Ruptures from ‘silent’ earthquakes deep in Earth’s crust can heal themselves within hours
Deep cracks in the Earth’s crust can glue back together within hours after some earthquakes, new research suggests.
Specifically, these cracks can heal quickly after what geologists call slow-slip events. This is when strain- and stress-induced movement between two sides of a fault occurs over days, weeks, or months, rather than seconds or, for the case the biggest earthquakesminutes.
“Slow-slip events and regular earthquakes can occur on the same major fault systems, but they typically occur at different depths and under different physical conditions,” Thomas told Live Science in an email. “What determines whether the fault slips slowly or suddenly is the frictional behavior of the fault and the effective stress exerted on the fault.”
Thomas and his colleagues studied slow-slip events deep within the Cascadia Subduction Zone, a “megafault” where the Juan de Fuca Plate slides beneath the North American Plate. Cascadia experiences a lot of these slow slip events and has an exceptional seismic monitoring network, making it one of the best places in the world to study this phenomenon, Thomas said.
The megafault is capable of triggering earthquakes of magnitude 8 and 9. “In subduction zones like Cascadia, large earthquakes occur in shallow, cooler rocks, while slow slips occur deeper, where temperatures and pressures are much higher and fluids are abundant,” Thomas explained.
Cascadia slow-slip events are unusual in that they sometimes rupture the same area multiple times during a single event. In just a few hours, a fault zone can rupture multiple times, suggesting both that stresses are rapidly recharging and that a “healing” process is occurring between ruptures. “This repeated reactivation is one of the puzzles that our study tried to explain,” Thomas said.
The results were published on November 19 in the journal Scientific advances.
Since the depths of Cascadia are inaccessible, the researchers recreated in the laboratory the conditions believed to exist deep in the subduction zone. They loaded a silver capsule with quartz powder and a trickle of water to mimic rocks and deep fluids, respectively. The team then welded the capsule together, heated it to about 930 degrees Fahrenheit (500 degrees Celsius), and subjected it to pressure 10,000 times greater than atmospheric pressure for up to 24 hours.
Next, the researchers used electron microscopy to determine what happened to the quartz powder. They found that the mineral grains had been welded together, even in samples that had only been “cooked” for a few hours.
“Healing of defects is highly dependent on temperature, pressure and the presence of fluids,” Thomas said. “In our experiments, these conditions produced measurable reinforcement within a few hours.”
Regular earthquakes typically occur in shallower regions of the crust, so it takes much longer – years or even decades – for fractures to heal. “Our results suggest that the same basic process can operate throughout the crust, but that the time frames change depending on the environment,” Thomas said.
The other part of the puzzle addressed in the study is how stress is recharged so quickly during slow-slip events in Cascadia. The subduction zone experiences low-frequency earthquakes, which are small seismic events triggered in bursts when the same zone ruptures over and over again. These bursts overlap with ocean tidal cycles, suggesting that tidal changes can cause a fault to re-rupture just hours after repair.
“In Cascadia, rapid healing means that parts of the deep fault can strengthen quickly enough to be reactivated several times during a slow slip cycle,” Thomas said. “This affects how we model slow slip and how we interpret the signals we use to monitor the deep fault.”
Fault repair is also an important consideration in shallower regions, including those known to cause major earthquakes. Repair processes should be included in the next generation of models because they could improve our understanding of seismic risks, Thomas said.
