Every so often, somewhere beneath our feet, rocks rupture, and an earthquake begins. With big enough ruptures, we might feel an earthquake as seismic waves radiate to or along the surface. However, a mere 15% to 20% of the energy needed to break rocks in the first place translates into seismicity, scientists suspect.
The remaining energy can dissipate as frictional heat, leaving behind melted planes of glassy rock called pseudotachylyte. The leftover energy may also fracture, pulverize, or deform rocks that surround the rupture as it rushes through the crust, said Erik Anderson, a doctoral student at the University of Maine. Because these processes occur kilometers below Earth’s surface, scientists cannot directly observe them when modern earthquakes strike. Shear zones millions of years old that now reside at the surface can provide windows into the rocks around ancient ruptures. However, although seismogenically altered rocks remain at depth, heat and pressure can erase clues of past quakes, said Anderson. “We need some other proxy,” he said, “when we’re looking for evidence of earthquakes in the rock record.”
Micas—sheetlike minerals that can stack together in individual crystals that often provide the sparkle in kitchen countertops—can preserve deformation features that look like microscopic chevrons. On geology’s macroscale, chevrons form in layered strata. In minuscule sheaves of mica, petrologists observe similar pointy folds because the structure of the mica leaves it prone to kinking, rather than buckling or folding, said Frans Aben, a rock physicist at University College London.
In a new article in Earth and Planetary Science Letters, Anderson and his colleagues argue that these microstructures—called kink bands—often mark bygone earthquake ruptures and might outlast other indicators of seismicity.
Ancient Kink Bands, Explosive Explanation
To observe kinked micas, scientists must carefully cut rocks into slivers thinner than the typical width of a human hair and affix each rock slice to a piece of glass. By using high-powered microscopes to examine this rock and glass combination (aptly called a thin section), Anderson and his colleagues compared kink bands from two locations in Maine, both more than 300 million years old. The first location is rife with telltale signs of a dynamically deformed former seismogenic zone, like shattered garnets and pseudotachylyte. The second location exposes rocks that changed slowly, under relatively static conditions.
Comparing the geometry of the kink bands from these sites, the researchers observed differences in the thicknesses and symmetries of the microstructures. In particular, samples from the dynamically deformed location display thin-sided, asymmetric kinks. The more statically deformed samples showcase equally proportioned points with thicker limbs.
Kink bands, said Aben, can be added to a growing list of indicators of seismic activity in otherwise cryptic shear zones. The data, he said, “speak for themselves.” Aben was not involved in this study.
To further cement the link between earthquakes and kink band geometry, Anderson and colleagues analyzed 1960s era studies largely driven by the development of nuclear weapons. During that time, scientists strove to understand how shock waves emanated from sites of sudden, rapid, massive perturbations like those produced at nuclear test sites or meteor impact craters. Micas developed kink bands at such sites, as well as in complementary laboratory experiments, said Anderson, and they mimic the geometric patterns produced by dynamic strain rate events—like earthquakes. “[Kink band] geometry,” Anderson said, “is directly linked to the mode of deformation.”
Stressing Rocks, Kinking Micas
In addition to exploring whether kinked mica geometry could fingerprint relics of earthquake ruptures, Anderson and his colleagues estimated the magnitude of localized, transient stress their samples experienced as an earthquake’s rupture front propagated through the rocks, he said. In other words, he asked, might the geometry of kinked micas scale with the magnitude of momentary stress that kinked the micas in the first place?
By extrapolating data from previously published laboratory experiments, Anderson estimated that pulverizing rocks at the deepest depths at which earthquakes can nucleate requires up to 2 gigapascals of stress. Although stress doesn’t directly correspond to pressure, 2 gigapascals are equivalent to more than 7,200 times the pressure inside a car tire inflated to 40 pounds per square inch. For reference, the unimaginably crushing pressure in the deepest part of the ocean—the Mariana Trench—is only about 400 times the pressure in that same tire.
By the same conversion, kinking micas requires stresses 8–30 times the water pressure in the deepest ocean. Because Anderson found pulverized garnets proximal to kinked micas at the fault-filled field site, he and his colleagues inferred that the stresses momentarily experienced by these rocks as an earthquake’s rupture tore through the shear zone were about 1 gigapascal, or 9 times the pressure at the Mariana Trench.
Aben described this transient stress estimate for earthquakes as speculative, but he said the new study’s focus on earthquake-induced deformation fills a gap in research between very slow rock deformation that builds mountains and extremely rapid deformation that occurs during nuclear weapons testing and meteor impacts. And with micas, he said, “once they’re kinked, they will remain kinked,” preserving records of ancient earthquakes in the hearts of mountains.
—Alka Tripathy-Lang (@DrAlkaTrip), Science Writer