Japan Lashed by Powerful Earthquake, Devastating Tsunami
The magnitude 9.0 earthquake that struck Japan on March 11, 2011, was unprecedented in the modern history of Japan, according to scientists at the U.S. Geological Survey (USGS). "This was a massive earthquake," said USGS research geophysicist Bill Ellsworth, "one of the largest earthquakes that we have ever recorded" and the largest instrumentally recorded earthquake ever to hit Japan.
The earthquake occurred on a fault that forms the boundary between the Pacific and North American tectonic plates. In this region, the Pacific plate is sliding (being "subducted") westward beneath the North American plate, on which northern Japan sits, at a rate of about 80 mm (3 inches) per year, beginning its westward descent at the Japan Trench. The earthquake's epicenter—the location on the Earth's surface directly above where rupture began—lies about 140 km (90 mi) west of the Japan Trench and about 130 km (80 mi) east of the city of Sendai on Honshu, Japan's main island.
Preliminary estimates indicate that a large area of the fault ruptured during the earthquake: about 400 km (250 mi) long by 150 km (90 mi) wide, with its long dimension parallel to the coast. (Download Earthquake Summary Poster at http://earthquake.usgs.gov/earthquakes/eqarchives/poster/2011/20110311.php.) Scientists estimate that tens of meters of slip took place along the rupture zone. This slip suddenly and permanently uplifted some areas of the seafloor and down-dropped others, causing corresponding vertical movement of the overlying water. Preliminary modeling by scientists at the University of California, Santa Barbara (UCSB), indicates that the March 11 earthquake may have raised parts of the seafloor as much as 9 m (30 ft). (See figure 5.1 at http://www.geol.ucsb.edu/faculty/ji/big_earthquakes/2011/03/0311/Honshu.html.) The tsunami was born as the energy of the displaced water moved away from the source in a series of waves. (For more information about how tsunamis form and move through the ocean, visit http://walrus.wr.usgs.gov/tsunami/basics.html.)
While the March 11 tsunami traveled throughout the Pacific Ocean basin, damaging harbors as far away as Crescent City and Santa Cruz, California, the most devastating effects were caused by the "local" tsunami, whose first crest may have reached Japan's northeast coast as quickly as 10 minutes after the earthquake. This local tsunami was widely documented in online videos that show dark water pushing through towns and over farmland, crushing buildings and carrying swirling masses of debris, including cars and houses. News accounts reported walls of water as high as 30 to 40 ft and waves that reached as far as 10 km (6 mi) inland. More precise measurements of inundation distances (the horizontal distance between the shoreline and the farthest point inland reached by the water) and runup heights (the difference between the ground elevation of the tsunami's farthest inland reach and sea level at the time of the tsunami) will be made when teams of scientists are able to conduct field surveys. Survey teams may be hard pressed to find structures on which to measure water levels because some coastal towns, such as Natori and Minamisanriku, were virtually obliterated by the tsunami. At this writing (March 15), the death toll is in the thousands and expected to rise.
Scientists are currently analyzing the wealth of data collected during the earthquake and tsunami, and their preliminary findings have shed some light on why the tsunami was so large. Several factors contributed to the tsunami's enormous size, including the magnitude, depth in the Earth's crust, and overlying water column of the triggering earthquake.
The March 11 earthquake "was one of the very largest earthquakes that we have ever recorded during the past hundred years of instrumental studies of earthquakes," according to Ellsworth. At magnitude 9.0, it is exceeded by only a few earthquakes, such as the 1964 Alaska earthquake. (See list of "Largest Earthquakes in the World Since 1900" at http://earthquake.usgs.gov/earthquakes/world/10_largest_world.php.) It was nearly as large as the magnitude 9.1 earthquake that struck Sumatra in 2004 and triggered the destructive Indian Ocean tsunami.
The depth of the earthquake in the Earth's crust also affects tsunami size. Preliminary estimates put the focus of the March 11 earthquake—the point where rupture began—approximately 24 km (15 mi) beneath the Earth's surface, making it a relatively "shallow" earthquake. (Subduction-zone earthquakes can occur as deep as 700 km below the Earth's surface.) The shallower the earthquake is in the Earth's crust, the more energy is transferred to the overlying water, and the larger the resulting tsunami.
The size of a tsunami is also determined by both the amount of elevation change on the seafloor and the depth of the overlying water column. As noted earlier, parts of the seafloor may have risen as much as 9 m (30 ft) during the March 11 earthquake. Preliminary modeling by UCSB scientists (http://www.geol.ucsb.edu/faculty/ji/big_earthquakes/2011/03/0311_v3/Honshu.html, fig. 4) indicates that the most extreme seafloor displacement during the March 11 earthquake occurred in fairly deep (approx 4,000 m) water. The deeper the water above the deformed seafloor, the greater the mass of displaced water and the larger the tsunami. (For a more detailed discussion of the effects of rupture location on tsunami size, visit http://soundwaves.usgs.gov/2010/04/research2.html and scroll down to the paragraph beginning "Earthquakes with the same magnitude….")
Subduction-zone earthquakes, like the March 11 event, commonly cause simultaneous uplift of offshore regions and subsidence of coastal land. Coastal subsidence is another factor that field-survey teams will investigate; preliminary observations suggest that parts of the Japanese coast subsided more than half a meter (about 2 ft) during the March 11 earthquake, making these areas even more vulnerable to the subsequent tsunami.
Measurements by field-survey teams will add to those already available from seismographs, tide gauges, deep-ocean tsunami-monitoring systems, GPS (Geographic Positioning System) stations, and other sensors to produce a huge dataset that scientists will study to improve our understanding of earthquakes and tsunamis and help devise better ways to prepare for them. That Japan—arguably the best-prepared country in the world for earthquakes and tsunamis—has been hit so hard by the March 11 event is a harsh reminder of how much we still have to learn.
Visit http://earthquake.usgs.gov/earthquakes/eqinthenews/2011/usc0001xgp/ for additional information about the earthquake. Visit http://walrus.wr.usgs.gov/tsunami/sendai11/ to view computer animations of the tsunami hitting the east coast of Japan. Visit http://walrus.wr.usgs.gov/news/field.html to view photographs and preliminary observations of the effects of the tsunami in California and Hawai‘i, plus links to Web sites with additional information and images.
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