HVSR

HVSR, or Horizontal-to-Vertical Spectral Ratio, is a passive seismic method used to characterize shallow subsurface structure by comparing the amplitude spectra of two horizontal ground-motion components with the vertical component. It is widely applied to assess site amplification potential, estimate sediment thickness, and provide constraints on shallow shear-wave velocity profiles.

The method relies on recording ambient seismic noise or weak earthquakes with three-component sensors. The Fourier spectra of the two horizontal components (commonly north-south and east-west) and the vertical component are computed and then combined to form the HVSR curve. A common approach is to compute the average horizontal spectrum and form the ratio HVSR(f) = H(f)/V(f), where H is the horizontal component spectrum and V is the vertical component spectrum. Smoothing and windowing are typically applied to reduce variability, and the peak frequency in the HVSR curve is interpreted as an impedance-contrast frequency, often related to a strong contrast between a superficial low-velocity layer and a higher-velocity substrate.

Interpretation of the HVSR curve yields the fundamental resonance frequency, f0, which can be related to the thickness and shear-wave velocity of subsurface layers. In simple two-layer models, f0 ≈ Vs/(4H). However, the method is non-unique and sensitive to the subsurface geometry, basin effects, and noise, so HVSR results are usually integrated with other geophysical data and geological information.

Applications include seismic microzonation, site effect assessment, and preliminary subsurface characterization for engineering projects. Practical use emphasizes data quality, station placement, adequate recording duration, and awareness of the method’s limitations.