Earthquake Faults Can Heal Themselves In Mere Hours, Adding Power to Disastrous Quakes
After faults deep in the Earth get shaken up from a stress-relieving event, they can be patched right back up to continue the endless earthquake cycle. However, this self-healing ability doesn’t apply to the quick and violent earthquakes that many people are familiar with; instead, it helps rocks recharge after a slower type of seismic event that we can’t even perceive.
A new study published in Science Advances has uncovered the way that faults repair themselves after a slow slip event, which occurs as built-up stress is released over the span of several days, weeks, or months. Even as this stress slowly leaks out, faults can partially restore their own power in a matter of hours and prepare for another re-rupture. Knowing the details of how this process works could help researchers understand how regenerating faults fuel major earthquakes.
Read More: How Worried Should You Be About Earthquakes in the US?
The Stress of Tectonic Movement
Earthquakes tend to develop in subduction zones, where a dense tectonic plate slides under another, less dense plate. As these plates scrape together, they sometimes get locked from friction. Stress then builds up until the plates finally manage to slip out of their stalemate, generating massive earthquakes as a result.
This can happen in a number of subduction zones around the world, like the Cascadia subduction zone off the Pacific Coast of North America. Here, the Juan de Fuca Plate slides beneath the North American plate.
The movement of these plates can even trigger “megathrust earthquakes,” which are powerful enough to exceed magnitude 9.0 on the Richter scale. However, megathrust earthquakes happen infrequently; only 13 have come from the Cascadia subduction zone over the past 6,000 years, each one taking place hundreds of years apart, according to Earthquakes Canada.
Recharging Earthquake Energy
Slow slip events (SSEs) occur beneath the “transition zone” where tectonic plates get locked, and they’re characterized by more gradual tectonic motion that happens over a long period of time, according to Earth Scope.
While the slow slip movement alleviates some stress on certain portions of a fault, it simultaneously aggravates stress in the parts where they’re locked. So while SSEs underneath the Cascadia subduction zone, for example, aren’t nearly powerful enough to be felt by anyone living on the Pacific coast, they still increase the risk of a powerful earthquake as they add stress to locked plates little by little.
Unlike a typical earthquake, SSEs don’t deplete all of their energy at once before starting the long process of rebuilding stress back at step one. After energy is released from an SSE, faults can reload stress in a short timespan, as researchers behind the new study observed from seismic data at the Cascadia subduction zone.
“We discovered that deep faults can heal themselves within hours,” said study author Amanda Thomas, professor of earth and planetary sciences at UC Davis, in a statement. “This prompts us to reevaluate fault rheological behavior, and if we have been neglecting something very important.”
Glued Back Together
To understand how faults heal themselves, the researchers replicated SSEs in experiments where they packed powdered quartz into a silver cylinder, welded it shut, and put it under pressure of 1 Gigapascal (10,000 times atmospheric pressure) at 500 degrees Celsius.
In essence, the researchers were simulating what happens in the aftermath of an SSE by “cooking” the quartz sample. After measuring how fast sound waves could move through the sample, they opened the cylinders and examined the sample’s structure via electron microscopy. They found that upon being compressed, the mineral grains welded together.
This shows that after an SSE, the rocks in faults glue themselves back together. The concept of cohesion, the researchers say, is something that may require more attention in earthquake models; it could potentially contribute to the stress at shallower faults and other faults that cause major earthquakes.
Read More: Mounting Pressure in the Tintina Fault Could Mean Dangerous Earthquakes
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