Laboratory experiments reveal that the ultra-fine crystals produced during fault motion can lead to earthquake instability, providing hints for understanding the potential global nucleation process of earthquakes. Credit: MIT
In the Earth’s crust, tectonic masses slide and fade into each other like massive ships separated from an anchor. Earthquakes are generated along these fault zones when enough pressure builds up for a mass to stick, and then slip suddenly.
These slips can be supported by several factors that reduce friction within the fault zone, such as higher temperatures or compressed gases that can separate lumps like pucks on an air hockey table. Decreasing friction enables one tectonic mass to accelerate against the other until it runs out of energy. Seismologists have long believed that this type of frictional instability could explain how all crustal earthquakes begin. But this story may not be complete.
In a study published today in Nature Communications, scientists Hongyu Sun and Matej Pec, of the Department of Earth, Atmospheric and Planetary Sciences (EAPS) at MIT, found that ultrafine crystals within fracture zones can behave like low-viscosity fluids. The discovery offers an alternative explanation for the instability that leads to crustal earthquakes. It also points to a link between earthquakes in the crust and other types of earthquakes that occur deep in the Earth.
Nanoparticles are commonly found in rocks from seismic environments along the smooth surface of “fracture mirrors”. These polished, reflective rock faces betray the slipping and sliding forces of past earthquakes. However, it wasn’t clear if the crystals were causing the earthquakes or if it was just a problem.
To better characterize how these crystals function within the rift, the researchers used a planetary ball milling machine to crush granite rock into particles similar to those found in nature. Like a super-powerful washer filled with ceramic balls, the machine pounded the rocks until all of their crystals were about 100 nanometers wide, each 1/2000 the size of an average grain of sand.
Packing nanopowder into postage stamp-sized cylinders coated with gold, the researchers then subjected the material to pressures and heat, creating miniature laboratory images of real fault zones. This process enabled them to isolate the effect of the crystals from the complexity of other factors involved in the actual earthquake.
The researchers reported that the crystals were very weak at the start of the shearing—an order of magnitude fainter than the most common microcrystals. But the nanocrystals became noticeably stronger when the rate of deformation accelerated. Pec, professor of geophysics and chair of Victor B. It is easy to stir in the honey slowly, but it becomes more difficult the faster you stir.
Experience suggests that something similar is happening in fault zones. When tectonic blocks slam into each other, the crystals glue things together like stir-fry honey in a seismic bowl.
Sun, lead author of the study and an EAPS graduate student, explains that their finding goes against the prevailing weak friction theory about how earthquakes start. This theory would predict that the fracture zone surfaces have material that weakens as the fault mass accelerates, and friction should decrease. The nanocrystals did just the opposite. However, the intrinsic weakness of the crystals can mean that when enough of them accumulate inside the fault, they can give way, causing an earthquake.
“We don’t completely disagree with the old theory, but our study really opens new doors to explaining the mechanisms of how earthquakes occur in the Earth’s crust,” Sun says.
The discovery also points to a previously unrecognized link between earthquakes in the Earth’s crust and earthquakes knocking hundreds of kilometers below the surface, where the same tectonic dynamics do not play a role. At this depth, there are no tectonic blocks that can grind against each other, and even if there were, the enormous pressure would prevent the kind of earthquakes observed in the crust that would require some stretching and void creation.
“We know that earthquakes happen all the way down to very great depths where this movement along a friction fault is essentially impossible,” Beck says. “And obviously there must be different processes that allow these earthquakes to happen.”
Possible mechanisms of these deep earthquakes include “phase transitions” that occur due to atomic rearrangements in minerals and are accompanied by a change in size and other types of metamorphic reactions, such as dehydration of water-bearing minerals, in which the released liquid is pumped through the pores and destabilizes the fault. All these mechanisms are distinguished by a weak coating that strengthens the rate.
If weak, rate-enhancing nanocrystals are abundant deep in the Earth, they could present another potential mechanism, Beck says. “Maybe crustal earthquakes are not a very different beast from deeper earthquakes. Maybe they have something in common.”
Rock crystals from depths give microscopic evidence of seismic Earth movements More information: Hongyu Sun et al, Nanofflow and seismic instability, Nature Communications (2021). DOI: 10.1038/s41467-021-26996-0 Provided by MIT
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