UT researchers study cause of slow slip earthquakes, shakes that cannot be seen or felt

Rylie Lillibridge, Senior News Reporter

A new study by UT researchers found the short-term strengthening of fault lines contributes to slow slip earthquakes, a type of earthquake that is not as destructive but occurs more frequently.

An earthquake occurs when stress on a fault line or area between plates builds up and ruptures. Study co-author Demian Saffer said slow slip events are similar to more severe earthquakes as they both occur due to fault line stress, but because they are so slow they cannot be felt or observed by the naked eye like these more destructive events. 

Fault lines repair themselves or are strengthened following a seismic event. However, because slow slips happen so often, the land plates do not have time to heal, causing frequent yet small movements.

“These rocks don’t heal in between slip events,” Saffer said. “So what that means is that it doesn’t take very long at all before they break again … the result is instead of every 100 years, something big, it’s like every two years, something that’s got much less energy.”

Geoscience researchers studied the Hikurangi fault line off the coast of New Zealand to learn more about slow slip events.

“Basically what they are is an earthquake in slow motion,” said Laura Wallace, one of the author’s of the study.

In a destructive earthquake, this rapid break of the fault line causes seismic waves to ripple through the Earth. Saffer said though slow slip events may release the same amount of energy as high-magnitude earthquakes, these earthquakes go undetected because the event occurs gradually, with some slow slip events taking months to rupture. 

“It’s so much slower, we don’t feel it,” Saffer said. “The only way we can detect them is with (GPS) monitoring … because it’s moving slowly and it doesn’t release any kind of waves that travel through the Earth that cause the shaking.”

Additionally, Saffer said slow slip events move less than larger earthquakes. 

“The energy is about the same, but they seem to move a larger area a smaller amount, whereas a big earthquake, for the same amount of energy, would give you a smaller region that’s moving, but it would move more,” Saffer said.

Wallace said this finding will help to uncover why slow slip events occur.

“It’s been a big mystery in the field of seismology and geophysics as to why these things exist,” Wallace said. “The ability of the fault rocks to heal and re-strengthen after they’ve ruptured is probably a very critical component of this whole thing, and that hasn’t really been clearly recognized before.”

Saffer said the next steps in the research are to apply the findings to other seismically active regions of the world such as the Pacific Northwest and offshore Central America. Team members will return to New Zealand in the upcoming weeks to collect data and samples by sending a robotic submarine down to the trench of a fault line, the shallow area where tsunamis are most effectively generated.

“The idea is to take this study on global tour,” Saffer said. “This is the kind of study that might help us make some predictions.”