Mapping the Shaking: How Coimbatore’s Soils Could Amplify the Next Earthquake

When an earthquake strikes, the ground beneath a city does not shake uniformly. Soft sediments can turn a moderate tremor into a destructive event, while stiff rock a few kilometers away may barely register the same waves. A new study published in the Bulletin of Earthquake Engineering has mapped this hidden variability across Coimbatore, a major industrial city in the southern Indian state of Tamil Nadu, revealing in fine detail which neighborhoods would experience the strongest shaking and which design spectra currently used by engineers may be misleading.

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