Earthquake Records Alone Can Now Reveal the Soil Beneath Seismic Stations

Every earthquake that rattles a city does so through the ground beneath it, and the character of that ground can make the difference between a building that survives and one that collapses. Soft sediments can amplify shaking dramatically, which is why engineers classify every seismic monitoring station according to the stiffness of the soil below it. The gold standard for this classification is a measurement of the shear-wave velocity, or Vs, profile: a vertical map of how fast seismic shear waves travel through each layer of soil and rock. Traditionally, obtaining that map requires expensive boreholes or invasive geophysical surveys, and for thousands of stations around the world, the data simply do not exist. A new study published in the Bulletin of Earthquake Engineering offers a striking alternative: reconstruct the entire velocity profile from earthquake records alone, using nothing more than a single seismic station and a clever inversion scheme.

The research, carried out by Nasser Laouami of the National Applied Research Centre in Earthquake Engineering in Algeria, tackles one of the most persistent gaps in seismic hazard assessment. Many strong-motion stations worldwide were installed decades ago, often on the roofs or basements of buildings, with little or no accompanying subsurface investigation. Drilling new boreholes at each site is costly and time-consuming, so the average shear-wave velocity of the top 30 metres, known as Vs30, remains unknown or poorly constrained for a large fraction of the global seismic network. Yet Vs30 is the single parameter that most building codes, including the American NEHRP provisions and Eurocode 8, use to assign a site class and therefore to determine the design forces a structure must withstand. Without reliable Vs30 values, the ground-motion predictions that feed into hazard maps carry an extra layer of uncertainty.

Laouami’s approach exploits a phenomenon that seismologists have long used as a diagnostic tool: the horizontal-to-vertical spectral ratio, or H/V. When earthquake waves arrive at a station, the horizontal components of motion are typically amplified relative to the vertical component at frequencies where the shallow soil column resonates. That resonance frequency depends on the thickness and stiffness of the sedimentary layers, while the shape and amplitude of the H/V curve encode information about the impedance contrasts between layers. In principle, the entire H/V spectrum is a fingerprint of the subsurface velocity structure. The challenge has always been to invert that fingerprint, meaning to find a layered velocity model whose predicted H/V curve matches the observed one, without falling into the trap of non-uniqueness, where many different models fit the data equally well.

What makes the new method notable is that it requires no prior information about the subsurface. Most inversion schemes in geophysics lean heavily on initial guesses: an approximate depth to bedrock, a rough velocity for the top layer, or a fixed number of layers. When such constraints are wrong, the inversion can converge to a misleading solution. The new technique instead assumes that the strong-motion wavefield at the station is dominated by vertically incident body waves, a reasonable approximation for stations recording earthquakes at regional distances, and then searches for the velocity profile that best reproduces the observed H/V spectrum. The inversion is statistical in character, exploring a wide space of possible layered models and identifying the family of solutions consistent with the data rather than committing to a single profile that may be an artefact of the starting conditions.

To test the method rigorously, Laouami turned to one of the best-characterized seismic datasets on Earth: Japan’s KIK-NET network, operated by the National Research Institute for Earth Science and Disaster Resilience. KIK-NET stations are paired with boreholes that reach stiff engineering bedrock, and the shear-wave velocity profile at each site has been measured directly. That makes the network an ideal proving ground, because the true answer is known. The study used strong-motion records and measured velocity profiles from 66 KIK-NET stations, spanning a wide range of soil conditions from soft alluvial basins to relatively stiff sites. The inverted profiles were compared layer by layer against the borehole measurements, and the key classification parameter, Vs30, was compared station by station.

The results were evaluated in two configurations. In the first, the inversion ran completely unconstrained, with no prior information whatsoever about the subsurface. In the second, a small piece of shallow information was supplied: the average shear-wave velocity of the top five metres, or Vs5, which can be obtained cheaply from non-invasive surface measurements. The comparison revealed a clear and useful pattern. Without any prior information, the method produced reliable velocity profiles for sites whose Vs30 values reach approximately 600 metres per second, which covers the softer end of the soil spectrum, including many of the most seismically hazardous sedimentary settings. For stiffer profiles beyond that threshold, the unconstrained inversion began to lose accuracy, but adding the modest Vs5 constraint restored performance, extending reliable estimates to stiffer sites as well.

