Paleoseismology helps scientists understand how frequently large earthquakes have occurred in the past — especially in regions with incomplete or no historical seismic records. By studying traces of ancient earthquakes preserved in the ground, researchers can determine when they occurred, how strong they were, how often they might recur, and how geological faults evolve over time.
This knowledge is crucial for improving building safety codes and designing effective earthquake mitigation strategies to better protect communities, according to scientists.
Radiocarbon dating has long been used to determine the age of organic materials in fossils and archaeological artefacts. Geologists also apply this technique to pinpoint the timing of major past earthquakes. Evidence of such seismic events — caused by the continuous movement of tectonic plates — is often preserved in sediments as distinctive features like sand dikes. With their seismogenic origin, scientists now say that dating these sand dikes can help precisely identify ancient earthquakes.
A collaborative team of Indian researchers from CSIR–National Geophysical Research Institute (NGRI), Hyderabad; Physical Research Laboratory (PRL), Ahmedabad; Indian Institute of Technology (IIT) Gandhinagar; Institute for Plasma Research (IPR), Gandhinagar; and Inter University Accelerator Centre (IUAC), New Delhi, has demonstrated that luminescence signals in quartz grains extracted from sand dikes offer a direct method to date past earthquakes.
What are sand dikes?
They are narrow, pointed, icicle-like structures formed during earthquakes in water-saturated sediments. These result from liquefaction — a process where intense seismic shaking causes sediment to lose strength and behave like a fluid. “Thus, a sand dike serves as clear evidence of a major earthquake,” said CSIR-NGRI Chief Scientist and corresponding author Devender Kumar.
Sand dikes form rapidly when a mixture of sand and water, behaving as a fluid, is violently injected into cracks opened by ground shaking. After injection, the water drains away, leaving clean sand trapped in the cracks.
“Our team proposed that inter-grain friction during this process generates enough heat, and this frictional heat can exceed 350C erasing previously accumulated geological luminescence in the quartz grains present in the dike sediments. Subsequently, the grains begin accruing a new luminescence signal, which can be measured to determine the timing of the dike formation — and thus, the earthquake’s occurrence,” he explained.
The researchers employed an ‘Optically Stimulated Luminescence’ (OSL) dating to estimate the age of the sand dikes. This method measures energy stored in quartz grains over time due to the natural radioactive decay of elements like thorium, uranium, and potassium.
Although luminescence can be affected by heat, light, and pressure, sand dikes — being underground — are naturally shielded from light. Laboratory experiments based on the temperature-dependent properties of quartz confirmed that temperatures during sand dike formation can reach or exceed 350C.
The findings were validated through analyses of sediment samples from five sand dikes in northeastern India. Most samples showed evidence of heating above 350C — sufficient to reset the luminescence signal in quartz grains — thereby providing a direct approach to date sedimentary features unequivocally formed by ancient earthquakes. The study, conducted by A.K. Tyagi, D. Kumar, M.K. Murari, R.N. Singh, and A.K. Singhvi, was recently published in Earth and Planetary Science Letters and has garnered significant international attention.
Published – October 10, 2025 05:50 pm IST
