At a magnitude of 9.0, the earthquake off the Japanese coast last March was already known as one of the most powerful ever recorded, killing (in large part due to the ensuing tsunami) almost 16,000 people and damaging or destroying more than 125,000 buildings. A recent study (available here; subscription required) now quantifies just how monumental the event was: the seafloor in the Japan Trench northeast of the mainland, where the quake originated, was jolted 50 meters horizontally and 10 meters vertically–movement that was “abnormally, extraordinarily huge,” according to Toshiya Fujiwara of the Japan Agency for Marine-Earth Science and Technology.
Fujiwara led the research that used multibeam bathymetric surveys to measure the depth of the water and contouring of the seafloor. He noted that the research team did not expect to be able to use such equipment to detect the crust movement,which during most earthquakes occurs in scales of millimeters or centimeters. For example, the 2005 Miyagi earthquake, which had a magnitude of 7.2, registered a crustal shift of 10 centimeters at a geodetic station near the Japan Trench. The 2011 earthquake had a shift of 15 meters at the same station. The study also found another vertical shift of at least 4-6 meters of a slab of ocean crust between the Japan Trench and the Japanese coastline, which may have contributed to the pulsating pattern of the tsunami waves that eventually struck the country.
The researchers believe that the fault that caused the quake may extend as far as the axis of the Japan Trench.
“Previously, we thought the displacement stopped somewhere underground,” Fujiwara said, “but this earthquake destroyed the entire plate boundary.”
As we posted previously, a number of presentations at the AMS Annual Meeting in New Orleans will cover the community response to the earthquake and tsunami, including Junichi Ishida of the Japan Meteorological Agency who will discuss the earthquake’s impact, the JMA’s response to it, and lessons learned from the disaster in the keynote address for the 28th Conference on Interactive Information Processing Systems (Monday, 11:00 a.m., Room 356).