Ruptures caused by “silent” earthquakes deep in the Earth's crust can heal themselves within hours

A new study suggests that deep cracks in the Earth's crust can stick together again within hours after certain earthquakes.

Specifically, these cracks can heal quickly after what geologists call slow slip events. This occurs when movement caused by deformation and pressure between the two sides of a fault occurs over days, weeks or months, rather than over seconds, or minutes in the case of the largest earthquakes.

Slow-slip events are “silent” earthquakes in which slip occurs much more slowly and does not radiate powerful waves that can make regular earthquakes dangerous, said Amanda Thomas, lead author of the study and a professor of geophysics at the University of California, Davis.

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“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 a fault slides slowly or suddenly is the behavior of the fault friction and the effective stress on the fault.”

Thomas and her colleagues studied slow-slip events deep within the Cascadia subduction zone, a “mega-rift” where the Juan de Fuca plate slides beneath the North American plate. Cascadia sees a lot of these slow-slip events and has an exceptional seismic network, making it one of the best places in the world to study the phenomenon, Thomas said.

The huge voltage is capable of causing earthquakes of magnitude 8 and 9. “In subduction zones like Cascadia, large earthquakes occur in shallow, cold rocks, while slow slip occurs at a greater depth where temperatures and pressures are much higher and fluids are available,” Thomas explained.

Cascadia slow slide events are unusual in that they sometimes rip the same area repeatedly during a single event. Within a few hours, one area of ​​a crack can break several times, indicating rapid stress reloading and a “healing” process occurring between ruptures. “Repeated reactivation is one of the mysteries that our study aims to explain,” Thomas said.

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The results were published on November 19 in the journal Science Advances.

Because the depths of Cascadia are difficult to access, researchers in the laboratory recreated conditions believed to exist deep within the subduction zone. They loaded a silver capsule with quartz powder and a little water to mimic deep rocks and fluids, respectively. The team then welded the capsule, heated it to about 930 degrees Fahrenheit (500 degrees Celsius), and placed it under pressure 10,000 times higher than atmospheric pressure for up to 24 hours.

Scanning electron microscope images show quartz fragments bonded together after 6 and 24 hours in a silver capsule. (Image credit: Watkins Lab, UC Davis)

Next, the researchers used electron microscopy to see what happened to the quartz powder. They found that the metal grains were welded together, even in samples that had been “cooked” for only a few hours.

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“Treatment of the defect depends strongly on temperature, pressure and the presence of fluid,” Thomas said. “In our experiments, these conditions produced a measurable enhancement within hours.”

Regular earthquakes usually occur in shallow areas of the Earth's crust, so it takes much longer — years to decades — for fractures to heal. “Our results suggest that the same basic process can operate across the cortex, but the timelines change depending on the environment,” Thomas said.

The other piece of the puzzle addressed by the study is how pressure reloads so quickly during slow-slip events in Cascadia. The subduction zone experiences low-frequency earthquakes, which are small seismic events caused by bursts when the same area ruptures over and over again. These eruptions interfere with ocean tidal cycles, suggesting that tidal changes could cause the rift to rupture again just hours after repairing itself.

“In Cascadia, rapid healing means that parts of a deep fault can strengthen again quickly enough to be reactivated several times during one slow slip cycle,” Thomas said. “This affects how we model slow slip and how we interpret the signals we use to monitor deep faulting.”

Fault remediation is also an important consideration in shallow areas, including those known to cause major earthquakes. Thomas said repairs should be included in the next generation of models because they could improve our understanding of earthquake risk.

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