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Japan's March 11, 2011 Earthquake and Tsunami
Travis Hudson, American Geological Institute, 4220 King Street, Alexandria, VA 22302

In the early afternoon of March 11, 2011 the largest earthquake in Japan’s history shook most of northern Honshu, Japan’s biggest island. The 9.0 magnitude earthquake’s epicenter was located on the seafloor 129 km (80 miles) off Honshu’s northeast coast.  Only three historical earthquakes, the 2004 Indonesia (M = 9.1), 1964 Alaska (M = 9.2) and 1960 Chile (M = 9.5) earthquakes were bigger.   

Japan has endured the devastating effect of large earthquakes for thousands of years. This is because Japan is near major boundaries between large moving blocks - tectonic plates - that form the outer solid part of Earth. Figure 1 shows the main (red star) and aftershock (circles) epicenters offshore Honshu. The red lines in Figure 1 mark the major boundaries between plates. The large Pacific Plate moves westward about 8 cm per year and sinks below Japan along an inclined boundary called a subduction zone. Much plate movement takes place along subduction zones around the rim of the Pacific Ocean and these movements are responsible for the world’s largest earthquakes.  

Earthquakes occur along subduction zones because the plate movements along them are not gradual and continuous. Instead, stress builds up in the rocks along the subduction zone as they slowly deform. Eventually, increased stress causes the rocks to rupture along the plate boundary. The March 11, 2011 Japan earthquake began about 32 km (20 miles) below the epicenter and migrated laterally, primarily to the north and south. It took about 2 minutes for this rupture to spread over about a 45,000 km2 (11 million acre) area. Strong shaking was felt on northern Honshu throughout this rupture period. The rupture resulted in rock displacements of up to 30 m (100 feet) along the plate boundary and the eastward movement of Honshu’s coast as much as 5 m (16 feet) in some places. Local stress release continued as hundreds of strong aftershocks (M > 5; see animation) occurred near the primary rupture zone for many days.    

Movements of the seafloor accompanied the March 11 earthquake and displaced huge amounts of overlying seawater. This produced the subsequent tsunami that was so devastating on Honshu. Figure 2 shows how a tsunami forms as a result of rupture along a subduction zone (excellent animations of this process are provided by PBS and the Woods Hole Oceanographic Institute).  Tsunami is a Japanese word that literally means “harbor wave”. Japan’s many bays and inlets along its east coast have been repeatedly inundated by tsunamis (NOAA), a consequence of being along a coast so close to a subduction zone plate boundary.

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map of March 2011 earthquakes in Japan
(click image to enlarge)
USGS

Figure 1. Map showing the major plate boundaries (red lines), main shock epicenter (red star), aftershock epicenters (circles), and main rupture area (pink) offshore Honshu, Japan (from Tohoku, Japan 2011 M9.0 Educational Slides).

diagram of how an earthquake can cause a tsunami
(click image to enlarge)
USGS

Figure 2. Diagram showing how rupture along a subduction zone displaces the seafloor and overlying seawater to form a tsunami. From Surviving a Tsunami.

Additional Resources: Surviving a Tsunami is an excellent overview of tsunamis around the Pacific Ocean and what people can do if they are threatened by one. The USGS’s March 11, 2011 Japan Earthquake  site is a great starting point.

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EarthNote No. 5 , © 2010-2014 American Geosciences Institute,
P. Patrick Leahy, Director, 4220 King Street, Alexandria VA 22302