The seafloor jumped about half the length of a football field when a magnitude-9 earthquake hit Japan in 2011, trig-gering a devastating tsunami and surprising scientists. “That’s the largest slip recorded–ever. The question was: Why?” said Emily Brodsky, a UC Santa Cruz geophysicist and co-author of three papers about the Tohoku-Oki earthquake published in Science.
The answer was in the temperature of the rocks beneath the sea. During earthquakes, tectonic plates generate friction when they grind against each other. High levels of friction create intense heat that takes several years to dissipate from rock. Those frictional forces can be calculated from temperature measurements, explained Patrick Fulton, a UCSC seismologist on the heat-seeking mission.
The UCSC scientists joined an international research team to get core samples from the fault zone. Then, they strung 55 temperature sensors across the fault to measure the background heat. Their findings: Three-tenths of a degree of excess temperature remained at the fault.
“That small change tells us the friction was very low, making that huge slip possible,” said Fulton.
Results from the two other Science papers showed a layer of clay at the fault zone also contributed to the slippery site.
Next, the researchers plan to take temperature measurements from other earthquakes to see if low readings always correspond to big jolts.