For the first time ever, scientists have observed a slow-motion earthquake as it happened along a major underwater fault off the coast of Japan.
Unlike the sudden, violent shaking of a typical earthquake, this one moved gradually over several weeks, unzipping the fault line like a ripple traveling through Earth’s crust.
The discovery sheds new light on how some earthquakes release stress in a much gentler way—and what that means for future hazards like tsunamis.
The groundbreaking study, published in Science, was led by researchers from The University of Texas at Austin.
They captured the event using advanced borehole sensors installed deep beneath the ocean floor in a region known as the Nankai Fault. This fault has a history of generating powerful earthquakes and tsunamis, including a magnitude 8 quake in 1946 that destroyed thousands of homes.
Slow-motion earthquakes, also called slow slip events, are relatively new to science. These events unfold over days or even weeks and don’t usually cause shaking felt at the surface.
Yet they play a major role in the earthquake cycle by gradually releasing built-up pressure between tectonic plates.
In this case, the slow earthquake happened in the tsunami-generating section of the fault, close to the seafloor.
Researchers described the event as a ripple of movement between two massive tectonic plates. They first detected the slow slip in 2015 and another similar event followed in 2020, both taking place about 30 miles off Japan’s coast and traveling some 20 miles along the fault before fading away.
The key to this discovery was the borehole sensors, which can detect movements as small as a few millimeters—motions too subtle for land-based GPS systems to catch.
The slow slip appears to have acted like a natural shock absorber, relieving pressure in the part of the fault that lies closest to the surface. That’s good news, because it means that this shallow region may not contribute to large, tsunami-triggering quakes. It also confirms something scientists have long suspected: that fluids trapped in the rocks deep underground may help trigger these slow slips.
Understanding how this part of the Nankai Fault behaves could help scientists better predict the risks posed by similar subduction zones around the Pacific Ring of Fire. One such fault, the Cascadia Subduction Zone off the U.S. Pacific Northwest, has not shown signs of slow slip at its tsunami-generating end—raising concerns that it may be more tightly locked and capable of a much more dangerous quake.
With this new insight from Japan, researchers hope to install more precise monitoring systems in other high-risk areas like Cascadia, offering a better chance of understanding—and possibly predicting—future megaquakes.
Source: University of Texas at Austin.
