In the earth’s crust, tectonic blocks glide and grind past each other like huge ships detached from anchor. Earthquakes are generated along these fault zones when sufficient stress builds up for a block to get stuck and suddenly slip.
These slips can be aided by several factors that reduce the friction within a fault zone, such as warmer temperatures or pressurized gases that can separate blocks like a puck on an air hockey table. The decreasing friction allows one tectonic block to accelerate toward the other until it runs out of energy. Seismologists have long believed that this type of frictional instability can explain how all earthquakes start. But that may not be the whole story.
In a study published today in Nature communication, researchers Hongyu Sun and Matej Pec of MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS) find that ultra-fine-grained crystals within fault zones can behave like low-viscosity liquids. The finding provides an alternative explanation for the instability leading to earthquakes. It also suggests a link between earthquakes in the earth’s crust and other types of noise that occur deep in the earth.
Nanocarns are commonly found in rocks from seismic environments along the smooth surface of “mirrors”. These polished, reflective rock surfaces betray the sliding, sliding forces of previous earthquakes. However, it was unclear whether the crystals caused earthquakes or were simply formed by them.
To better characterize how these crystals behaved in a fault, the scientists used a planetary ball mill to pulverize granite stones into particles similar to those found in nature. Like a super-powered washing machine filled with ceramic balls, the machine knocked on the stone until all its crystals were about 100 nanometers in width, each grain 1 / 2,000 the size of an average grain of sand.
After packing the nanopowder in stamp-sized cylinders lined with gold, the researchers exposed the material to stresses and heat, creating laboratory miniatures of real fault zones. This process allowed them to isolate the effect of the crystals from the complexity of other factors involved in an actual earthquake.
The researchers report that the crystals were extremely weak when clipping was started – an order of magnitude weaker than more common microcrystals. However, the nanocrystals became significantly stronger as the deformation rate was accelerated. Pec, a professor of geophysics and Victor P. Starr Career Development Chair, compares this property, called “velocity enhancement”, to stirring honey in a jar. It is easy to stir the honey slowly, but it gets harder the faster you stir.
The experiment suggests that something similar is happening in fault zones. While tectonic blocks accelerate past each other, the crystals chew things between them like honey stirred in a seismic pot.
Sun, the study’s lead author and EAPS graduate student, explains that their findings contradict the dominant friction weakening theory of how earthquakes start. This theory would predict that surfaces in a fault zone have material that becomes weaker as the fault block accelerates and the friction should be decreasing. The nanocrystals did just the opposite. However, the inherent weakness of the crystals can mean that when enough of them accumulate in a fault, they can give way and cause an earthquake.
“We do not completely disagree with the old theorem, but our study really opens new doors to explain the mechanisms of how earthquakes occur in the crust,” says Sun.
The find also suggests a previously unknown connection between earthquakes in the crust and the earthquakes that rumble hundreds of kilometers below the surface, where the same tectonic dynamics are not at stake. So deep there are no tectonic blocks to grind against each other, and even if there were, the enormous pressure would prevent the type of earthquake observed in the crust, which necessitates some dilatation and void creation.
“We know that earthquakes happen all the way down to really great depths, where this movement along a friction fault is basically impossible,” says Pec. “And it’s clear that there have to be different processes that allow these earthquakes to happen.”
Possible mechanisms for these deep earthquakes include “phase transitions”, which occur due to atomic rearrangement in minerals and are accompanied by a volume change and other forms of metamorphic reactions, such as dehydration of aquifers, where released fluid is pumped through the pores and destabilizes a fault. These mechanisms are all characterized by a weak, velocity-enhancing layer.
If weak, velocity-enhancing nanocrystals are abundant in the deep Earth, they may present another possible mechanism, Pec says. “Maybe earthquakes are not a completely different beast than the deeper earthquakes. Maybe they have something in common.”
Rock crystals from the depths provide microscopic clues to earthquake movements
Hongyu Sun et al., Nanometric flow and earthquake instability, Nature communication (2021). DOI: 10.1038 / s41467-021-26996-0
Provided by the Massachusetts Institute of Technology
Citation: Nanograins: Study finds that strange properties of tiny crystals have traces of earthquake formation (2021, 24 November) retrieved 25 November 2021 from https://phys.org/news/2021-11-nanograins-curious-properties-tiny -crystals. html
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