Researchers from a renowned U.S. university captured a slow slip earthquake in motion. It was captured during the act of releasing tectonic pressure on a major fault zone at the bottom of the ocean.
A team from the University of Texas at Austin recorded the slow earthquake spreading along the tsunami-generating portion of the fault off the coast of Japan, behaving like a tectonic shock absorber. The team described the event as the slow unzipping of the fault line between two of the Earth’s tectonic plates.
A type of slow-motion seismic event that takes days or weeks to unfold, slow slip earthquakes are relatively new to science and are thought to be an important process for accumulating and releasing stress as part of the earthquake cycle. The new measurements, made along Japan’s Nankai Fault, appear to confirm that.
Borehole sensors were placed to detect slow-motion seismic event
“It’s like a ripple moving across the plate interface,” said Josh Edgington, who conducted the work as a doctoral student at the University of Texas Institute for Geophysics (UTIG) at UT Austin’s Jackson School of Geosciences.
Researchers captured the earthquake with the help of borehole sensors that were placed in the critical region far offshore, where the fault lies closest to the seafloor at the ocean trench.
UTIG Director Demian Saffer, who led the study, stressed that sensors installed in boreholes can detect even the slightest motions, as small as a few millimeters.
Such movement on the shallow fault is all but invisible to land-based monitoring systems such as GPS networks.
The slow slip earthquake, captured by the team’s sensors in the fall of 2015, traveled along the tail of the fault — the region close to the seafloor where shallow earthquakes can generate tsunamis, easing tectonic pressure at a potentially hazardous location. A second slow tremor in 2020 followed the same path.
“In this work, we analyze formation pore pressure records from three offshore borehole observatories at the Nankai subduction zone, Honshu, Japan, to capture detailed slip-time histories of two slow slip events (SSEs) along the outermost reaches of the plate boundary,” said researchers in the results published in the journal Science recently.
“Slip initiates ~30 kilometers landward of the trench; migrates seaward at 1 to 2 kilometers per day to within a few kilometers of, and possibly breaching, the trench; and coincides with the onset and migration of tremor and/or very-low-frequency earthquakes. The SSE source region lies in a zone of high pore fluid pressure and low stress, which provides clear observational evidence linking these factors to shallow slow earthquakes.”
Borehole sensors tracked unzipping motion
The two events, which have only now successfully been analyzed in detail, appear as ripples of deformation traveling through Earth’s crust. Originating about 30 miles off the coast of Japan, borehole sensors tracked this unzipping motion along the fault as it moved out to sea before dissipating at the edge of the continental margin, according to a press release.
Researchers also pointed out that although the Nankai Fault is known to generate large earthquakes and tsunamis, the discovery suggests that this part of the fault does not contribute energy to these events, acting more like a shock absorber. The results will help researchers home in on the behavior of subduction zone faults across the Pacific Ring of Fire, the tectonic belt that spawns the planet’s largest earthquakes and tsunamis.
Events took several weeks to travel 20 miles
Each event took several weeks to travel 20 miles along the fault, and each one happened in places where geologic fluid pressures were higher than normal. The finding is important because it is strong evidence that fluids are a key ingredient for slow earthquakes. This is an idea widely circulated in the scientific community, but finding a direct connection has been elusive until now, as per the release.
Researchers also revealed that 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, recurring slow slip earthquakes.
The team stressed that 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.
Using these details, scientists can begin to probe other regions of the fault to better understand the overall hazard it poses. Saffer underlined that knowledge is also vital for understanding other faults.
