Largest Fault Slip Ever Responsible For 2011 Japan Earthquake and Tsunami
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redOrbit Staff & Wire Reports – Your Universe Online
The magnitude 9.0 Tohoku-Oki earthquake that occurred off the Japanese coast in March 2011, triggering a tsunami and ultimately resulting in more than 15,000 deaths, was the result of the largest fault slip ever recorded, according to a trio of studies published Thursday in the journal Science.
According to Jane J. Lee of National Geographic, researchers calculated that the boundary between two tectonic plates in the Japan trench slipped by as much as 164 feet. Other earthquakes of similar magnitude, including the 9.1 event in Sumatra nine years ago, resulted in a 66-foot to 82-foot fault slip, she added.
“The Tohoku fault is more slippery than anyone expected,” stated co-author and University of California, Santa Cruz professor of Earth and planetary sciences Emily Brodsky. “It’s been difficult to get this measurement because the signal is weak and it dissipates over time, so we needed a big earthquake and a rapid response.”
All three studies were based on the findings of the international Japan Trench Fast Drilling Project (JFAST), which drilled across the Tohoku fault last year. A team of 27 scientists from 10 different countries participated in the 50-day expedition, which took place on a Japanese drilling vessel, drilling three holes in the trench to study the rupture zone.
The JFAST team, of which Brodsky was a part, installed a temperature observatory in one of the three boreholes, which were located approximately seven kilometers beneath the ocean’s surface. The effort allowed temperature readings, other data, and core samples from across the fault, the authors said.
According to McGill University, one of the institutions involved in the research, the largest fault slip before 2011 occurred off the coast of Chile in 1960. During that seismic event, seafloor plates were displaced by an average of 20 meters, while in the Tohoku earthquake, the slip amounted to 30 to 50 meters. In addition, it actually grew larger as the subterranean rupture approached the seafloor, ultimately triggering the tsunami.
“This gives us some unprecedented insights into how earthquakes actually work. No one really knows how much frictional resistance there is to slip and this for the first time gives us some idea,” said co-author and Oregon State University geology and geophysics professor Robert Harris.
“The project itself was an engineering feat and an amazing one at that,” he added. “To reach the fault, the team had to drill through 800 meters of the seafloor – at a depth of nearly 7,000 meters below the ocean’s surface. It pushed the limits of that technology as far as they can go.”
Analysis of the data collected during the JFAST project demonstrated a 0.31 degree Celsius anomaly with the surrounding temperatures at the plate’s fault boundary. As tectonic plates rub together, the friction creates heat, and by measuring changes to the temperature, scientists can calculate the amount of energy generated during the earthquake. In this case, the temperature anomaly corresponds to 27 million joules (27 megajoules) per square meter of dissipated energy, the researchers reported.
“Even though the earthquake produced a 1,100° to 2,200°F (600° to 1,200°C) temperature increase, the amount of friction that had to be overcome to produce the fault slip wasn’t as large as researchers expected,” Lee said. “This helped confirm the fact that something else was going on” – specifically, clay lubrication. The minerals can trap water and become slippery, the researchers told National Geographic, and this phenomenon was likely responsible for the slip.
Image 2 (below): An international team of scientists has concluded that an unusually thin and slippery geological fault where the North American plate rides over the edge of the Pacific plate caused a massive displacement of the seafloor off the coast of Japan in March 2011, touching off the devastating tsunami that struck the Tohoku region. Credit: JAMSTEC/IODP