📄 earth_science

Regional seismic site response assessment in the deep Bengal Basin, Bangladesh, from simulated far-field earthquake ground motions

This study demonstrates that deep soft sediments in the Bengal Basin significantly amplify long-period ground motions from far-field earthquakes, revealing that the current V_S30-based seismic site characterization in Bangladesh is insufficient and must incorporate engineering bedrock depth to accurately assess seismic risk.

Original authors: Atikul Haque Farazi, José Piña-Flores, Md. Shakhawat Hossain, Nasim Ferdous, Sukhen Goswami, Md Wabidur Rahman

Published 2026-09-08

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Atikul Haque Farazi, José Piña-Flores, Md. Shakhawat Hossain, Nasim Ferdous, Sukhen Goswami, Md Wabidur Rahman

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Earthquakes are not just about how hard the ground shakes; they are also about how the ground shakes. When seismic waves travel from a distant fault line, they pass through layers of rock and soil before reaching the surface where people live. These layers act like a filter, changing the speed and rhythm of the shaking. In some places, thick piles of soft, loose sediment can trap these waves, causing them to bounce around and amplify, much like sound resonating in a large, empty hall. This phenomenon is particularly dangerous for tall buildings, which sway naturally at slow, long intervals. If the ground shakes at that same slow rhythm, the building can begin to sway violently, potentially leading to collapse. Understanding this interaction between deep soil and earthquake waves is critical for designing safe cities, especially in regions built on massive sedimentary basins.

In Bangladesh, a country situated on one of the world's largest and deepest sedimentary basins, this issue is urgent. The ground beneath much of the country consists of thousands of meters of soft, unconsolidated sand and clay, deposited over millions of years by rivers flowing from the Himalayas. For years, engineers and scientists have assessed earthquake risk in Bangladesh by measuring the speed of seismic waves in the top 30 meters of the ground. This standard measurement, known as the average shear-wave velocity, helps classify soil stiffness and predict how much the ground might shake. However, this method looks only at the shallow surface. It often misses the influence of the deep, soft layers far below, which can significantly alter the rhythm of the shaking, particularly for distant, powerful earthquakes.

A recent study by researchers from universities in Bangladesh and Mexico sought to understand what happens when a massive earthquake occurs far away, sending waves through this deep, soft basin. The team was motivated by a recent event in a neighboring region, where a powerful earthquake hundreds of kilometers away caused severe swaying in high-rise buildings in a deep basin, highlighting a risk that shallow measurements might miss. To investigate this, the researchers did not wait for a real earthquake to strike Bangladesh. Instead, they used computer simulations to model how the ground would respond to two specific, large earthquakes that occurred in Nepal. They took the recorded shaking from a hard-rock station in India and mathematically passed it through the deep soil profiles of nineteen different locations across Bangladesh. These profiles, which extend thousands of meters deep, were derived from a previous study that analyzed the natural hum of the earth to map the underground layers.

The researchers found that the deep soil acts as a powerful tuner for earthquake waves. While the simulated shaking at the surface was not as intense in terms of raw force as what is typically expected from nearby quakes, the rhythm of the shaking changed dramatically. At many locations, particularly in the central and northern parts of the basin, the ground began to sway with a dominant period of about one second. This is a slow, rolling motion. In contrast, the standard building codes currently used in Bangladesh, which rely on the shallow soil measurements, predict peak shaking at much faster, shorter intervals. The study suggests that the current codes may be underestimating the risk to medium- and high-rise buildings, which naturally sway at these slower, one-second intervals. If a distant earthquake sends waves that match this rhythm, the deep soft soil could amplify the motion, causing tall structures to resonate dangerously.

The study also compared their deep-soil models with data from standard drilling tests taken from the ground at twelve of the sites. While the shallow soil classifications from the drills and the deep models agreed on the general type of soil, the deep models revealed a crucial difference: the depth to the hard rock layer. The researchers discovered a clear pattern: the deeper the hard rock was, the longer the period of the shaking. Sites with very deep soft sediments, some reaching depths where hard rock lies far below, produced the longest, slowest swaying motions. This relationship confirms that the thickness of the soft layer is a primary driver of how the ground responds to distant quakes. The findings indicate that relying solely on the top 30 meters of soil is insufficient for a country built on such a deep basin.

Ultimately, the research suggests that the deep, soft sediments of the Bengal Basin have the potential to amplify long-period ground motions from far-field earthquakes, a risk that is not fully captured by current building standards. The study does not claim that an earthquake will definitely destroy buildings, but rather that the physics of the basin creates a specific vulnerability for tall structures during distant, large-magnitude events. The authors propose that future updates to the national building code should explicitly account for the depth of the engineering bedrock and the thickness of the soft sediment layers. By incorporating these deep geological factors, engineers could better design cities that are resilient not just to the immediate shaking of nearby faults, but also to the slow, rolling waves that travel across the landscape from distant seismic sources. This shift in perspective is essential for protecting the growing number of high-rise buildings in Bangladesh and ensuring that the country's infrastructure can withstand the complex realities of its geological setting.

