EGU2020-2606
https://doi.org/10.5194/egusphere-egu2020-2606
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Seismic Microzonation using 6C Measurements

Sabrina Keil, Joachim Wassermann, and Heiner Igel
Sabrina Keil et al.
  • Geophysik, Ludwig-Maximilians-Universität, München, Germany

Microzonation is one of the essential tools in seismology to mitigate earthquake damage by estimating the near surface velocity structure and developing land usage plans and intelligent building design. The number of microzonation studies increased in the last few years as induced seismicity becomes more relevant, even in low risk areas. While of vital importance, especially in densely populated cities, most of the traditional techniques suffer from different short comings. The microzonation technique presented here tries to reduce the existing ambiguity of the inversion results by the combination of single-station six-component (6C) measurements, including three translational and three rotational motions, and more traditional H/V techniques. By applying this new technique to a microzonation study in Munichs (Germany) inner city using an iXblue blueSeis-3A rotational motion sensor together with a Nanometrics Trillium Compact seismometer we were able to estimate Love and Rayleigh wave dispersion curves. These curves together with H/V spectral ratios are then inverted to obtain shear wave velocity profiles of the upper 100 m. The resulting 1D velocity profiles are used to estimate the local shaking characteristics in Munich. In addition, the comparison between the estimated velocity models and the borehole-derived lithology gives a positive correlation, indicating the applicability of our method.

How to cite: Keil, S., Wassermann, J., and Igel, H.: Seismic Microzonation using 6C Measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2606, https://doi.org/10.5194/egusphere-egu2020-2606, 2020

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Presentation version 1 – uploaded on 30 Apr 2020
  • CC1: Comment on EGU2020-2606, Hans-Balder Havenith, 03 May 2020

    Dear Sabrina,

     

    you write that for the 6C the noise level (for the rotational component)  is reached below 5Hz .. it that site-dependent? ..becaus you're in a city, ...

    or general (which would be a mjor limitation to its application). Do you have a comparison with a classical array analysis?

    ..you can leave the answer for tomorrow for the chat if you participate.

    yours

    Hans-Balder

     

  • CC2: Comment on EGU2020-2606, Daniel Bowden, 04 May 2020

    Nice study, it will be interesting to see how this develops!
    As a question - you claim that velocities decrease above 5Hz because there's less signal. But could it be that velocities should decrease in shallow sediments? (Higher freq -> shallower eigenfunctions -> slower sediments?).
    As long as you're confident you're seeing Love waves, the amplitudes of those signals shouldn't affect the measurement. Maybe I need to go read some of the references, as to how you actually make the dispersion curves?

    • AC1: Reply to CC2, Sabrina Keil, 04 May 2020

      Thank you for your questions.

      I think there is a little misunderstanding: I claim that the velocities decreas BELOW 5Hz because the rotation rates are too small to be recorded by the sensor. In the PSD plot it can be seen that the rotation traces are flat below 5Hz because it reached the self-noise level of the rotational sensor, therefore, the ratio between translational and rotational motion (which gives us the phase velocity as shown in the formulas) decreases and thus the dispersion curve drops for frequencies < 5Hz. Lower frequencies are more sensitive to deeper structures and therefore the velocity in this frequency range shouldn´t be influenced by the shallow sediments. For higher frequencies this is of course true.

      The relation between translational and rotational motion is based on the assumption of fundamental mode surface waves. In reality we could have a more complex wavefield, which could influence our results.

      The methodology for the analysis of the 6C data is given in Wassermann et al. (2016) (shown on the reference slide).

      Don´t hesitate if you have further questions.

      • CC3: Reply to AC1, Daniel Bowden, 04 May 2020

        You're absolutely right, I misread! I agree that a decrease below 5Hz is odd.

        And as for the Rayleigh-waves vs. other waves biasing your observation, I'll take a look at the Wassermann reference. Thanks!