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Seismic response of unstable rockslopes: Topographic site effect?

Seismic response of unstable rockslopes: Topographic site effect?. J. Burj ánek, G. Gassner- Stamm, V. Poggi, D. F äh. Swiss Seismological Service, ETH Z ü rich. in collaboration with. J. Moore, V. Gischig, F. Yugsi. Geological Institute, ETH Z ü rich. Switzerland. Sites overview.

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Seismic response of unstable rockslopes: Topographic site effect?

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  1. Seismic response of unstable rockslopes: Topographic site effect? J. Burjánek, G. Gassner-Stamm, V. Poggi, D. Fäh Swiss Seismological Service, ETH Zürich in collaboration with J. Moore, V. Gischig, F. Yugsi Geological Institute, ETH Zürich

  2. Switzerland Sites overview • Walkerschmatt (potential rockfall) • Randa (rockfall) Visp Walkerschmatt Randa Zermatt

  3. Walkerschmatt site 1 day: 12 LE3D-5s sensors = 2 arrays (23 measuring points) 100m

  4. Time-frequency polarization analysis • We characterize the particle motion for each time step and scale by an ellipse (tilted in 3D space). We look at three parameters of such an ellipse: • strike (F) – an azimuth (in horizontal plane) of the main axis of the ellipse • dip (d) – an inclination (in vertical plane) of the main axis of the ellipse • ellipticity (tan e) – ‘thickness’ of the ellipse (equal to 1 for circular polarization and to 0 for linear polarization) • All three parameters depend on time and scale! (Burjanek et al., 2010)

  5. Statistics over time

  6. Comparison with directional H/V

  7. Comparison with directional H/V TF polarization analysis clearly outperforms directional H/V!

  8. Results for Walkerschmatt Stable Unstable (Burjanek et al., to be submitted to JGR)

  9. Polarization analysis (1-2 Hz)

  10. Site-to-reference spectral ratios

  11. Site-to-reference spectral ratios Z

  12. Site-to-reference spectral ratios

  13. Site-to-reference spectral ratios Horizontal c. Vertical c.

  14. High-resolution f-k analysis

  15. Summary for Walkerschmatt • Polarization analysis identified a single frequency, where the ambient vibrations is polarized in same direction (perpendicular to the main cracks on the surface) for all stations on unstable part. • Same resonant frequency was identified from the site-to-reference spectral ratios (on noise). • Amplitude of site-to-reference ratio at resonant frequency increases toward the cliff. • Result of high-resolution beamforming presents sharp increase of apparent velocity close to resonant frequency. • Can be explained by presence of 2D/3D resonance.

  16. Randa site

  17. Measurements overview 100 m

  18. Site-to-reference spectral ratios – azimuth 135

  19. Site-to-reference spectral ratios – azimuth 135

  20. Site-to-reference spectral

  21. Structural model

  22. Numerical model 2D in-plane model Moore et al., submitted to BSSA

  23. Numerical simulations - results

  24. Concluding remarks • Almost no amplification due to local topography changes with respect to our reference sites. • The strong directional site response observed is likely caused by the fractured nature of the medium. • The density of measurement points is critical for interpretation of the seismic response. • TF polarization analysis clearly outperforms directional H/V. • Noise polarization analysis could be performed with single-station measurements.

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