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

The research, led by Manoharan Sambath and Sembulichampalayam Sennimalai Chandrasekaran of Vellore Institute of Technology together with Sandeep Maithani of the Indian Institute of Remote Sensing, forms part of a broader effort to assess urban seismic risk in India, a country where rapid construction often outpaces detailed geotechnical knowledge. Funded by the Indian Space Research Organisation through its RESPOND program, the study combined borehole records, geophysical surveys and computational modeling to produce some of the most granular ground-motion estimates yet available for the city.

At the heart of the work lies a technique called Site-Specific Seismic Ground Response Analysis, or SSGRA. The method takes an earthquake signal defined at bedrock depth and propagates it upward through the local soil column, accounting for how each layer stiffens, softens and dissipates energy as it is deformed. Because soil is not a simple elastic medium, the team ran two parallel formulations: an equivalent linear approach, which uses averaged stiffness and damping values, and a more computationally demanding nonlinear analysis that tracks the true stress-strain behavior of the sediments under strong cyclic loading.

The input side of the calculation was equally rigorous. The researchers drew on a probabilistic seismic hazard analysis of the Coimbatore region, which they had previously developed using a logic-tree framework, to define five distinct hazard zones based on bedrock peak ground acceleration. For each zone, they constructed a Uniform Hazard Response Spectrum and then spectrally matched eleven suitable recorded ground motions to it, ensuring that the input signals carried the right energy content across the frequency range that matters for buildings. These matched motions were then applied for two hazard levels, corresponding to return periods of 475 and 2475 years, roughly the shaking expected once in 475 and once in 2475 years respectively.

Characterizing the soils themselves required fieldwork across the city. The team used 40 geospatially distributed bore logs, which record the layering and index properties of the subsurface, together with active Multichannel Analysis of Surface Waves tests. MASW surveys measure how seismic surface waves disperse through the ground, allowing engineers to compute the average shear-wave velocity in the top 30 meters, a parameter known as Vs30 that has become the international standard for classifying site conditions. Soils with low Vs30 values, typically soft clays and loose sands, amplify ground motion far more than dense gravels or rock.

The results paint a strikingly uneven picture of Coimbatore’s seismic vulnerability. The neighborhoods of Rathinapuri, Avinashi, Gandhipuram and NGGO Colony showed a broad variation in surface peak ground acceleration, meaning that even within these areas, the intensity of shaking could differ substantially from one site to the next depending on local soil conditions. Such block-by-block variability has direct consequences for building codes, which often assume a single design value across an entire city.

Perhaps the most consequential finding concerns spectral acceleration, the quantity that most directly governs structural design. The central part of the city exhibits the highest surface spectral accelerations at spectral periods of 0.5 and 1.0 seconds, ranges that coincide with the natural vibration periods of many mid-rise reinforced concrete buildings. In an earthquake, structures resonate with ground motion at their own natural periods, so elevated spectral acceleration in this band implies that a significant share of Coimbatore’s building stock could experience forces well above what a uniform hazard map would suggest.

The study also found a clear geographic pattern in amplification, the ratio of surface motion to bedrock motion. The western region of the city experienced higher amplification factors for the 475-year return period event, indicating that its subsurface conditions selectively boost moderate-intensity shaking. Interestingly, the relationship between amplification and shaking intensity is not constant: as input motions grow stronger, soils soften and damp more, which is precisely why the team ran both linear-equivalent and nonlinear analyses across the two return periods rather than relying on a single scenario.

The comparison with codified design spectra carries a cautionary message for practitioners. The site-specific Acceleration Design Response Spectrum proposed in the study was found to be overestimated, particularly for the second group of bedrock PGA-based hazard zones, when set against the nonlinear surface response spectra computed by the analysis. In other words, for those hazard zones, the design spectrum is conservative, demanding more seismic resistance than the physics of the soil column requires. While conservatism is not inherently dangerous, it imposes unnecessary construction costs; the flip side is that in zones where the spectrum is not conservative, the margin of safety may be thinner than assumed.

For Coimbatore, a city whose economy is anchored by textile manufacturing, engineering industries and a dense fabric of both formal and informal construction, the maps produced by this study offer a practical template for risk-sensitive planning. The work also demonstrates a methodology that other Indian cities can replicate: combine probabilistic hazard analysis with dense geotechnical sampling, geophysical profiling and dual-formulation site response modeling to move from city-wide averages to neighborhood-level truth. As India’s seismic monitoring network expands and urban populations grow, studies of this kind convert abstract hazard curves into concrete guidance about where the ground will shake hardest, and where engineers should look most carefully before the next earthquake tests their assumptions.

Subject of Research: Site-specific seismic ground response analysis and geospatial mapping of surface ground motion parameters for Coimbatore city, India

Article Title: Geospatial variation of surface ground motion parameters using the 1-D Site-Specific Seismic Ground Response Analysis (SSGRA) for Coimbatore city, Tamil Nadu, India

Article References: Geospatial variation of surface ground motion parameters using the 1-D Site-Specific Seismic Ground Response Analysis (SSGRA) for Coimbatore city, Tamil Nadu, India. (n.d.). https://doi.org/10.1007/s10518-026-02705-4

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02705-4

Keywords: seismic hazard, ground response analysis, Coimbatore, shear-wave velocity, MASW, peak ground acceleration, spectral acceleration, site amplification, soil dynamics, earthquake engineering, India, borehole data