Seismic Microzonation

It has long been recognized that the intensity of ground shaking during earthquakes and the associated damage to structures are significantly influenced by local geologic and soil conditions (Seed and Schnabel, 1972). Unconsolidated sediments are found to amplify ground motion during earthquakes and are hence more prone to earthquake damage than ground with hard strata. Structures built on soft sediments are especially vulnerable to damage caused by amplified ground motions.

The Seismic Microzonation is the process of estimating the response of soil layers under earthquake excitations and thus the variation of earthquake characteristics on the ground surface (Finn, 1991), and it is important in sites prepared for essential or critical structures located in areas with medium and high seismic activities.

Dynamic characteristics of a site such as predominant period, amplification factor, shear wave velocity, and standard penetration test values can be used for seismic microzonation purposes. However, the more direct method to make a fast and precise seismic microzonation is using Environmental Seismic Vibrations (known as microtremors or seismic noise) measurement for determining some dynamic properties of soil strata (Nogoshi and Igarashi, 1970; Nakamura, 1989; Molnar et al., 2018, 2022; Bonnefoy-Claudet et al., 2006). This method is extensively used for microzonation, mainly because it is easy to perform and can be applied to any location.

The Seismic Microzonation is very important for a precise evaluation of seismic risk, which in some cases helps to reduce the seismic risk (Schell et al., 1978), because the damage incurred in an earthquake depends not only on the earthquake magnitude (source) but also, to a large extent, on the medium through which the seismic waves propagate (site effects).

This service uses records of Environmental Seismic Vibrations (microtremors or seismic noise) to apply the Horizontal-to-Vertical Spectral Ratio (HVSR) technique to study seismic site effects in the project area. The HVSR applied to microtremor data has been proven to be an effective and practical tool for assessing local site effects (Perdhana and Nurcahya, 2019; Lermo and Chávez-García, 1993; Imposa et al., 2004; Rosa-Cintas et al., 2017; Souriau et al., 2007; Bonnefoy-Claudet et al., 2009). From this technique, the site’s predominant frequency or fundamental resonance frequency (f₀) and the site amplification factor (A₀) can be obtained and mapped.

The HVSR method uses a very sensitive three-component seismometer that measures seismic noise in three orthogonal directions (two horizontal and one vertical). The raw seismic-trace data show the amplitudes of seismic noise in the time domain for each direction. Seismic-noise amplitude is time-varying, and recorded traces are often filtered to remove high-frequency impulse and anthropogenic noise before being transformed to the frequency domain (Koller et al., 2004).

The two horizontal components are merged into a single frequency spectrum, which is used to compute the ratio of horizontal-to-vertical (H/V) spectra. This H/V spectral ratio remains consistent despite time-varying factors and can be reliably used to estimate the fundamental resonance frequency.

The Seismic Microzonation Survey can help correct the Peak Ground Acceleration (PGA) estimate for a site using the site amplification factor. It is an additional service and not necessary in all cases but is very important for locations with large soil layers or sites situated in sedimentary basins (see figure below).

The figure above shows the variation of the peak ground acceleration ratios between observed and predicted values and mean ratios for rock, hard-, medium-, and soft-soil sites (figure adapted from Fukushima and Tanaka, 1990). The large variations are mainly related to the site characteristics, which can be estimated using the Seismic Microzonation service.

References

  • Bonnefoy-Claudet, S., Baize, S., Bonilla, L. F., Berge-Thierry, C., Pasten, C., Campos, J., Volant, P., & Verdugo, R. (2009). Site effect evaluation in the basin of Santiago de Chile using ambient noise measurements. Geophysical Journal International, 176(3), 925–937. https://doi.org/10.1111/j.1365-246x.2008.04020.x
  • Bonnefoy-Claudet, S., Cornou, C., Bard, P.-Y., Cotton, F., Moczo, P., Kristek, J., & Fäh, D. (2006). H/V ratio: A tool for site effects evaluation. Results from 1-D noise simulations. Geophysical Journal International, 167(2), 827–837. https://doi.org/10.1111/j.1365-246x.2006.03154.x
  • Finn, W.D.L. (1991). Geotechnical Engineering Aspects of Microzonation. Proc. 4th International Conference on Seismic Zonation, 1, 199–259.
  • Imposa, S. et al. (2004). Site effects close to structural lineaments in eastern Sicily (Italy). Engineering Geology.
  • Koller, M.G.; Chatelain, J.-L.; Guillier, B.; Duval, A.-M.; Atakan, K.; Lacave, C.; Bard, P.-Y. and SESAME participants (2004). Practical user guidelines and software for the implementation of the H/V ratio technique: measuring conditions, processing method and results interpretation. 13th World Conference on Earthquake Engineering, Vancouver, Canada, WCEE, 10 p.
  • Lermo, J. and Chavez-Garcia, F.J. (1993). Site Effect Evaluation Using Spectral Ratios with Only One Station. Bulletin of the Seismological Society of America, 83(5), 1574–1594.
  • Molnar, S., Cassidy, J. F., Castellaro, S., Cornou, C., Crow, H., Hunter, J. A., Matsushima, S., Sánchez-Sesma, F. J., & Yong, A. (2018). Application of microtremor horizontal-to-vertical spectral ratio (MHVSR) analysis for site characterization: State of the art. Surveys in Geophysics, 39(4), 613–631. https://doi.org/10.1007/s10712-018-9464-4
  • Molnar, S., Sirohey, A., Assaf, J., Bard, P.-Y., Castellaro, S., Cornou, C., Cox, B., Guillier, B., Hassani, B., Kawase, H., Matsushima, S., Sánchez-Sesma, F. J., & Yong, A. (2022). A review of the microtremor horizontal-to-vertical spectral ratio (MHVSR) method. Journal of Seismology, 26(4), 653–685. https://doi.org/10.1007/s10950-021-10062-9
  • Nakamura, Y. (1989). A method for dynamic characteristics estimations of subsurface using microtremors on the ground surface. Quarterly Report Railway Technical Research Institute, 30(1), 25–33.
  • Nogoshi, M., and Igarashi, T. (1970). On the amplitude characteristics of microtremor (part 1). Zisin (Journal of the Seismological Society of Japan, 2nd Series), 23(4), 281–303. https://doi.org/10.4294/zisin1948.23.4_281
  • Perdhana, R. and Nurcahya, B.E. (2019). Seismic microzonation based on microseismic data and damage distribution of the 2006 Yogyakarta Earthquake. The 4th International Conference on Science and Technology, E3S Web of Conferences, 76, 03008. https://doi.org/10.1051/e3sconf/20197603008
  • Rosa-Cintas, S. et al. (2017). Characterization of the shear wave velocity in the metropolitan area of Málaga (S Spain) using the H/V technique. Soil Dynamics and Earthquake Engineering.
  • Schell, B. A. et al. (1978). Seismotectonic Microzonation for Earthquake Risk Reduction. Proc. of Second International Conference on Microzonation for Safer Construction – Research and Application, vol. I, pp. 571–583.
  • Seed, H. B. and Schnabel, P. B. (1972). Soil and Geological Effects on Site Response During Earthquakes. Proc. of First International Conference on Microzonation for Safer Construction – Research and Application, vol. I, 61–74.
  • Souriau, A., Roulle, A., & Ponsolles, C. (2007). Site effects in the city of Lourdes, France, from H/V measurements: implications for seismic-risk evaluation. Bulletin of the Seismological Society of America, 97(6), 2118–2136. https://doi.org/10.1785/0120060224