On the Computation of H/V and its Application to Microzonation and Seismic Design

The H/V ratio is the square root of the ratio of horizontal to vertical energies of ground motion. It has been observed that the frequency of the main peak is well suited for the characterization of site effects and had been widely used for micro-zonation and seismic structural design. Historically that ratio was made from the average of individual H/V ratios obtained from noise autocorrelations. Nevertheless, it has been recently pointed out that the H/V ratio should be calculated differently as the ratio of the average of H over the average of V. This calculation is based on the relation between the directional energies (the imaginary part of Green’s function) and the noise autocorrelations. In general, the average of ratios is different from the ratio of averages.

Although the frequency of the main response was correctly obtained, the associated amplification factor has generally been badly predicted, having little matching with the amplification observed during strong earthquakes. The unexpected decay behavior of such ratios at high frequency and the lack of stability and reproducibility of the H/V ratios are other problems that face the method.

These problems are addressed here from the point of view of normalization of noise correlations. In fact, several normalization techniques have already been proposed in order to correctly retrieve the Green’s function. Some of them are well suited for the retrieval of the surface wave contribution, while others are more appropriate for bulk wave incidence. Since the H/V ratio may be used for various purposes like surface wave tomography, micro-zonation or seismic design, different normalizations are discussed in functions of the objectives. The H/V obtained from local historical earthquakes on top or far away from the subduction zone are also discussed.

 ACKNOWLEDGEMENT This research has been partially supported by DGAPA-UNAM under Project IN104712 and the AXA Research Fund.