The quantitative agreement is impressive for a technique that requires no drilling. For most of the 66 stations, the relative error between the observed and predicted Vs30 was less than 20 percent, and the error in the predominant frequency of the H/V curve, the frequency at which the site resonates most strongly, was around 15 percent. In geotechnical and geophysical practice, both levels of accuracy are generally considered acceptable for preliminary site characterization. Perhaps more importantly, the method captured the overall shape of the velocity profiles, including the depth of major impedance contrasts, rather than merely matching a single averaged number. That matters because site response is not determined by Vs30 alone; the full layering controls how ground motion is amplified across the frequency range that affects buildings of different heights.

The implications extend well beyond Japan. Dense seismic networks exist in many earthquake-prone countries, but borehole measurements are far rarer, particularly in developing regions where instrumentation has expanded faster than site characterization budgets. A technique that extracts velocity profiles from recordings that stations are already producing, at no additional field cost, could rapidly fill the characterization gap. It could also serve as a first-pass screening tool: stations flagged by the H/V inversion as sitting on unusually soft or strongly resonant soils could be prioritized for follow-up borehole investigation. In regions governed by seismic codes that rely on Vs30-based site classes, such as the Algerian RPA2024 regulations or the NEHRP provisions, even a preliminary estimate with 20 percent accuracy can meaningfully improve hazard maps and design decisions.

The method also fits into a broader scientific conversation about what the H/V spectral ratio actually measures. Earlier theoretical work, including diffuse-field formulations of the H/V ratio developed by researchers such as Kawase, Sánchez-Sesma and Matsushima, showed that the ratio can be interpreted through the properties of the wavefield and the layered medium beneath the receiver. Laouami’s contribution is to demonstrate, against ground truth, that a body-wave assumption applied to strong-motion records is sufficient to recover the velocity structure without prior constraints, at least for a substantial range of site stiffness. The technique complements rather than replaces established non-invasive methods such as surface-wave analysis, which suffers from its own well-documented non-uniqueness problems, and it inherits the practical advantage of requiring only a single three-component station rather than an array of sensors.

Limitations remain, and the author is careful about them. The assumption of vertically incident body waves may weaken at sites with complex three-dimensional structure or strong lateral heterogeneity, and the accuracy degrades for very stiff sites unless shallow information is supplied. The validation set, while diverse, comes from a single national network with high-quality borehole data, so performance in other tectonic and sedimentary environments will need independent confirmation. Nevertheless, the core result stands: for a wide range of soil profiles, the ground beneath a seismic station can be mapped from its own earthquake recordings, with errors small enough to support site classification. In a field where every unknown parameter translates directly into uncertainty in shaking estimates and building safety margins, a reliable, non-invasive, zero-drilling method for recovering shear-wave velocity profiles is a genuinely valuable addition to the earthquake engineering toolkit, and one that could be applied to thousands of existing stations worldwide almost immediately.

Subject of Research: Non-invasive estimation of S-wave velocity profiles from single-station H/V spectral ratio inversion of strong-motion records

Article Title: Estimating S-wave velocity profiles from single-station horizontal-to-vertical spectral ratio inversion without prior information: application for KIK-NET accelerometric stations

Article References: Laouami, N. (2026). Estimating S-wave velocity profiles from single-station horizontal-to-vertical spectral ratio inversion without prior information: application for KIK-NET accelerometric stations. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02660-0

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02660-0

Keywords: shear-wave velocity, H/V spectral ratio, inversion, Vs30, site characterization, KIK-NET, strong-motion records, seismic station classification, earthquake engineering, geophysics, non-invasive methods, soil amplification