Why does shear wave velocity matter so much? When an earthquake strikes, the ground does not shake uniformly everywhere. The speed at which shear waves, the transverse waves that produce most of the destructive shaking in buildings, propagate through the upper layers of soil fundamentally controls how strongly the ground amplifies seismic motion. Soft, loose sediments can slow these waves dramatically and trap their energy, turning a moderate distant earthquake into a locally violent shaking event. Hard, dense soils and rock, by contrast, transmit the waves quickly and tend to amplify them less. This is why modern seismic design codes around the world, from ASCE 7-16 in the United States to Eurocode 8 in Europe, classify building sites primarily by the average shear wave velocity in the top thirty meters, a parameter known as VS30. Getting this number right is one of the first and most consequential steps in designing an earthquake-resistant structure.

The gold-standard way to measure shear wave velocity is geophysical: techniques such as the Multichannel Analysis of Surface Waves, or MASW, record seismic waves at the ground surface and invert their dispersion to recover a velocity profile with depth. In the one-dimensional MASW variant used in this study, a seismic source generates Rayleigh surface waves, an array of geophones records them, and the analysis exploits the fact that different frequency components sample different depths. The method is non-invasive, relatively fast and reliable, and its good practice has been codified in international guidelines such as the InterPACIFIC project recommendations. But in many urban investigations, particularly in developing regions, budgets and schedules do not always allow dedicated geophysical surveys at every site of interest. Boreholes with penetration testing, however, are drilled routinely for ordinary building projects.

That is where the Standard Penetration Test enters the story. The SPT, standardized as ASTM D1586, is one of the most widely performed in-situ geotechnical tests in the world. A split-barrel sampler is driven into the soil at the bottom of a borehole by a standardized hammer, and engineers count the number of blows required to advance the sampler a specified distance, yielding the N value. This humble number encodes a great deal about soil density and consistency: loose sands and soft clays yield low blow counts, while dense gravels and stiff clays demand many more. Because SPT data already exist in thousands of archived borehole logs, an empirical correlation linking N values to shear wave velocity would allow engineers to retrofit velocity estimates onto decades of existing geotechnical records, effectively converting old borehole data into modern seismic site classifications without drilling a single new hole.

Correlations of this kind have been published for many cities and soil types worldwide, from Lucknow and Kolkata in India to Korean alluvial deposits and Norwegian clays, but the literature carries a consistent warning: these relationships are strongly region-specific. Soil genesis, mineralogy, stress history and cementation all influence both penetration resistance and shear stiffness, and a formula calibrated in one basin can be badly biased when exported to another. For Cochabamba, a city of more than a million people sitting in a seismically active intermontane valley of the Bolivian Andes, no locally calibrated relationship existed for its fine-grained soils. The Bolivian seismic design norm and the national geotechnical study standard both demand site characterization, yet practitioners lacked a validated tool tailored to local ground conditions. The new study set out to close that gap.

The researchers assembled their dataset by pairing two independent measurement streams at coincident locations across three representative zones of the city. On the geophysical side, they performed one-dimensional MASW surveys to extract shear wave velocity profiles. On the geotechnical side, they collected SPT records from boreholes in the same areas. The overlap produced 164 data points, each tying a measured velocity to a penetration resistance at a comparable depth. With this dataset in hand, they fitted power-law models of the form VS equals a coefficient multiplied by N raised to an exponent, the classic functional form used in essentially all such correlations, reflecting the nonlinear stiffening of soil as density increases. Crucially, they evaluated the models twice: once using the raw field blow count and once using the energy-corrected value N60, which normalizes the measured blow count to a standard hammer energy of sixty percent of the theoretical maximum, following the correction framework established in the liquefaction literature since the 1980s.

The verdict on energy correction was clear. The model built on N60 outperformed the one based on uncorrected field values, and the team’s recommended relationship is VS equals 129.59 times N60 raised to the power of 0.2836, with velocities in meters per second. The statistical indicators are respectable for an empirical soil correlation: a coefficient of determination of 0.6713, a correlation coefficient of 0.8209 and a root-mean-square error of 30.81 meters per second. In practical terms, the equation predicts that a soft soil with a corrected blow count of 5 would have a shear wave velocity of roughly 210 meters per second, while a stiff soil with N60 of 30 would reach around 320 meters per second, differences large enough to shift a site from one seismic soil class to another. The exponent of about 0.28 sits within the range reported in international studies, suggesting the Cochabamba soils behave in a broadly familiar way, while the local coefficient captures the specific character of the valley’s fine-grained deposits.

The implications for Cochabamba itself are significant. The city’s valley, whose geology and hydrogeology have been documented in German-Bolivian cooperative studies, is filled with alluvial and lacustrine fine-grained sediments that can amplify earthquake shaking, and the region has a documented seismic hazard that motivates the national design code. With the new correlation, engineers reviewing borehole logs for a school, hospital or apartment block can now estimate the shear wave velocity profile and hence the site class directly from the SPT data that every geotechnical campaign already produces. That means faster, cheaper preliminary seismic screening across the urban fabric, and a way to populate site-class maps from the enormous archive of existing penetration test records. The authors position the model as a reliable estimation tool that contributes to the local seismic characterization of the Cochabamba valley, complementing rather than replacing direct geophysical measurement where critical structures demand it.

The study also carries a broader lesson for earthquake engineering in data-scarce regions. The workflow demonstrated here, fusing routine geotechnical borehole data with targeted geophysical surveys and validating the result with transparent statistics, is replicable in cities across the Andes and beyond that face similar combinations of seismic exposure, soft soils and limited measurement budgets. Comparable efforts in Peru, Ecuador and India have shown how local correlations sharpen regional hazard assessments, and the Cochabamba equation now joins that growing library. The authors, who conducted the fieldwork and analysis at Universidad Mayor de San Simón with support from the university’s geotechnical laboratory, note that the underlying data will be made available on request, an important step for independent verification and future meta-analyses.

For a city where a single well-placed hospital or bridge must stand for decades, the difference between assuming a generic imported correlation and using one calibrated on 164 local measurements could translate into real margin of safety. The equation itself is compact enough to fit on a single line of a design report, but behind it lies an unglamorous and essential truth of earthquake engineering: the ground beneath a building is as much a part of the structure as its columns and beams, and the more precisely that ground is characterized, the better the odds when the next earthquake arrives. Cochabamba’s engineers now have one more tool, rooted in their own valley’s soils, to tip those odds in their favor.

Subject of Research: Empirical correlation between shear wave velocity and SPT blow counts for seismic site characterization of fine-grained soils in Cochabamba, Bolivia

Article Title: Empirical correlation between shear wave velocity (VS) and SPT results using 1D MASW in fine-grained soils of Cochabamba, Bolivia

Article References: Copa, P. A. D. C., Padilla, A. F. C., & Rocha, J. H. A. (2026). Empirical correlation between shear wave velocity (VS) and SPT results using 1D MASW in fine-grained soils of Cochabamba, Bolivia. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02663-x

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02663-x

Keywords: shear wave velocity, Standard Penetration Test, MASW, seismic site characterization, Cochabamba, Bolivia, fine-grained soils, geotechnical engineering, earthquake engineering, N60 correction, power-law correlation, VS30