Scientists have identified a thin layer of slippery clay beneath the Japan Trench that allowed the 2011 earthquake to rupture all the way to the seafloor. The discovery explains the massive seafloor displacement that generated the deadly tsunami. Researchers reached this conclusion through record-breaking deep ocean drilling.
A team led by Christine Regalla of Northern Arizona University recovered sediment samples from about 26,000 feet below the ocean surface. The expedition, conducted aboard the research vessel Chikyu, was recognized by Guinness World Records as the deepest scientific ocean drilling project completed to date.
The samples revealed a 100-foot-thick layer of soft pelagic clay sandwiched between stronger rocks. This weak layer concentrated the fault rupture along a narrow path during the magnitude 9.1 earthquake, enabling the seafloor to shift 130 to 200 feet.
"That's equivalent to the entire area between Los Angeles and San Francisco moving 130 to 200 feet in just six minutes," Regalla said. Co-author Patrick Fulton of Cornell University noted that the geologic layering at the trench predetermines where the fault forms.
The study, published in Science, suggests the clay layer extends for hundreds of miles and may make the region more prone to shallow-slip events. Researchers say the findings could help improve forecasts and preparedness for future earthquakes and tsunamis.