Uncovering Earth’s Past: How Paleoseismology Measures Ancient Quakes
Paleoseismology is the science of peering into Earth’s seismic history when written records are scarce or absent. In regions where historical earthquake catalogs fade into the distant past, researchers turn to the ground itself to piece together a sequence of damaging events. By studying traces preserved in sediments, scientists can estimate when large earthquakes occurred, how powerful they were, and how often they might recur. This information is critical for refining building codes and seismic mitigation strategies that protect communities today.
Sand Dikes: The Seismic Footprints Beneath Our Feet
One of the most informative records of past quakes lies in sand dikes. These narrow, icicle-like intrusions form during strong ground shaking in water-saturated sediments through a process called liquefaction. When seismic energy disrupts the sediment, it behaves like a fluid. The mixture of sand and water is violently injected into cracks opened by the shaking. After the surge, the water drains away, leaving clean sand trapped within the fractures. A sand dike thus serves as a clear, tangible witness to a major earthquake.
From Fractures to Dating: How We Pinpoint Ancient Quakes
The breakthrough comes from dating the quartz grains that make up the sand dikes. Geologists have long used radiocarbon dating for organic materials, but here the key lies in luminescence dating. The team from India’s CSIR–NGRI, PRL, IIT Gandhinagar, IPR, and IUAC demonstrated that the heat generated during dike formation can reset the luminescence clock in quartz grains. When friction during liquefaction reaches temperatures around 350°C, the previously accumulated luminescence signal in the quartz is erased. The grains then begin accumulating a new luminescence signal as they are exposed to ambient radiation since burial. Measuring this fresh signal can reveal when the dike formed—and by extension, when the earthquake occurred.
Optically Stimulated Luminescence (OSL): Reading the Earth’s Timeline
To translate luminescence into calendar time, scientists use Optically Stimulated Luminescence (OSL) dating. OSL assesses the energy stored in quartz grains over time, a product of natural radioactive decay from elements like thorium, uranium, and potassium in the surrounding environment. When the sand is shielded from light underground, the clock continues to run. Exposure to light during lab measurements releases the stored energy in a controlled way, producing a luminescence signal that can be interpreted as an elapsed time since the last exposure to sunlight. In the case of sand dikes, a strong, recent signal indicates the moment of dike formation and the associated earthquake event.
Why This Matters for Today’s Safer Communities
Accurate reconstructions of seismic history help engineers and policymakers anticipate how often large earthquakes may strike a given region. This knowledge informs building safety codes, land-use planning, and mitigation measures designed to reduce risk for homes, schools, and critical infrastructure. In parts of the world where historical records are incomplete or gaps exist, sand dikes and OSL dating provide a robust toolkit for filling in the blanks of Earth’s seismic timeline.
Laboratory Verification: Proving the 350°C Reset
The researchers conducted temperature-dependent experiments on quartz to confirm how heat from dike formation would reset luminescence signals. Their results supported the hypothesis that inter-grain friction during rapid liquefaction can reach or exceed 350°C, enough to erase prior luminescence. This verification is essential to ensure the dating method yields reliable ages for the dikes and the earthquakes that created them.
A Collaborative Leap Forward in Geoscience
This breakthrough results from a multidisciplinary collaboration across several Indian institutions. By combining field observations of sand dikes with controlled luminescence measurements, the team established a practical approach to dating ancient earthquakes. The method enhances our historical seismic record and strengthens the scientific basis for future hazard assessments and infrastructure resilience planning.
