New studies show that rocks can cement themselves together under the conditions found on a subducting fault. Scanning electron microscope images of quartz powder before (top) and after six and 24 hours of treatment at 1 Gigapascal and 500°C. Credit: James Watkins/ UC Davis.
A new study has found that earthquake faults deep in the Earth can glue themselves back together following a seismic event.
The research, led by scientists at the University of California Davis, adds a new factor to our understanding of the behaviour of faults that can give rise to major earthquakes.
Corresponding author on the paper, Amanda Thomas, professor of earth and planetary sciences at UC Davis, said: “We discovered that deep faults can heal themselves within hours.
“This prompts us to re-evaluate fault rheological behaviour, and if we have been neglecting something very important.”
Thomas, UC Davis colleague Professor James Watkins and collaborators studied slow slip events or SSEs, a type of seismic movement that is like an earthquake in slow motion.
Earthquakes occur when the stresses that build up as the Earth’s tectonic plates rub against each other over centuries or millennia are released in a few seconds of violent shaking.
In about 2002, Thomas said, seismologists identified a new, slower type of seismic event. In a slow slip event, stresses built up over months to years are released in movements of a few centimetres over days, weeks or months.
Slow slip events in Cascadia fault zone
To characterise deep slow slip events, the team analysed seismic data from the Cascadia Subduction Zone of the Pacific Northwest, where the Juan de Fuca plate dives under the North American plate.
Slow slip events do not behave quite like earthquakes; the same section of fault can re-rupture within hours or days.
This suggests both that the fault has at least partly repaired itself and that stresses have been reloaded on a short timescale.
Small stress changes from the tides show that the fault is reloaded on short timescales, Thomas said.
The gravitational forces of the Sun and Moon act on the Earth’s crust just as they act on the oceans to make tides rise and fall. At the same time, the weight of moving sea water also acts on rocks beneath it.
The missing part of the puzzle is how faults can heal on a short timescale.
Under pressure
Watkins is a geochemist who studies what happens to minerals under high heat and pressure. His laboratory has equipment to simulate conditions deep in the crust or under a volcano.
Watkins and Thomas developed 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°C.
He said: “We’re simulating what happens in the aftermath of a slow slip event. We cook it and look at it.”
They measured how fast sound waves could move through the ‘cooked’ sample, then opened the cylinders and used electron microscopy to study the structure.
After compression, the researchers found the mineral grains were welded together.
Thomas said: “It’s like quick set fault glue; it’s really fast and you can get significant strength recovery.”
This cohesion – the ability of faults to repair themselves – may be important elsewhere, including in shallower faults and those known for causing major earthquakes.
Thomas added: “Cohesion is neglected in most models. Under certain conditions, cohesion may be more important than we thought.”
Thomas and Watkins recently received a new grant from the National Science Foundation to further study cohesion on earthquake faults.
Watkins said: “It links events on the microscopic scale to major thrust earthquakes on a scale of hundreds of kilometres,” Watkins said.
The work is published in Science Advances.
