Small kinks record ancient earthquakes

Every now and then, somewhere under our feet, rocks rip and an earthquake begins. With large enough ruptures, we may feel an earthquake as seismic waves radiate to or along the surface. However, scientists suspect that a mere 15% to 20% of the energy needed to break the rock in the first place translates into seismicity.

Mica’s structure makes it susceptible to bending rather than twisting or folding, and the remaining energy can dissipate as frictional heat, leaving behind molten levels of glassy rock called pseudotachylyte. The remaining energy could fracture, crush or deform the rock that surrounds the rupture as it rushes through the crust, Eric Anderson, a doctoral student at the University of Maine, said. Because these processes occur kilometers below the Earth’s surface, scientists cannot directly observe them when modern earthquakes strike. Shear zones that are millions of years old and are now at the surface can provide windows into rocks around ancient ruptures. However, even though seismically shifting rocks remain deep, heat and pressure can erase evidence of past earthquakes, Anderson said. “We need another proxy,” he said, “when we’re looking for evidence of earthquakes in the rock record.”

Mica—the plate-like minerals that can clump together into individual crystals that often provide sparkle in kitchen countertops—can maintain discoloring features that look like microscopic blades. On the geological scale of geology, chevrons form in multi-layered strata. In small bundles of mica, petrologists have noticed similar pointed folds because the mica’s structure makes them susceptible to buckling, rather than kinking or folding, said Frans Eben, a rock physicist at University College London.

In a new article in Earth and Planetary Science Letters, Anderson and colleagues argue that these minute structures — called lattice bands — often indicate ancient seismic ruptures and may withstand other seismic indicators.

Old retinal teams, explosive explanation

To observe twisted mica, scientists must carefully cut the rock into fragments thinner than the typical width of a human hair, and glue each rock slice to a piece of glass. Using high-powered microscopes to examine this mixture of rock and glass (aptly called the thin section), Anderson and colleagues compared lattice bands from two Maine sites, both more than 300 million years old. The first site is replete with telltale signs of a previous, dynamically distorted seismic zone, such as shredded chalcedony and pseudotachylyte. The second site reveals rocks that changed slowly, under relatively constant conditions.

By comparing the geometry of the kink bands from these sites, the researchers observed differences in the thickness and symmetry of the microstructures. In particular, samples from the dynamically deformed site display thin-sided asymmetric kinks. More consistently deformed samples show evenly proportional points with thicker tips.

Kink teams could be added to a growing list of indicators of seismic activity in coded shear zones, Eben said. He said the data “speaks for itself”. Eben was not involved in this study.

To strengthen the link between earthquakes and grating band engineering, Anderson and colleagues analyzed 1960s-era studies largely driven by the development of nuclear weapons. During that time, scientists have sought to understand how shock waves are emitted from sudden, rapid, and massive disturbance sites such as those at nuclear test sites or meteorite impact pits. Mica has developed lattice bands at such locations, as well as in complementary lab experiments, Anderson said, and they mimic the geometric patterns produced by dynamic rate stress events — such as earthquakes. “[Kink band] Anderson said the geometry is directly related to the deformation pattern.

Tightening the rocks, Kinking Micas

In addition to exploring whether zigzag mica geometry can fingerprint the effects of earthquake ruptures, Anderson and colleagues estimated the magnitude of the transient localized stress their samples experienced when the earthquake frontal rupture propagated through the rock. In other words, he wondered, could the geometry of the zigzag mica fit the magnitude of the instantaneous pressure that caused the mica to distort in the first place?

Extrapolating data from previously published lab experiments, Anderson estimated that fracturing rocks at the deepest depths where earthquakes can intend would require up to 2 gigapascals of stress. Although the pressure does not directly correspond to pressure, 2 GPa is more than 7,200 times the pressure inside a tire inflated to 40 psi. For reference, the unimaginably crushing pressure in the deepest part of the ocean – the Mariana Trench – is only about 400 times the pressure in the same frame.

With mica, once intertwined, it will remain twisted, preserving ancient seismic records in mountain cores. With the same conversion, twisted mica requires pressures 8-30 times greater than that of water in the deepest ocean. Because Anderson found crushed garnets close to twisted mica at the faulty field site, he and colleagues concluded that the stresses that these rocks were temporarily subjected to when the earthquake ruptured through the shear zone were about 1 gigapascal, or 9 times the pressure at the Mariana Trench.

Eben called the earthquake transient stress estimate speculative, but said the new study’s focus on earthquake-induced deformation fills a research gap between the very slow deformation of mountain-building rocks and the very fast deformation that occurs during nuclear weapons testing and the effects of meteorites. With mica, he said, “once it becomes twisted, it will still be twisted,” preserving ancient seismic records in mountain cores.

—Alka Tripathy-Lang (DrAlkaTrip), science writer

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