On September 29, 2009, at 17:48 UTC (6:48 a.m. local time), a magnitude 8.1 earthquake struck midway between Samoa and American Samoa, a U.S. territory. The earthquake generated tsunami waves of up to 22 meters (72 feet) that engulfed the shores, killing at least 192 people; 149 in Samoa, 34 in American Samoa, and 9 in Niuatoputapu, Tonga.
The devastation extended beyond human casualties with houses destroyed, cars swept out to sea and some villages being virtually annihilated. With over $200 million dollars in damages, the islands were ravaged both physically and economically.
With over 30 deaths in the U.S. territory of American Samoa, the 2009 event caused the largest number of deaths in America due to a tsunami in the 21st Century.
Cars in a parking lot were destroyed after the first wave hit in Pago Pago, American Samoa, in September 2009. Courtesy of NOAA NCEI.
An Uncommon Event
Most large earthquakes occur in subduction zones, where one plate of Earth’s crust dives beneath another plate. In this region, the Pacific plate subducts westward beneath the Australia plate at the Tonga Trench.
The deadly 2009 tsunami was triggered by at least two separate earthquakes occurring within 2–3 minutes of each other near the Tonga TrenchThe deadly 2009 tsunami was triggered by at least two separate earthquakes occurring within 2–3 minutes of each other near the Tonga Trench, one of the most seismically active areas in the world. This is an extremely rare event, known as a “doublet.”
Since the earthquakes occurred so close in time, scientists have not been able to distinguish which earthquake occurred first, or which caused a bigger tsunami. However, the events of September 29 involved a magnitude 8.1 earthquake on a normal fault within the outer rise; and the other magnitude 8.0 earthquake occurred on the subduction zone as a thrust event.
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