higher weight on the second one.

Presentation version 1 – uploaded on 03 May 2020 , no comments