The teams also measured topography and bathymetry near the shore, including reef-flat elevations. The tsunami that was triggered by a magnitude 8.1 earthquake on September 29, 2009, caused significant damage and loss of life on Samoa, American Samoa, and Tonga. Crustal stresses caused by earthquakes on the inter-plate thrust fault in subduction zones can also be transferred to the outer rise, triggering earthquakes on At the Tonga trench, the Pacific plate entering the subduction zone is particularly old and dense, resulting in a steep angle of descent and many normal faults near the trench. The Samoa tsunami of September 29, 2009, was the fifth tsunami studied by U.S. Geological Survey (USGS) field teams in 15 years, and yet it presented many surprises. Fringing reefs surround much of American Samoa and Samoa. The 1933 Sanriku tsunami generated from a Outer-rise earthquakes are caused by stresses in the subducting, oceanic plate induced by bending as the plate enters the trench.Flexure of the plate elevates the sea floor, creating an oceanic feature known as an "outer rise" that parallels the oceanic trench. Reliant on remittances and tourism, the economy of Samoa shrunk by 5.2 percent in 2009, as things got tougher in sending countries, and the nation’s only major manufacturer—an automotive components plant—laid off more than 1,550 workers. And the tsunami struck exactly when Samoa was already facing the full force of the global economic crisis. The loss of life could have been much worse had residents of the islands not been prepared for this 2009 event. There have only been a few verified instances of tsunamis generated by outer-rise earthquakes, but those that have occurred have been devastating. [Buildings were destroyed in Pago Pago, American Samoa. Because tsunamis have long wavelengths (as much as hundreds of kilometers), they are affected by the depth of the water they traverse from the time they are generated until they inundate land.
As a result, tsunami waves change direction as underwater topography refracts their travel path. Another possible cause for the strong return flow is the shape of the waves in the tsunami wave train; this factor is being investigated using hydrodynamic models. September 29, 2009 Samoa Tsunami, Southwest Pacific Ocean. The Samoa tsunami in September 2009 had many effects which changed the countries natural features both on the coastline and further inland. In order to improve tsunami-inundation models—which predict the behavior of tsunamis and are vital for helping communities design evacuation routes, build stronger structures, and determine the best areas for building—the USGS survey teams collected data that included: Click on a pin on the map to see more information. The combination of trapped water and gravity resulted in strong offshore flows. USGS teams collected data in American Samoa from October 5 to 22 and November 5 to 12, 2009, and in Samoa from October 14 to 22, 2009. The thought that the work we do helps to save lives focused our tsunami survey teams on the task at hand. The September 2009 Samoa tsunami was generated by an unusual type of earthquake that occurs near ocean trenches.