Deep within the Earth’s crust, fractures caused by slow-moving earthquakes can mend themselves within a matter of hours, according to new research published in Science Advances on November 19. This remarkable self-repair mechanism occurs under extreme conditions of heat and pressure deep beneath tectonic plates. The study provides critical insights into how faults behave, and has implications for understanding both slow-slip events and the potential for larger, more destructive earthquakes.
Silent Earthquakes and Rapid Fracture Repair
Geologists have long known about “silent” earthquakes – slow-slip events where the ground deforms over weeks or months instead of rupturing violently in seconds. Unlike typical quakes, these don’t generate strong seismic waves, making them less immediately dangerous. However, they occur on the same major fault systems as large earthquakes, leading researchers to question why some areas slip slowly while others snap suddenly.
The key, according to lead author Amanda Thomas of UC Davis, lies in the physical conditions: fault frictional behavior and effective stress. “What determines whether the fault slips slowly or suddenly is the fault frictional behavior and the effective stress on the fault.” Slow slips happen deeper down, where temperatures are high and fluids are abundant.
The Cascadia Subduction Zone: A Natural Laboratory
The research focused on the Cascadia subduction zone, a region where the Juan de Fuca Plate slides under North America. This “megafault” is capable of producing magnitude 8-9 earthquakes, but also experiences frequent slow-slip events. Cascadia is unique because its extensive monitoring network allows scientists to study these phenomena in detail.
The study observed that certain areas of the Cascadia fault repeatedly rupture and then re-seal within the same slow-slip cycle. This rapid re-activation suggests stress builds up quickly, but also that the fault material can somehow “heal” between breaks. The research team sought to explain this puzzling behavior.
Lab Simulations Recreate Deep Earth Conditions
To investigate, researchers recreated the extreme environment of the subduction zone in a laboratory setting. A silver capsule was loaded with powdered quartz and water, then subjected to temperatures of 500°C and pressures 10,000 times higher than atmospheric pressure. Electron microscopy revealed that even after just a few hours, the quartz grains had fused together.
“Fault healing depends strongly on temperature, pressure, and the presence of fluids,” Thomas explained. The experiments proved measurable strengthening within hours under these conditions. This contrasts sharply with shallower crustal fractures, which take years to decades to heal.
Tidal Influence and Rapid Stress Reloading
The study also uncovered a link between the rapid healing process and ocean tides. The Cascadia fault experiences bursts of low-frequency earthquakes that align with tidal cycles. This suggests that changes in pressure from the tides can trigger re-ruptures just hours after the fault has repaired itself.
The results indicate that deep faults in Cascadia can strengthen quickly enough to be repeatedly stressed and re-activated within a single slow-slip cycle. This understanding is crucial for accurately modeling fault behavior and interpreting seismic data.
Implications for Earthquake Risk
Fault healing is not limited to deep subduction zones; it also occurs, though more slowly, in shallower regions where major earthquakes originate. The researchers argue that including repair processes in future earthquake models will improve risk assessment. Understanding how quickly faults can re-strengthen could refine predictions about where and when the next large quake will strike.
“Our results suggest the same basic process can operate across the crust, but the timescales change depending on the environment.” — Amanda Thomas, UC Davis
