UT researchers published a study in
Science
on June 26 that provided insight into the behavior of
slow slip earthquakes
, which release energy gradually and produce no detectable shaking.
Regular earthquakes occur when two sides of a fault slip in a sudden release of energy, said Joshua Edgington, a UT doctoral student who worked on the research. However, he said slow slip earthquakes release energy over days, making them difficult to detect with seismometers, which measure the shaking of the Earth.
Rather than measuring movement on the surface, the researchers detected the slow slip earthquake using three borehole sensors embedded into a shallow section of the Earth’s crust at the ocean trench of the Nankai Fault close to Japan.
Demian Saffer, director of the
UT Institute for Geophysics
, said the area of the slow slip earthquake builds up stress and then releases it in slow, small earthquakes, which acts like a “shock absorber.” The borehole sensors let the researchers detect even millimeters of movement.
“These measurements that we’re using now are much more sensitive than what we had before,” said Charles Williams, a research collaborator from New Zealand.
Saffer said the research is an important part of “hazard assessment.” The findings help refine the models used to understand the probability of future earthquakes or tsunami events, he said.
“Regions that have slow slip typically will not initiate an earthquake, but if all of the stress is not relieved, an earthquake can still propagate into that region, and that’s ultimately why we want to figure this out,” Williams said.
The borehole sensors on the Nankai Fault also showed that the central area where the slip occurs is actually “migrating” over time, Edgington said. This finding provides information on the redistribution of stress inside of the Earth. The slow slip earthquake, while relieving stress, can also likely lead to more stress being redistributed laterally along the fault, he said.
“Now that we know these things are moving, we can get a much better grasp on when, where and how much slip is happening,” Edgington said.
A slow slip earthquake can only occur under certain conditions. Edgington said factors such as temperature, minerals and presence of pore fluid impact the interaction of tectonic plates. For example, he said the pore fluid, or water in the Earth’s crust, acts like a “lubricant” between the plates and can lead to slow slip earthquakes.
“If you have elevated pore pressure along your fault interface, it’s going to make it less likely that the two pieces of the fault are going to stick together,” Edgington said.
Saffer said the most important target to understand is the Cascadia Fault, located in the U.S. Pacific Northwest, which is geologically similar to the Nankai Fault. If an earthquake were to occur there, it could have major economic impacts in Washington, Oregon and North California.
“The ultimate end game is earthquake prediction, right?” Edgington said. “If we get that, I’m out of a job, which will be unfortunate for me, but useful for all of humanity.”
