Swirls in a 15-Million-Year-Old Fault Show Clay Flowed Nearly Like Water in a Quake
Osaka researchers say vortex-like structures on Japan's Boso Peninsula point to a sudden drop in friction that can send a fault slipping far enough to trigger a tsunami.
TOKYO — Swirls frozen inside an ancient fault in Japan suggest that clay in the fault zone once flowed almost like water during an earthquake about 15 million years ago, a finding that could sharpen how scientists judge the tsunami threat from quakes under the sea. The work, led by Professor Tetsuro Hirono of Osaka Metropolitan University, was published in Scientific Reports.
The 2011 earthquake in eastern Japan is the textbook case. That magnitude 9.1 quake shook its epicenter for almost six minutes. Its effects did not stop there, because the movement of the plates displaced the seafloor enough to cause a devastating tsunami. Understanding how such earthquakes generate tsunamis is a central part of building early-warning systems.
One proposed mechanism starts with friction. When plates move rapidly, the sliding generates large amounts of heat. As the temperature climbs, water trapped in the pores of the rock heats up and its pressure rises, which makes the fault boundaries increasingly slippery. The fault then slips an exceptionally large distance, and the seafloor above it moves enough to push a tsunami. The process is known as thermal pressurization.
Testing that idea directly is hard, so researchers often turn to similar events preserved in rock from Earth's past. Hirono's group studied rocks exposed on the Boso Peninsula in Japan that were once part of an ancient seafloor environment. In an ancient fault there, they identified vortex-like structures. The rock layers showed wavy and curved laminations, which the team described as "turbulent-flow-like." The pattern suggests the material underwent fluid-like flow during an earthquake roughly 15 million years ago.
The researchers then ran fluid-dynamical analyses of the structures. Their findings indicated that thermal pressurization had been enough to fluidize the clay in the fault. "Our analysis suggests that the viscosity approached that of water," 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 study, with Tatsuru Fukuta as first author, is titled "Coseismic turbulence-like flow of fault material along the shallow portion of a plate-subduction-related fault." The word coseismic refers to something that happens during the shaking itself, and the shallow part of a subduction fault is the zone closest to the seafloor, where slip does the most to lift water.
The group hopes to carry the same method over to faults that are active today. If the viscosity of fault material during slip can be estimated from rock structures, scientists may be able to judge more accurately how large past slips were. "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," Hirono concluded.
The result does not predict when a quake will strike. It adds physical evidence for how a fault can turn from sticky to slick within moments, and why some earthquakes move the seafloor far more than others.
