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

Trypanosoma cruzi, the parasite that causes Chagas disease, infects more than seven million people worldwide, with the heaviest burden concentrated in Latin America. In recent decades, migration and travel have turned the infection into a public health concern well beyond its traditional geographic boundaries, with cases now identified in the United States, Europe and Japan. Although the parasite often remains silent for years, roughly 30 to 40 percent of infected individuals eventually develop Chagas disease, which can lead to serious and potentially life-threatening heart or digestive complications. Despite the scale of the problem, fewer than one percent of infected people are ever diagnosed and treated, a gap driven by weak screening programs, stigma, and the poor tolerability of the drugs that exist.

Only two medicines are available against T. cruzi: benznidazole and nifurtimox. Both were developed in the 1960s and 1970s, and both are still administered according to dosing schedules that have barely changed since then. The standard benznidazole course requires patients to take the drug twice a day for 60 days, a demanding commitment for people who may live far from clinics or have jobs that make midday dosing difficult. More importantly, these prolonged regimens frequently cause adverse reactions ranging from skin rashes and digestive complaints to neurological symptoms, and up to 31 percent of patients abandon treatment before finishing it. Because treatment completion is essential for curing the infection, side effects and pill burden translate directly into treatment failure at the population level.

TESEO was designed to test whether alternative durations and dosing schedules could improve that equation. According to Igor Almeida of the University of Texas at El Paso, corresponding author and co-senior author of the study alongside Faustino Torrico of CEADES and Joaquim Gascón of ISGlobal, the trial was the first randomized clinical study to evaluate benznidazole and nifurtimox head-to-head under identical conditions, with sustained follow-up designed to detect parasites in the blood by PCR for three years. This long observation window matters because the parasite can persist at very low levels after treatment, and confirming sustained clearance requires patient monitoring that many earlier studies simply could not provide.

The phase 2b trial enrolled 450 adults at three centers in Bolivia, a country where Chagas infection remains highly endemic and where the research team has decades of clinical experience. Participants were randomly assigned to one of six treatment groups. Three groups received benznidazole, either as the 60-day standard regimen or as shortened 30-day and extended 90-day courses, while three parallel groups received nifurtimox on the same three schedules. Throughout the study, investigators recorded every drug-related adverse event and periodically tested participants’ blood for parasite DNA using quantitative PCR, a molecular technique sensitive enough to detect residual infection that microscopic or serological methods would miss. The statistical design and analysis were led by the ISGlobal Biostatistics Unit, which also supported an independent board responsible for monitoring patient safety throughout the trial.

The safety results pointed clearly in one direction. Only the 30-day benznidazole regimen significantly reduced side effects: 37 percent of participants on that schedule experienced a drug-related adverse event, compared with 60 percent of those on the standard 60-day course, a reduction of nearly 40 percent. Tolerability translated directly into adherence. On the shorter regimen, 83 percent of patients completed treatment without interruption, versus 60 percent on the standard schedule. An additional practical advantage is that the 30-day course requires only one dose per day rather than two, simplifying life for patients and health systems alike. The researchers also observed that most side effects appeared within the first two weeks of treatment regardless of how long the drug was continued, and that almost all of these reactions resolved, suggesting that the later weeks of the standard course add toxicity without adding early-warning signals.

Crucially, the shorter course did not sacrifice efficacy. Three years after treatment ended, the parasite remained undetectable in the blood of 94 percent of participants in the 30-day benznidazole group, compared with 95 percent in the standard 60-day group. Because the trial was designed as a non-inferiority study, this narrow difference met the prespecified statistical criterion, meaning the shorter regimen could be considered as effective as the established one. Among the six strategies tested, the 30-day benznidazole course therefore offered the best overall balance between benefits and harms. The nifurtimox arms performed less well across the board, with parasite clearance rates ranging from 81 to 90 percent, reinforcing benznidazole’s position as the preferred first-line option where it is available and tolerated.

The implications extend beyond individual patients to the economics and logistics of public health programs. Because the 30-day regimen uses a quarter of the total drug required by the standard course, the same supply of benznidazole can treat four times as many people. Torrico highlighted this point directly, noting that four patients can now be treated with the amount of drug previously needed for one. Gascón added that the improved safety profile, combined with once-daily dosing, has the potential to substantially increase the proportion of patients who complete treatment, which is precisely the bottleneck that has limited the impact of Chagas therapy for decades. For national control programs in Latin America and for growing patient populations in the United States and Europe, a shorter, cheaper and better-tolerated cure could make routine treatment far more feasible.

The authors caution that larger phase 3 trials are needed to confirm the results before the 30-day regimen can be written into official treatment guidelines, and they emphasize that people currently receiving therapy for Chagas disease should continue the regimen prescribed by their doctor. Even so, the trial represents a milestone for a disease long classified as neglected. By demonstrating that a 50-year-old drug can be given more briefly, more conveniently and more safely without losing efficacy, TESEO provides the strongest evidence yet that the barriers keeping millions of people from a cure can be substantially lowered. The study was funded by the National Institute of Allergy and Infectious Diseases, part of the US National Institutes of Health, and is registered as ClinicalTrials.gov NCT03981523. The authors declare no competing interests.

Subject of Research: A randomized phase 2b trial comparing alternative benznidazole and nifurtimox regimens for chronic Trypanosoma cruzi infection

Article Title: A shorter, better-tolerated treatment for chagas disease is possible

Article References: A shorter, better-tolerated treatment for chagas disease is possible. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Chagas disease, Trypanosoma cruzi, benznidazole, nifurtimox, TESEO trial, randomized clinical trial, neglected tropical diseases, ISGlobal, treatment adherence, parasitology, Bolivia, The Lancet Infectious Diseases