The horizontal to vertical spectral ratio (HVSR) of ambient noise provides information on the fundamental natural frequency of the local sediments when there is a large acoustic impedance contrast between sediments and underlying rocks.

The HVSR curve, computed for a seismic station, in effect represents station’s site response and may contain peaks at resonant frequencies associated with local sediments. For stations at high-latitudes (like TA stations in Alaska) these HVSR peaks may be affected by seasonal temperature variations due to impedance contrasts between frozen and thawed ground (Lee et al., 2014).

Estimate of natural frequency of the deposits obtained by HVSR technique also has engineering applications. McNamara et al. (2014) computed HVSR peak frequencies for 218 seismic stations in the eastern US and demonstrated that HVSR can be used as a proxy to estimate Vs30* in eastern US.

The HVSR package provides a set of tools to compute and plot HVSR curves using combination of IRIS's MUSTANG (http:https://service.iris.edu/mustang/) noise-psd web service (returns Power Spectral Density estimates for seismic channels) and MUSTANG’s noise-pdf web service (returns Probability Density Functions for seismic channels). The package also adopts SESAME** 2004 guidelines for automated identification of potential HVSR’s peaks corresponding to the fundamental natural frequency (f0) of the deposits.

Lee, R.F., R. E. Abbott, H. A. Knox, and A. Pancha, 2014, Seasonal Changes in H/V Spectral Ratio at High Latitude Seismic Stations, Abstract S41A-4438 presented at 2014 Fall Meeting, AGU, San Francisco, Calif., 15-19 Dec. http:https://adsabs.harvard.edu/abs/2014AGUFM.S41A4438L

McNamara, D.E., Stephenson, W.J., Odum, J.K., et al., 2014, Site response in the eastern United States: A comparison of Vs30 measurements with estimates from horizontal:vertical spectral ratios, in Geological Society of America Special Papers, published online September 30, 2014;doi:10.1130/2015.2509(04). https://profile.usgs.gov/myscience/upload_folder/ci2014Oct0210281844121McNamara-2015.2509_04.pdf

SESAME, 2004, Guidelines for the implementation of the H/V spectral ratio technique on ambient vibrations: measurements, processing and interpretation. European Commission – Research General Directorate Project No. EVG1-CT-2000-00026. ftp:https://ftp.geo.uib.no/pub/seismo/SOFTWARE/SESAME/USER-GUIDELINES/SESAME-HV-User-Guidelines.pdf

* Vs30 is the time-weighted average shear-wave velocity between 0 and 30-meters depth and is in wide-spread use for characterizing site response for earthquake resistant design of buildings and large structures.

** European Site EffectS assessment using AMbient Excitations