Ancient fault rocks reveal earthquake slip turned clay fluid-like

When the magnitude 9.1 earthquake struck off eastern Japan in 2011, shaking continued for nearly six minutes, and the sudden displacement of the seafloor unleashed a devastating tsunami. Understanding exactly how earthquakes generate such massive seafloor displacement is central to improving early-warning systems, and one proposed mechanism involves frictional heating: rapid plate movement generates intense heat along the fault, raising the pressure of water trapped in rock pores until the fault boundaries become exceptionally slippery, allowing a huge slip that displaces the seafloor enough to launch a tsunami. Now, a research group led by Professor Tetsuro Hirono of Osaka Metropolitan University has found direct evidence in ancient rocks that this mechanism, known as thermal pressurization, can go even further, reducing fault material to a fluid-like state during an earthquake.

Examining rocks exposed on the Boso Peninsula in Japan, which were once part of an ancient seafloor environment, the researchers identified vortex-like structures within an ancient fault that suggest the fault material underwent fluid-like flow during an earthquake approximately 15 million years ago. The team described the rock layers as "turbulent-flow-like", because they displayed wavy and curved laminations along with vortex-like structures, the kinds of swirl patterns familiar from flowing liquids. When the researchers performed fluid-dynamical analyses of these structures, their findings suggested that thermal pressurization had been sufficient to fluidize the clay within the fault.

"Our analysis suggests that the viscosity approached that of water," Professor Hirono explained. "This would have been a dramatic reduction. The fluidized state would have acted as a form of dynamic lubrication, causing the fault to weaken dramatically.

The resulting large-scale slip would have produced a large displacement on the fault near the seafloor, potentially causing a tsunami." The significance of the finding lies in what it implies about the physics of large earthquakes on subduction zones, where one tectonic plate slides beneath another. Such large slips near the seafloor are precisely what drive the most destructive tsunamis, so recognizing the conditions under which fault material fluidizes could sharpen estimates of tsunami potential for modern subduction margins.

Because the evidence comes from rocks that preserved the aftermath of a single ancient earthquake, the study is observational in nature, and the researchers' conclusions rest on interpreting the preserved structures through fluid-dynamical analysis rather than direct measurement of a modern event. Even so, ancient exhumed faults offer something instrumental records cannot: a close-up view of the physical state of fault material at the moment of slip, frozen into the rock record millions of years ago. The group now hopes to apply the same methodology to modern faults.

"Understanding the change in viscosity in fault-slips may allow us to more accurately recognize the magnitude of fault slips, which would support tsunami mitigation efforts," Professor Hirono concluded. If the turbulent-flow signatures identified on the Boso Peninsula can be recognized or modeled in active fault zones, they could become a tool for estimating how far a fault is likely to slip in a future earthquake, a key input for tsunami forecasting and early-warning systems. The findings were published in Scientific Reports on 27 June 2026, in a paper titled "Coseismic turbulence-like flow of fault material along the shallow portion of a plate-subduction-related fault".

Subject of Research: Fluid-like flow of fault material during earthquakes and its role in tsunami generation

Article Title: Earthquakes make fault material flow like a liquid

Article References: Earthquakes make fault material flow like a liquid. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: earthquakes, fault mechanics, thermal pressurization, tsunami, subduction zone, fluidization, clay, viscosity, Boso Peninsula, ancient fault rocks, Osaka Metropolitan University, Scientific Reports