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The tsunami that hit Samoa, American Samoa, and Tonga on September 29, 2009, was generated by an unusual type of earthquake that occurs near oceanic trenches, called an “outer-rise” earthquake. Unlike typical tsunamigenic earthquakes that occur on the thrust fault that separates tectonic plates in a subduction zone (termed the interplate thrust), outer-rise earthquakes occur within the subducting or downgoing plate before it enters the subduction zone.

There have been only a few verified instances of tsunamis generated by outer-rise earthquakes, but those that have occurred have been devastating. The 1933 Sanriku tsunami generated from a magnitude 8.6 outer-rise earthquake resulted in more than 3,000 deaths in Japan and significant damage on the Island of Hawai‘i. The 1977 Sumba magnitude 8.2-8.3 outer-rise earthquake resulted in 189 deaths in Indonesia. The September 29, 2009, Samoa outer-rise earthquake, of magnitude 8.1 according to the National Earthquake Information Center (NEIC), resulted in comparable fatalities. It was the fourth largest outer-rise earthquake to have been instrumentally recorded since 1900.

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For several years, the U.S. Geological Survey (USGS) Menlo Park Tsunami Sources Working Group has been reexamining the 1933 Sanriku earthquake and tsunami and its tectonic setting, as well as identifying trends among other large outer-rise earthquakes worldwide. This investigation is a part of an overall effort to evaluate the tsunamigenic potential of the world's subduction zones.

Outer-rise earthquakes are caused by stresses in the subducting oceanic plate induced by bending as the plate enters the trench (see subduction-zone diagram, above). Flexure of the plate elevates the sea floor, creating an oceanic feature that parallels the oceanic trench and is known as the "outer rise." As the plate flexes, tensional stresses in the oceanic crust can create large normal faults, in which rock on one side of the fault moves down and away from rock on the other side. Crustal stresses caused by earthquakes on the interplate thrust fault in subduction zones can also be transferred to the outer rise, triggering earthquakes on normal faults that are already close to failure.

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 2009 Samoa earthquake occurred east of the Tonga trench, near the northern terminus of the Tonga volcanic arc, where the trench takes a sharp bend to the west (see map). Correspondingly, normal faults in the outer-rise and trench slope change orientation from northeast-southwest trends near the main part of the Tonga trench to east-west trends near the east-west-trending part of the Tonga trench. The fact that the normal faults are nearly parallel to the trench suggests that the faults occur primarily in response to bending stresses in the oceanic plate. These and other tectonic characteristics in outer-rise regions where great earthquakes occur are also found in many other subduction zones in the western Pacific and northeastern Indian Oceans.

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When a fault ruptures beneath the seafloor, the rocks surrounding the fault are permanently uplifted in some areas and downdropped in others, with the ocean going along for the ride to generate the tsunami (see “Life of a Tsunami,” panel 1). Rupture of an interplate thrust at a subduction zone typically occurs below a substantial thickness of sediment, and the rupture does not reach the seafloor (see subduction-zone diagram, top of page). In contrast, outer-rise normal faults typically rupture brittle oceanic basalt in a region that has very little sediment cover, and so the rupture commonly reaches the seafloor. For this reason, it is likely that the fault that ruptured during the 2009 Samoa earthquake can be mapped using bathymetric techniques.

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Particularly detailed images of the seafloor can be obtained using multibeam mapping systems, which send out a fan of sound energy and then record sound reflected from the seafloor through a set of narrow receivers aimed at different angles. Dawn Wright (professor of geography and oceanography at Oregon State University) and colleagues conducted a multibeam survey of the Tonga trench in 1996 that includes images of the seafloor near the epicenter of the September 29, 2009, earthquake. In a map created from their data and data collected by Scripps Institution of Oceanography, several prominent, curved normal faults are visible entering the trench at an oblique angle where the trench curves around the northern Tonga arc (see map of multibeam bathymetry). The orientation of these faults is similar to the faulting geometry of the September 29 earthquake as determined from analysis of seismic waveforms, and it may well be one of these faults that ruptured during the recent earthquake. Preliminary analysis by Gavin Hayes (NEIC) of how much the fault slipped during the September 29 earthquake indicates that there was a large amount of slip (as much as 14 m) near the seafloor on a steeply dipping rupture, further suggesting that the earthquake may have produced a mappable step, or scarp, where the ruptured fault intersects the seafloor. New mapping data from this area could be compared with existing multibeam bathymetry to look for a seafloor scarp produced by the September 29 fault rupture.

Many of the aforementioned characteristics of outer-rise earthquakes can explain why the tsunami was so large. The maximum fault slip for this earthquake (approximately 14 m) is much higher than for an interplate thrust earthquake of comparable magnitude (typically 3-8 m). Greater slip translates into greater vertical movement of the seafloor, affecting the entire ocean above the rupture zone. Moreover, tsunami generation by this outer-rise earthquake occurred in much deeper water than the more typical tsunami generation above an interplate-thrust earthquake (see subduction-zone diagram, top of page). When a tsunami travels from deep water to shallow water, the speed of the wave crest or trough slows, the wavelength decreases, and the amplitude (and wave height) increases. This process is sometimes referred to as “shoaling amplification” (see “Life of a Tsunami,” panel 3). A tsunami that starts off in deeper water will be more amplified by the time it reaches shore than a comparable tsunami that starts off in shallower water. Preliminary field-survey data indicate that the tsunami runups (height above mean sea level) in American Samoa reached more than 15 m, which is higher than for most tsunamis generated by magnitude 8.1 earthquakes on the interplate thrust (typically 2-10 m).

For additional information, including animations of the generation and propagation of the September 29 tsunami, visit Preliminary Analysis of the 2009 Samoa Tsunami.

It is hoped that continued research on the nature and occurrence of outer-rise earthquakes around the world will help identify potential sites for future outer-rise earthquakes of this size and help mitigate the tsunami hazard associated with such rare but devastating events.