September 29, 2009 Somoa Quake and Tsunami

Updated 10/1/2009

The Samoa Islands were hit by a 7.9 M earthquake at 17:48 UTC (6:48 AM local time) on September 29, 2009. The epicenter was 139 miles SW of Pago Pago, American Somoa. The resulting tsunami caused considerable damage in both Somoa and in American Somoa. Over 150 people are estimated to have been killed as the coastal areas of all the Samoan islands were inundated. Casualties were reported from Somoa, American Somoa and Tonga. American Somoa is a US territory and has been declared a disaster area by President Obama.

Reports from the scene were that the quake was strong enough to rock buildings and cause rockslides in the mountains. It was distinguished more by its long duration of over a minute than by the strength of the shaking. This is indicative of a very strong quake that is distant enough to dilute its strength. Within 5 minutes the sea receded as the tsunami approached then swept ashore 10-15 feet deep. Boats were swept inland and cars were swept out to sea. Most people rushed to higher ground. A number of people were killed when they went down to the shore to gather stranded fish after the first wave receded, only to be caught by the second wave. Altogether there were 4 waves in the course of 20 minutes. There was extensive damage throughout the region. Although Somoa has a tsunami warning system, the tsunami hit too soon after the quake to allow the warning to take effect.

The earthquake occurred on the boundary between the Pacific and Australian Plates. The Pacific Plate subducts under the Australian Plate at the Tonga Trench. The epicenter was located near a bend in the plate boundary, at the northern end of the subduction zone. The magnitude has been reported as anywhere from 7.9 M to 8.3M. An earthquake of this size is catagorized as a "Great" earthquake and is capable of extensive damage. This is about the size of the 1906 earthquake that devastated San Francisco. In this case the fact that the quake occurred at sea over 100 miles from any populated area greatly reduced the damage from the quake itself. However, an earthquake of this size underwater does have the potential to create a tsunami, as this one did. The tsunami can travel great distances and in this case, the tsunami appears to have caused the most damage.

(See the Plate Tectonics page for more information on these processes.) The US Geological Survey (USGS) is an invaluable resource in understanding and tracking earthquakes. See the USGS summary of this quake.

See also the following news stories:

* Deadly Tsunami Strikes in Pacific (BBC 9/30/09)
*Somoa tsunami: survivors tell of giant walls of water (guardian.co.uk 10/1/09)
* Somoa tsunami lesson: Early warning system too slow (Christian Science Monitor 9/30/09)