For scientists, understanding the frequency of powerful earthquakes in the past is vital, especially for regions lacking thorough historical seismic records. By meticulously examining the remnants of ancient quakes preserved within the Earth, researchers can uncover critical details about their timing, intensity, recurrence patterns, and how geological faults have evolved over millennia.
This historical seismic knowledge is incredibly valuable. It’s essential for refining modern building safety codes and developing more effective earthquake mitigation strategies, ultimately helping to better protect communities from future seismic events.
Historically, radiocarbon dating has been a go-to method for determining the age of organic materials in fossils and archaeological finds. Geologists have also adapted this technique to pinpoint major past earthquakes. Evidence of these powerful seismic events, which are driven by the Earth’s constantly moving tectonic plates, often survives in sediments as distinctive features known as sand dikes. With their undeniable link to seismic activity, scientists are now asserting that precisely dating these sand dikes can accurately mark the occurrence of ancient earthquakes.
A pioneering collaboration among Indian researchers from institutions including CSIR–National Geophysical Research Institute (NGRI) in Hyderabad, Physical Research Laboratory (PRL) in Ahmedabad, Indian Institute of Technology (IIT) Gandhinagar, Institute for Plasma Research (IPR) in Gandhinagar, and Inter University Accelerator Centre (IUAC) in New Delhi, has revealed a direct new method for dating past earthquakes: by analyzing luminescence signals in quartz grains extracted from these very sand dikes.
Unveiling the Secrets of Sand Dikes: Earth’s Own Earthquake Recorders
Sand dikes are fascinating, narrow, icicle-shaped geological structures. They form during earthquakes in sediments that are saturated with water, a process triggered by liquefaction. During liquefaction, intense seismic shaking causes sediment to temporarily lose its strength, behaving much like a fluid. As CSIR-NGRI Chief Scientist and lead author Devender Kumar explains, “Thus, a sand dike serves as clear evidence of a major earthquake.”
These dikes are created almost instantly when a fluid-like mixture of sand and water is forcefully injected into cracks that open up in the ground due to violent shaking. Once injected, the water drains away, leaving behind clean sand permanently trapped within these cracks.
Kumar’s team theorized that the intense friction between sand grains during this rapid injection generates substantial heat. This frictional heat can surge above 350°C, effectively “erasing” any previous geological luminescence accumulated in the quartz grains within the dike sediments. After this thermal reset, the quartz grains begin to accumulate a new luminescence signal, which can then be measured to precisely determine the age of the dike’s formation – and, by extension, the timing of the earthquake itself.
The researchers utilized ‘Optically Stimulated Luminescence’ (OSL) dating for this purpose. This method works by measuring the energy that quartz grains naturally store over time, primarily from the radioactive decay of elements like thorium, uranium, and potassium present in the surrounding environment.
While luminescence signals can typically be influenced by factors like heat, light, and pressure, sand dikes are uniquely positioned deep underground, providing a natural shield from light exposure. Controlled laboratory experiments confirmed that the temperatures achieved during sand dike formation indeed reach or exceed 350°C. This critical temperature is sufficient to completely reset the luminescence signal in quartz grains, making them perfect “clocks” for dating seismic events.
These remarkable findings were rigorously validated through detailed analyses of sediment samples taken from five different sand dikes found across northeastern India. The majority of these samples showed clear evidence of heating above 350°C, confirming that the luminescence signal in their quartz grains had been successfully reset. This breakthrough offers a direct and unequivocal method to date sedimentary features unequivocally formed by ancient earthquakes. The study, authored by A.K. Tyagi, D. Kumar, M.K. Murari, R.N. Singh, and A.K. Singhvi, was recently published in the prestigious Earth and Planetary Science Letters and has since attracted significant international attention.