Site-Specific Relationships between Bedrock Depth and HVSR Fundamental Resonance Frequency Using KiK-NET Data from Japan

Abstract

It is commonly accepted that the horizontal-to-vertical spectral ratio (HVSR) technique enables the detection of the fundamental resonance frequency ($f_{\text{HVSR}}$) of a given site. The utility of this $f_{\text{HVSR}}$ is analyzed using the nonlinear regression relationships between $f_{\text{HVSR}}$ and bedrock depth ($h$). The derived relationships are mostly site-specific, so that the present paper consists of two main parts. The first is a literature review for the available empirical relationships between $f_{\text{HVSR}}$ and $h$. The aim of this part is to highlight the practical limitations of these established relationships and to make fair comparisons. The second is to generate new relationships, taking advantage of the very wide range of available lithological, geophysical, and geotechnical borehole drilling data of the 697 KiK-NET seismic stations in Japan. For this purpose, HVSR are calculated using 10,000 weak earthquakes or linear events recorded at KiK-NET stations to determine the $f_{\text{HVSR}}$ and correlate it with the corresponding $h$. The overlying layers/bedrock interface falling within sedimentary, igneous, or metamorphic layers significantly affect the derived frequency–depth relationships. In addition, these relationships are strongly reproduced by the $V_{\text{p}} /V_{\text{s}}$ ratio of the bedrock in the range of 1.6–2.2. Interestingly, it is found that $f_{\text{HVSR}}$ less than 1 Hz corresponding to $h$ more than 100 m leads the trend of the overall frequency–depth relationship.