The earthquake of March, 2011 which struck off the Japanese island of Honshu killing over 15,000 people was, at M9.0, one of the largest on record. Earthquakes are caused by the rupture of the earth’s crust as stress builds up and now new research has revealed the extent of the displacement of the sea floor caused by the earthquake
Earthquakes and Tsunamis
Earthquakes are one of several mechanisms which generate tsunamis, although not all earthquakes do so. Key factors, according to NOAA, are that the earthquake must ‘occur underneath or near the ocean, be large and create movements in the sea floor’. Even if all these conditions are met a tsunami will not necessarily result, as the displacement of the sea floor must be sufficient to set large volumes of seawater in motion.
Typically, displacement caused by a subterranean earthquake doesn’t generate a major threat, and research by Noson et al has suggested that a magnitude of at least 7.5 is required for a tsunami to occur. Even then, other factors (both physical and human) determine how damaging such an event will be. It is worth noting that there was no tsunami damage caused by a foreshock of M7.3 or an aftershock of M7.9 on either side of the M9.0 Japan earthquake.
The Extent of Displacement of the March 2011 Japan Earthquake
Research undertaken by Toshiya Fujiwara and colleagues has shown that the rupture zone (the area of the sea bed deformed by the earthquake) extended across an area measuring approximately 500km by 200 km. The epicentre of the earthquake was in shallower water to the seaward side of the Japan Trench (where the Pacific Plate disappears beneath the North American Plate) and the research showed that the rupture reached as far as the trench axis.
Perhaps more significantly than the extent of the surface affected, there was also significant vertical displacement of the sea floor – and this was the cause of the devastating tsunami. The coseismic displacement caused by the 2011 earthquake was the ‘largest on record since state of the art … sea floor surveys have been conducted,’ Dr Fujiwara told Decoded Science.
The research shows that the extent of the slippage was more extensive than first thought, with horizontal slip of around 50m and vertical movement of up to 10m. Initial studies based on GPS and geodetic measurements had suggested horizontal displacement of over 20 meters and vertical movement of 3 meters (Sato et al).
The report is important, in that it is the first time that seafloor movement caused by tsunamigenic earthquakes has been detected. But perhaps most significantly, it gives a measurement of the actual movement caused by a major earthquake, and an indication of the power of the earth. In the words of Dr Fujiwara, ‘we have never known such a huge coseismic displacement’.
Noson, Qamar, and Thorsen. Washington State Earthquake Hazards. (1988). Washington Division of Geology and Earth Resources Information Circular 85. Accessed 1 December, 2011.
Fujiwara, T, et al. The 2011 Tohoku-Oki Earthquake: Displacement Reaching the Trench Axis. (1 December, 2011). Science. Accessed 1 December, 2011.
NOAA. Tsunami: the Great Waves. Accessed 1 December, 2011.
Sato, M. et al. Displacement Above the Hypocenter of the 2011 Tohoku-Oki Earthquake. Science. (June 2011). Accessed 1 December, 2011.
USGS. Magnitude 9.0 – near the east coast of Honshu, Japan. Accessed 1 December, 2011.
Decoding Science. One article at a time.