Soil Classification using the H/V Technique
by Rania Bedair, Ph.D. Eng
Today, statistical estimates of the probability of a large earthquake and the identification of the highest seismic hazardous areas are available. However, the lack of information about local subsoil conditions and the vulnerability of buildings limits the usefulness of these estimates. In fact, identifying the sites of local seismic amplification and the buildings which would undertake the most damage under the seismic shaking is crucial.
Buildings damaged by the 1999 earthquake in the Izmit area, Turkey (image: U.S. Geological Survey)
The effect of geology and soil properties on the intensity of ground motions during earthquakes is signiﬁcant. It may lead to very significant amplification at specific frequencies at some sites in the affected area and, consequently, great damage during earthquakes. Shaking is increased in soft, thick, wet soils. In certain cases, the ground surface may settle or slide. One of the most important seismic properties is the resonance frequency of the subsoil (the frequency at which the maximum-amplitude oscillation occurs).
In contrast to active seismic methods (e.g., refraction, reflection or surface-wave), which use an artificial source such as an explosive charge or hammer blow to excite a seismic response from the subsurface, the H/V method is a passive method that uses three-component measurements of ambient seismic noise (microtremors induced by wind, ocean waves, anthropogenic activity, etc.) to determine and evaluate a site’s fundamental seismic resonance frequency.
With highly sensitive vibration sensors, applying the H/V technique is considered a low cost, reliable and effective tool in estimating resonance frequency (f), which depends on the shear wave velocity (v) and the thickness of the resonating layer (h) where n is the number of modes.
In the case of higher modes, for example, the peak with the lowest frequency on the H/V curve is the fundamental mode (generally the bedrock cover limit) and the other peaks correspond to other geological limits which also cause seismic motion amplification.
Geological limits found in H/V plot peaks
(image: U.S. Geological Survey)
The H/V technique has been used for microzonation studies to predict site response to earthquake seismicity and to estimate the unconsolidated sediment thickness, map the bedrock surface and find inter-fault locations. This method is most effective in the case of soft soil sites when there is a large impedance contrast with the underlying bedrock. According to the Site Effects Assessment using Ambient Excitations (SESAME) by the European Research Project, the method is especially recommended in areas of low and moderate seismicity where significant earthquake recordings are limited as compared to high seismicity areas.
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