Spin Cycle for Seismology: Centrifuge Tests Validate a New Way to Model How Earthquake Shaking Varies Across a Site

When an earthquake strikes, the ground does not shake uniformly. Two points on the surface only tens of meters apart can experience noticeably different motions, a phenomenon engineers call spatial incoherency. For large, extended structures such as dams, bridges, tunnels, and nuclear power facilities, these differences matter enormously: they generate differential stresses that a uniformly shaking assumption would completely miss. The standard way to capture this effect has been to deploy dense arrays of seismometers at real sites and derive empirical coherency models from the recordings. That approach works, but it is slow, expensive, and only possible at the handful of locations where such arrays exist. A new study published in the Bulletin of Earthquake Engineering offers a way out of that bottleneck, showing that a carefully constructed numerical simulation, anchored by laboratory measurements of soil variability and validated against physical centrifuge experiments, can reproduce site-specific seismic coherency with impressive fidelity.

The research, led by Dongyeon Lee of Hanyang University together with colleagues from Korea Hydro and Nuclear Power and the Korea Advanced Institute of Science and Technology, tackles a long-standing gap in earthquake engineering practice. Spatial coherency describes how the similarity between ground motions at two locations decays as the separation between them grows and as the frequency of the shaking increases. Empirical models built from dense arrays such as those at Lotung in Taiwan or Chiba in Japan have long served as the reference standard, but they are inherently site-specific. Extending them to a new site means installing instruments and waiting for earthquakes to happen, which is neither practical nor economical for most engineering projects. The Korean team’s alternative is to predict coherency from first principles, using the measured spatial statistics of the soil itself as the input.

The physical basis of the new framework is the recognition that incoherency at a single site arises largely from the heterogeneity of the subsurface. Shear waves traveling upward through soil encounter pockets and layers of varying stiffness, and each encounter scatters and delays the wavefield slightly differently at every location. If the spatial distribution of shear wave velocity, denoted Vs, is known in statistical terms, the resulting distortion of the wavefield can, in principle, be computed. The researchers therefore set out to measure Vs variability directly and feed those statistics into a two-dimensional finite-element model of the soil deposit, generating Gaussian random fields whose coefficient of variation and correlation lengths match the field measurements.

Validating such a framework requires ground truth, and this is where the centrifuge enters the story. Geotechnical centrifuges allow engineers to test scaled physical models of soil deposits under properly scaled gravitational stress, reproducing the stress-dependent stiffness and damping behavior that governs wave propagation at full scale. The team constructed two soil models in the centrifuge, one a single-layered deposit and the other a double-layered deposit, and subjected them to recorded earthquake motions using an on-board earthquake simulator. Crucially, they characterized the spatial variability of the models themselves using bender elements and cone-penetration tests at multiple locations, obtaining the coefficient of variation and correlation lengths needed to build representative random fields. This pairing of physical testing and statistical characterization is what gives the study its unusual rigor.

On the computational side, each physical model was represented by a two-dimensional finite-element mesh, and the researchers ran a substantial campaign of simulations: ten bedrock motions recorded during the centrifuge tests were combined with thirty random-field realizations for each model, producing three hundred time-history analyses per deposit. Each realization represents one plausible arrangement of the measured soil heterogeneity, and the ensemble captures the range of responses that variability can produce. From the simulated surface motions at multiple locations, the team computed coherency functions in the same manner used for array recordings, binning results by separation distance so that they could be compared directly with the coherency derived from the centrifuge sensors.

The agreement reported is striking. The numerical models closely reproduced the measured response spectra and the shear stress-strain behavior observed in the centrifuge, confirming that the wave propagation physics was being captured correctly. More importantly for the study’s central claim, the coherency models computed from the simulations showed strong agreement with the centrifuge-derived coherency across all separation-distance bins. In other words, once the spatial variability statistics of the soil are known, the simulation does not merely approximate the general character of incoherency; it tracks the measured decay of coherency with distance and frequency in detail.

The implications extend well beyond the laboratory. Sites hosting critical infrastructure, particularly nuclear facilities, increasingly demand site-specific coherency models for soil-structure interaction analysis, and regulators in several countries have pressed for models tailored to local conditions. In Korea, for example, recent work has focused on generating coherency models suitable for nuclear power plant sites where no dense arrays exist. A validated simulation-based framework means that a project team could characterize a site with conventional geotechnical investigation tools, bender element tests and cone-penetration soundings are routine in practice, and then compute a coherency model numerically rather than waiting years for array data to accumulate. The study’s authors present this as a practical alternative to empirical models whenever site-specific spatial variability parameters are available.

The work also connects to a broader trend in geotechnical engineering: the treatment of soil spatial variability as a quantifiable random field rather than a nuisance to be averaged away. Probabilistic approaches using random-field theory have transformed reliability analysis of slopes, foundations, and liquefaction assessments over the past two decades. What this study adds is a bridge between that statistical tradition and the wave-propagation community concerned with spatially varying earthquake ground motion. By demonstrating that measured Vs statistics suffice to predict coherency, the researchers effectively show that the two communities’ toolkits can be combined, with site investigation data flowing directly into ground-motion incoherency predictions.

There are, of course, boundaries to what the framework has so far demonstrated. The validation was performed on idealized laboratory deposits in a centrifuge, where the soil variability could be measured exhaustively and the input motions were fully known. Real field sites present additional complications, including three-dimensional subsurface topography, deeper and more variable layering, and uncertainty in bedrock input motions. The authors note that their findings establish the feasibility of simulation-based coherency estimation, and extending the approach to field conditions is the natural next step. Even so, the core result stands: the physics linking soil heterogeneity to ground-motion incoherency is now demonstrably reproducible in a validated numerical setting.

For engineers designing the next generation of long-span bridges, lifeline networks, and nuclear structures, the message is that coherency analysis need no longer be the exclusive privilege of instrumented sites. A well-characterized soil profile, a finite-element model, and a modest computational campaign of random-field realizations can deliver a defensible, site-specific coherency model, one that has now been checked against physical reality at centrifuge scale. As seismic design codes continue to move toward performance-based criteria that explicitly account for the spatial character of shaking, tools of this kind are likely to move from research papers into standard practice, changing how the profession thinks about what lies beneath the foundations of everything it builds.

Subject of Research: Centrifuge-validated numerical simulation of spatial seismic ground-motion coherency in heterogeneous soil deposits

Article Title: Centrifuge-validated numerical framework for estimating seismic coherency

Article References: Centrifuge-validated numerical framework for estimating seismic coherency. (n.d.). https://doi.org/10.1007/s10518-026-02714-3

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02714-3

Keywords: seismic coherency, spatial variability, shear wave velocity, centrifuge testing, finite element analysis, Gaussian random fields, site response, soil-structure interaction, earthquake engineering, geotechnical engineering, dense seismic arrays, Bulletin of Earthquake Engineering