Technical Summary: Regional Seismic Site Response Assessment in the Deep Bengal Basin

Problem Statement
Bangladesh is situated within the active Eurasia–India–Burma plate tectonic system and is underlain by the Bengal Basin, one of the world's largest and deepest sedimentary basins, containing soft sediments up to 20–22 km thick. While the country is vulnerable to earthquakes from regional sources (e.g., the Himalayan and Indo-Burmese systems), current seismic hazard assessment relies heavily on the Bangladesh National Building Code (BNBC, 2020), which utilizes $V_{S30}$ (average shear-wave velocity in the top 30 meters) for site classification. The authors argue that $V_{S30}$ alone is insufficient for deep sedimentary basins where engineering bedrock often lies well below 30 meters. Consequently, the influence of deep, soft sedimentary sequences on long-period ground motion amplification remains underestimated. This gap is critical given recent observations, such as the 2025 MW 7.7 Myanmar earthquake, which caused severe long-period shaking in the distant Bangkok Basin, highlighting the potential for thick sedimentary basins to amplify far-field seismic waves and damage high-rise structures.

Methodology
This study presents the first regional assessment of seismic site response across the Bengal Basin using a simulation-based approach. The methodology involves:

Input Motions: Two far-field subduction-zone earthquake records (MW 6.7 and MW 7.3, both aftershocks of the 2015 Gorkha sequence) recorded at a hard-rock reference station (SHL) in Shillong, India, approximately 1,162 km from the study area.
Site Models: One-dimensional (1D) linear elastic site response analyses were conducted at 19 seismic stations. The subsurface models utilized deep shear-wave velocity ( $V_S$ ) profiles extending to ~3000 m depth, previously derived by Farazi et al. (2023) through the inversion of ambient noise Horizontal-to-Vertical Spectral Ratio (HVSR) data.
Simulation: The study employed the reflectivity method to compute 1D transfer functions for vertically propagating SH waves. These transfer functions were convolved with the reference earthquake records in the frequency domain to generate synthetic accelerograms and elastic response spectra (5%-damped pseudo-spectral acceleration, PSA).
Validation: To address the non-uniqueness of HVSR inversion, the reliability of the derived $V_{S30}$ values and seismic site classes was validated against Standard Penetration Test (SPT) borehole data from 12 nearby sites. $V_{S30}$ values were estimated from SPT-N values using empirical correlations developed for Dhaka soils, and site classes were assigned according to BNBC (2020) criteria.

Key Results

Site Classification: Based on HVSR-derived $V_{S30}$ , 12 of the 19 stations were classified as Site Class SC (stiff soil/soft rock) and 7 as Site Class SD (stiff soil). There was generally good agreement (approx. 67%) between HVSR-derived classifications and those derived from SPT data, supporting the reliability of the velocity models.
Transfer Functions and Amplification: Transfer function amplitudes varied significantly across stations, ranging from 4.37 to 17.40. Stations in the Surma Basin and Central Bengal Foredeep generally exhibited higher amplification factors compared to the Chittagong-Tripura Fold Belt (CTFB).
Dominant Periods: A critical finding is the prevalence of dominant spectral periods approaching 1.0 second (ranging from ~0.95 to 1.20 s) at many stations, particularly in the Surma Basin and Central Bengal Foredeep. These periods are significantly longer than the peak periods typically represented in the BNBC (2020) design spectra for the corresponding site classes.
Bedrock Depth Correlation: The study identified a systematic relationship where the dominant spectral period increases with the depth of the engineering bedrock. Stations with deeper bedrock (thicker sedimentary sequences) exhibited lower peak frequencies and longer dominant periods.
PGA vs. Spectral Acceleration: While Peak Ground Accelerations (PGAs) were relatively low due to the large source-to-site distance, the Pseudo-Spectral Accelerations (PSA) showed significant spatial variability, with some stations experiencing spectral demands that could resonate with medium- to high-rise structures.

Significance and Claims
The paper claims that the deep soft sediments of the Bengal Basin have the potential to significantly amplify long-period ground motions from far-field earthquakes, a phenomenon not fully captured by current $V_{S30}$ -based site characterization. The authors assert that:

Limitations of Current Codes: The BNBC (2020) framework, while adequate for characterizing near-surface stiffness, fails to account for the influence of engineering bedrock depth on long-period amplification.
Resonance Risk: The identified dominant periods (~1 s) coincide with the natural vibration periods of medium- to high-rise buildings, suggesting a potential for structural resonance and damage during future large-magnitude far-field earthquakes.
Recommendation: Future revisions of the BNBC should explicitly incorporate engineering bedrock depth and deep sedimentary effects alongside $V_{S30}$ to improve seismic hazard assessment and earthquake-resistant design.
Regional Baseline: This study provides a foundational baseline for understanding regional period-dependent amplification, motivating policymakers and engineers to consider deep soil effects in thick sedimentary basins.

The authors acknowledge limitations, noting that the study uses 1D linear elastic modeling and specific far-field scenarios, which do not capture nonlinear soil behavior, 3D basin effects, or near-fault directivity. However, they maintain that the results offer a physically consistent basis for evaluating regional trends in long-period site response.

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