Using Microseisms for Seismic Microzoning

The efficiency of using microseisms for seismic microzoning (SMZ) is discussed based on experimental field data in different geological conditions. The microseism method is in the list of recommendations and codes and is widely used because of its low cost. In SMZ, microseisms are used to estimate the shaking intensity increment by calculating the amplitude spectra at seismic stations and resonance characteristics of ground layers, usually in combination with other methods. Difficulties in taking into account numerous local noise sources may significantly reduce the accuracy of the results. This has to be considered when planning field works for each object (site). In some cases (high anthropogenic noise level), it is better to refrain from using the spectral amplitude ratio in the high frequency band (above 3 Hz). Calculation of intensity increments individually in the 0.5–2 and 2–3 Hz bands, taking as the final increment the maximum of both calculations, guarantees the due level of conservatism; hence, the microseism method is also applicable to critical facilities. The spectral ratio of the horizontal and vertical components of seismic motion (H/V) can be used to estimate the resonance characteristics of soils and structure of the upper part of the soil profile, in particular, the depth of the soil/bedrock boundary.