A new study has identified that a frictional phenomenon could be key to understanding how often and how violently tectonic plate boundary faults move.
It’s called “frictional healing” – the ability of a fault to restrengthen and store energy between earthquakes.
They found that a fault that “heals” more slowly is more likely to release energy harmlessly in what are known as “slow slip events.” A fault that heals quickly is more likely to stick until it breaks all at once in a large, damaging earthquake.
While this discovery alone won’t allow scientists to predict when the next big earthquake will strike (the forces behind them are a bit more complex than that), it does give them a new way to investigate potential for them to occur.
“This doesn’t get us any closer to actually predicting earthquakes, but it does tell us whether a fault is likely to slip silently with no earthquakes, or have large ground-shaking earthquakes,” says Srisharan Shreedharan, a geophysicist at Utah State University in the US.
The boundaries between the tectonic plates of the Earth’s crust are made up of a system of faults. The movement of these plates causes pressure to build up in these fault zones and the sudden release of this pressure causes earthquakes.
But sometimes this pressure isn’t released all at once; slow slip events (SSEs) occur when the fault slips slowly over a period of days, weeks, or even months – instead of in seconds like in typical earthquakes.
This means they don’t usually cause a lot of shaking and generally release the pent-up energy in non-damaging ways.
“As surfaces slide against each other, we observe frictional properties, including frictional healing that describes the degree of fault restrengthening between earthquakes,” says Shreedharan.
“However, we know little about how this phenomenon may affect future slip events, including earthquakes.”
The researchers measured the frictional properties of rocks in the Hikurangi subduction zone in offshore New Zealand.
This is a convergent plate boundary where the Pacific plate subducts (or slides beneath) the Australian plate along the northern Hikurangi Trough at a rate of 50 to 60 millimetres per year.
Megathrust earthquakes, the planet’s most powerful earthquakes, occur at convergent plate boundaries. But it’s well documented that SSEs occur regularly at the Hikurangi subduction zone instead.
By squeezing the rock samples in a hydraulic press, they found that the clay-rich rocks were very slow to “heal” and slipped easily. Plugging the data into a computer model of the fault revealed a small, slow-motion tremor every two years – nearly exactly matching real life observations.
“We show that the very low healing rates of fault zone materials along the megathrust, which are typical of clay-rich faults in both subduction zones and other geological settings, are a key ingredient for the occurrence of recurring, frequent, small-stress-drop, slow slip transients,” the authors write in the paper.
This gives seismologists a new window into the working of subduction zone faults that they didn’t have before.
“The same physics and logic should apply to all different kinds of faults around the world,” says co-lead author Professor Demian Saffer, director of the University of Texas Institute for Geophysics in the US.
“With the right samples and field observations we can now start to make testable predictions about how big and how often large seismic slip events might occur on other major faults.”