'Slow-Motion' Earthquakes witnessed in real time

 For the First Time, Scientists Witness 'Slow-Motion' Earthquakes in Real Time

Slow slip events can teach seismologists more about earthquake prediction. A deep-sea fault off Japan was caught in the act of slowly unzipping, offering a rare glimpse into the hidden mechanics of earthquake cycles. Seafloor sensors caught a rare slow quake in action. It hints at how Earth’s stress is quietly released. For the first time, scientists have directly observed a slow slip earthquake in the midst of relieving tectonic stress along a major underwater fault zone. Slow-motion earthquakes, as you might guess from the name, involve the release of pent-up geological energy over the course of days or weeks rather than minutes, and scientists have now recorded some as they were happening. This gradual seismic event was tracked as it moved through the tsunami-prone segment of the fault off Japan’s coast, where it appeared to act as a natural buffer, absorbing pressure. Researchers at the University of Texas at Austin likened the phenomenon to a fault line slowly unzipping along the boundary between two tectonic plates. These quakes, also known as slow slip earthquakes or just slow earthquakes, are typically too gentle to cause immediate danger. However, they can help scientists predict full-speed earthquakes or tsunamis, which can of course be far more dangerous.

A team led by researchers from the University of Texas Institute for Geophysics (UTIG) tracked two separate slow slip events (SSEs) in real time, one in 2015 and another in 2020. “It’s like a ripple moving across the plate interface,” said Josh Edgington, who led the research while completing his doctorate at the University of Texas Institute for Geophysics (UTIG), part of UT Austin’s Jackson School of Geosciences. Unlike sudden earthquakes, slow slip events unfold over days or weeks. Though only recently recognized by scientists, these events are believed to play a key role in how stress accumulates and is released along faults. The new observations from Japan’s Nankai Fault provide compelling evidence supporting this idea. Special borehole sensors were positioned deep underwater, close to the Nankai Trough subduction zone off the coast of Japan. There, the Philippine Sea plate is pushing under the Eurasian plate. The researchers describe the activity of the slow quakes as being like a tectonic shock absorber. 

This significant scientific advancement was made possible by borehole sensors installed far offshore, in the area where the fault lies closest to the seafloor near the ocean trench. According to UTIG Director Demian Saffer, who led the research, these borehole instruments are capable of detecting extremely subtle ground movements, as small as a few millimeters. Such fine-scale motion in the shallow portion of the fault cannot be captured by conventional land-based systems like GPS networks. The measurements confirm what scientists had previously thought about these slow-motion earthquakes, which were only recently discovered: that they can be significant in releasing (or building) stress around a faultline. This subduction zone is part of the Pacific Ring of Fire, an extensive collection of volcanoes and faults surrounding the Pacific Ocean. It's responsible for many of the largest earthquakes and tsunamis on record. Sensors and observation instruments being lowered into a borehole off the coast of Japan nearly 1,500 feet below the seafloor during an International Ocean Discovery Program mission in 2016. Sensors like these transmit data in real time to researchers in Japan and at the University of Texas Institute for Geophysics, and enabled researchers to detect and describe a slow slip earthquake in motion in a new study in Science. 

The team’s instruments recorded a slow slip earthquake during the fall of 2015 as it moved along the fault’s outermost section near the seafloor, an area known for generating tsunamis during shallow quakes, where it appeared to release tectonic strain in a potentially high-risk zone. A similar event followed the same trajectory in 2020. And the findings here, about the shock absorber effect, will be crucial in understanding when and where future earthquakes could hit. Other faults lack this kind of tectonic protection, including Cascadia off the western coast of North America. "This is a place that we know has hosted magnitude 9 earthquakes and can spawn deadly tsunamis," says geophysicist Demian Saffer, from UTIG. "Are there creaks and groans that indicate the release of accumulated strain, or is the fault near the trench deadly silent?" While the Nankai Fault has a history of producing major earthquakes and tsunamis, these observations suggest that this particular segment may not contribute energy to such destructive events. Instead, it may act as a seismic buffer. These findings offer valuable insights into the mechanics of subduction zone faults across the Pacific Ring of Fire, a region responsible for some of the world’s most powerful earthquakes and tsunamis. "Cascadia is a clear top-priority area for the kind of high-precision monitoring approach that we've demonstrated is so valuable at Nankai."

The two slow slip events, only recently examined in detail, manifested as waves of deformation moving through the Earth’s crust. Originating roughly 30 miles off Japan’s coast, the unzipping motion was traced by the sensors as it progressed seaward before fading at the continental margin. It's only possible to measure these SSEs because of advances in sensor technology, meaning shakes of much lower strength, sometimes only shifting the ground a few millimeters at a time, can be detected. Through their analysis, the researchers were able to determine that slow earthquakes may be related to high geologic fluid pressures, and that the upper part of the fault can release pressure independently of the rest of it. The so-called Ring of Fire is an area surrounding the Pacific tectonic plate where many of the world’s earthquakes and volcanic eruptions occur.  Both quakes took several weeks to advance 20 miles along the fault, and both occurred in zones where underground fluid pressure was higher than usual. This detail is especially significant, as it provides compelling evidence that high fluid pressure plays a critical role in triggering slow earthquakes, an idea long theorized by geophysicists but until now lacking clear observational proof.

All of this helps to inform models predicting earthquakes and tsunamis, with the potential to save thousands of lives. The last time Japan’s Nankai Fault produced a significant earthquake was in 1946. The magnitude 8 earthquake destroyed 36,000 homes and killed over 1,300 people. Although another large earthquake is expected in the future, the observations suggest the fault releases at least some of its pent-up energy harmlessly in regular, re-occurring slow slip earthquakes. The location is also important, because it shows that the part of the fault nearest the surface releases tectonic pressure independently of the rest of the fault. Predicting earthquakes isn't an exact science, with a host of variables involved, but it's something we're getting better at. With each study and technological upgrade, seismologists are improving their models, and adding in data from slow earthquake activity could help greatly. Armed with that knowledge, scientists can begin to probe other regions of the fault to better understand the overall hazard it poses. The knowledge is also vital for understanding other faults, Saffer said. For instance, Cascadia, a massive earthquake fault facing the Pacific Northwest, appears to lack Nankai’s natural shock absorber. Although some slow slip has been detected at Cascadia, none has been detected at the tsunami-generating, tail end of the fault, which suggests that it may be strongly locked to the trench, Saffer said.

"The patterns of strain accumulation and release along the offshore reaches of subduction megathrusts are particularly important toward understanding hazards associated with shallow coseismic slip and tsunamigenesis," wrote the researchers. “This is a place that we know has hosted magnitude 9 earthquakes and can spawn deadly tsunamis,” Saffer said. “Are there creaks and groans that indicate the release of accumulated strain, or is fault near the trench deadly silent? Cascadia is a clear top-priority area for the kind of high-precision monitoring approach that we’ve demonstrated is so valuable at Nankai.” The borehole observatories used in the Japan study were installed by the Integrated Ocean Drilling Program and funded by the US National Science Foundation. Other data were supplied by ocean floor cable observatories operated by Japan Agency for Marine-Earth Science and Technology (JAMSTEC).