EGU2020-12063, updated on 12 Jun 2020
https://doi.org/10.5194/egusphere-egu2020-12063
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Studies of seismic velocities in subduction zones from continuous OBS data

Weiwei Wang1, Martha Savage1, Alec Yates1, Shu-Huei Hung2, Yinhe Luo3, Tim Stern1, Pei-Ying Patty Lin4, Hsin-Ying Yang5, Ban-Yuan Kuo6, Bill Fry7, and Spahr Webb8
Weiwei Wang et al.
  • 1School of Geography, Environment and Earth Sciences, Victoria University of Wellington, Wellington, New Zealand
  • 2Department of Geosciences, National Taiwan University, Taipei, Taiwan
  • 3Institute of Geophysics & Geomatics, China University of Geosciences (Wuhan), Wuhan, China
  • 4Department of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan
  • 5School of Earth and Space Sciences, University of Science and Technology of China, Hefei, China
  • 6Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan
  • 7GNS Science, Wellington, New Zealand
  • 8Lamont-Doherty Earth Observatory, Columbia University, New York, US

In recent years, Ocean Bottom Seismometers (OBSs) have become widely used to expand the coverage of seismic networks onto the ocean. This study takes advantage of offshore observations at the northern end of the Hikurangi margin and southwestern Okinawa Trough to study the tectonics in both regions.

In the Hikurangi subduction zone, slow slip events (SSEs) have been observed, which are caused by the subduction of the Pacific Plate under New Zealand. The behaviour of SSEs and how they influence the physical properties of Earth materials are open to question. From 2014 to 2015, 15 OBSs were deployed offshore Gisborne, New Zealand on the Hikurangi margin. Ambient noise data from the OBSs are used to study velocity changes related to SSEs. Single station cross-component correlations and auto-correlations are computed, from which coda waves are used to monitor the velocity changes before, during and after the SSEs in 2014 and 2015 to analyse the slow earthquake behaviour and its relation to stress changes. Different rotation on horizontal components is tested by rotating horizontal components to N-E direction and parallel-perpendicular to the coastline. The dv/v computed by different components or rotation show different changes. The averaged dv/v displays a 0.1% velocity decrease during the SSE in October 2014.

The southwestern Okinawa Trough tapers towards Taiwan. How the back-arc crust accommodates the narrowing processing remains to be understood. At various times between 2010 and 2017, 22 OBSs on a small scale (~0.2°×0.3°) were deployed in Southwestern Okinawa Trough offshore northeast Taiwan. Ambient noise recorded on vertical velocity and pressure sensors is used to retrieve Scholte waves for studying shear wave velocity structure. Phase velocities are forward-modeled according to a model proposed by Kuo et al. 2015 and shear strength and density results from ODP1202. Phase velocity dispersion curves are measured from cross-correlations and unwrapped according to the modeled phase velocities. The fundamental mode phase velocities averaged from different station pairs are 0.62 km/s at 3 s period and 1.56 km/s at 6.5 s period. A 3D inversion will be conducted for a shear wave velocity structure from the basin center to the edge.

How to cite: Wang, W., Savage, M., Yates, A., Hung, S.-H., Luo, Y., Stern, T., Lin, P.-Y. P., Yang, H.-Y., Kuo, B.-Y., Fry, B., and Webb, S.: Studies of seismic velocities in subduction zones from continuous OBS data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12063, https://doi.org/10.5194/egusphere-egu2020-12063, 2020

Comments on the presentation

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Presentation version 1 – uploaded on 02 May 2020
  • CC1: Comment on EGU2020-12063, Anne Sheehan, 04 May 2020

    Hi Weiwei, Thanks for posting your EGU presentation! Is it important to use the horizontal components to determine dv/v, or are the vertical components sufficient? Thanks, Anne Sheehan

    • AC1: Reply to CC1, Weiwei Wang, 04 May 2020

      Hi Anne,

      Thanks for the question. Most results from single station cross-component ZN, ZE, and NE are better than auto-correlations like ZZ. In our case, the horizontal components are important as a part of the cross-component correlations. Our assessment is to compute all the available correlations and compare their lag-time dependent SNR and coherence, and use the ones with high SNR and coherence. In our case, the auto-correlations are noisy and the cross-component correlations have high SNR and therefore they are used to compute dv/v.

      If the ZZ correlations have high SNR (> 2 for at least 70 s window), maybe they are sufficient to determine dv/v but it's still better to compare with cross-components because the dv/v computation is hard to be stable and so it's difficult to tell if the velocity variations are true or not. Try to include more data (like more components) will be helpful.

      Cheers, Weiwei

      • CC2: Reply to AC1, Anne Sheehan, 04 May 2020

        Thanks for the quick reply. Did you try to obtain dv/v by two-station methods? Why the emphasis on single station methods - is that a more typical approach?

        • AC2: Reply to CC2, Weiwei Wang, 04 May 2020

          Single-station is used is because the clock issue on OBSs can bias the dt computation. I also tried to compute dv/v by station pairs using the stations with good timing, the results show a similar pattern to the dv/v computed by single-station but it's a bit noisy. 

  • CC3: Comment on EGU2020-12063, Xiaozhou Yang, 05 May 2020

    Hi, Weiwei. Thank you for your wonderful presentation! It's very helpful for my work.

    Could you briefly tell me what is the clock issue of OBSs?

    • AC3: Reply to CC3, Weiwei Wang, 05 May 2020

      Hello, Yuan dundun,

      Because OBSs are deployed under the sea, they are not able to be equipped with GPS to have an accurate clock. The pressure and temperature changes in the deep sea can bias the timing of the clock. 

      Cheers, www