Understanding the frequency of powerful historical earthquakes is critical, especially in regions lacking detailed seismic records. This field, known as paleoseismology, involves studying the geological evidence of ancient tremors. By examining features preserved deep within the earth, scientists can pinpoint when these events occurred, gauge their intensity, estimate their recurrence rates, and track the long-term evolution of geological faults.
Such knowledge is invaluable for updating building codes and developing robust strategies to mitigate earthquake risks, ultimately safeguarding communities. Traditionally, radiocarbon dating has been a go-to method for aging organic materials, and geologists have adapted it to date major past earthquakes. Often, evidence of these tectonic plate movements is found in distinctive sedimentary features like sand dikes. Now, researchers believe that directly dating these sand dikes can precisely reveal the timing of ancient seismic activity.
A collaborative research team from India, involving experts from CSIR–National Geophysical Research Institute (NGRI), Physical Research Laboratory (PRL), Indian Institute of Technology (IIT) Gandhinagar, Institute for Plasma Research (IPR), and Inter University Accelerator Centre (IUAC), has successfully demonstrated that luminescence signals within quartz grains from sand dikes can directly date prehistoric earthquakes.
What Exactly Are Sand Dikes?
Sand dikes are unique, narrow, icicle-shaped geological structures that form during earthquakes in water-saturated sediments. They are created through a process called liquefaction, where intense seismic shaking causes sediment to momentarily lose its solid strength and behave like a fluid. As CSIR-NGRI Chief Scientist and lead author Devender Kumar explains, “A sand dike thus serves as unequivocal evidence of a significant earthquake.”
These dikes emerge rapidly when a mixture of sand and water, acting as a fluid, is violently injected into cracks opened up by ground shaking. Once injected, the water drains away, leaving behind clean sand trapped within these fissures. Dr. Kumar further elaborated, “Our team hypothesized that the friction between sand grains during this forceful injection generates enough heat, potentially exceeding 350°C. This intense frictional heat effectively erases any previously accumulated geological luminescence in the quartz grains found within the dike sediments. Subsequently, these grains begin to acquire a new luminescence signal, which can then be precisely measured to determine the age of the dike’s formation—and, consequently, the earthquake’s occurrence.”
To determine the age of these sand dikes, the researchers utilized Optically Stimulated Luminescence (OSL) dating. This sophisticated technique measures the energy stored in quartz grains over time, which accumulates due to the natural radioactive decay of elements like thorium, uranium, and potassium within the surrounding environment.
While luminescence can be influenced by heat, light, and pressure, sand dikes are naturally protected from light underground. Laboratory tests, based on the temperature-sensitive properties of quartz, confirmed that temperatures during sand dike formation can indeed reach or surpass 350°C. This level of heat is sufficient to ‘reset’ the luminescence signal in quartz grains.
The groundbreaking findings were rigorously verified through analyses of sediment samples taken from five different sand dikes in northeastern India. The majority of these samples showed clear indications of heating above 350°C, confirming that the luminescence signal in their quartz grains had been effectively reset. This provides a direct and reliable method to date these sedimentary features that are undeniably 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 journal Earth and Planetary Science Letters and has garnered considerable international recognition.