The 2009 Samoa Earthquake and Tsunami
On September 29, 2009 at 6: 48a.m. a powerful earthquake struck the South Pacific island nation of Samoa, which triggered a tsunami.

Figure 1: Map of Samona Islands.
Figure 1: Map of Samona Islands.

Background Information:
On Tuesday September 29, 2009 there was a submarine earthquake and tsunami located in the Samoa Islands region that struck about 20 miles below the ocean floor. The earthquake lasted about two to three minutes and there were at least three aftershocks that reached at least 5.6 magnitudes. A destructive basin-wide tsunami was created by an earthquake of a magnitude between 8.0 and 8.3 that occurred southwest of American Samoa. This ran through the regions of Hihifo, Tonga, APIA, Samoa, NUKU’ALOFA, Tonga, and Auckland, New Zealand. There was a widespread damage and most of the damages were felt in American Samoa, Samoa and northern Tonga. American Samoa was hit by the tsunami about 25 minutes after the earthquake. In addition, the day after Sumatra Indonesia was hit by a 7.6 magnitude earthquake at 5:16 p.m., America Samoa and Samoa both experienced four tsunami waves and the waves traveled at least one mile inland.

Figure 2:The blue box shows where the earthquake took place.
Figure 2:The blue box shows where the earthquake took place.

About Earthquakes and Tsunamis
Earthquakes are defined as an unexpected movement of the earth’s crust caused by release of tension that is build up along geologic faults or by volcanic activity. Where a tsunami is a very large wave caused by an underwater earthquake or volcanic eruption. Both of these natural disasters share common attibutes but in the end are very different from each other. Both of these definitions help explain the reason to why the September 2009 Samoa earthquake and tsunami occurred.

Figure 3:Graph showing the measurement of the wavelengths at Pago Pago.

Tectonic Summary
The 2009 Samoa tsunami was produced my an abnormal type of earthquake that occurs near ocean trenches. The relation of the convergence of the Pacific Australia plates control the broad-scale tectonics of the Tonga region, with the Westward subducting Pacific plate under the Australian plate at the Tonga trench. On September 29, 2009 the Pacific plate moved westward which was caused by the Australian plate. The plate moved at a velocity of 86 mm/yr which is a long term average, not during the earthquake. The most active earthquake areas in the world are the Australian/Pacific plate boundary which you can see below. The earthquake took place near the northern end of a 3,000 km long subdivision of the Pacific/Australia plate boundary that trends north-northeast. The earthquake of September 29 came about as a normal fault break close to or on the outer rise of the subducting Pacific plate. Typical tsunami earthquakes occur on the thrust fault that separates tectonic plates in a subduction zone, but in the Samoa earthquake and tsunami an outer-rise earthquake occurred. The earthquakes were caused by tension in the subducting, oceanic plate induced by bending as the plate enters the trench. The few outer-rise earthquakes that have occurred in the past have been very devastating.

Figure 4:The earthquake occured close to the subduction zone in the Pacific Ring of Fire.

Figure 5:Subduction zone, which shows the outer rise and tensional pressure within the subducting plate.

Damages and people killed
A tsunami was produced which caused substantial damage and loss of life in Samoa, American Samoa, and Tonga. There were at least 149 people that were killed in Samoa, nine people killed and 4 injured in Tonga, and 34 people killed in American Samoa. The Pacific Ocean earthquake generated soaring tsunami waves that cleared ashore on Samoa. Cars and homes were being swallowed by floodwaters. There were power outages which it made it difficult to evaluate the losses and damages. Tonga suffered some coastal damage caused by 13- foot waves. There was practically nothing left in the Samoan capital and schools and businesses were closed.

Figure 6:Cars being taken away by the huge water masses in America Samoa.

Figure 7:Damages from the Earthquake off Samoa.
Figure 7:Damages from the Earthquake off Samoa.

American Samoa
The National Park of American Samoa was hit by four tsunami waves they were anywhere from 15-20 feet high waves, and reached up to one mile inland shortly after the earthquake. The water streamed inland about 90 meters before moving back and left some cars struck in the mud. There were many Samoan people that were left injured or homeless due to this destruction. Pago Pago the largest city in America Samoa had continuous damage in the tsunami, which consisted of its main street to be flooded, cars were upside down, and shoreline business was damaged. The earthquake also caused really big damages to the American Samoa’s electrical infrastructure. The main electricity producer which is in Satala had been damaged (Figure 5). It is predicted for the Satala electrical plant to be out of service anywhere from a month or more until repairs have been made. The water lines were also damaged in account of the earthquake. The supplies of fresh water to eastern parts of American Samoa have been disturbed due to the broken water lines. However, water will be brought to the affected villages in tanker trucks. American Samoa will be receiving help from federal funds to be used for rescues, life support, and public health and safety measures.

Figure 8:Crews working on the damages that were left behind caused by the earthquake and tsunami at the National Park of American Samona.

In conclusion the September 2009 Samoa earthquake and tsunami had an unusual type of faulting associated with the earthquake. Although only a few of these types of tsunamis have occurred in the past we should still have some knowledge into what it is that can causes these events and how we can be prepared for them. They have caused huge amount of damages and have all been very devastating and ruined lives, by being a little more prepared we can hopefully not lose lives of innocent people.


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Tides & Currents. NoAA, 30 Sept. 2009. Web. 5 Nov. 2009.
USGS science for a changing world., 6 Oct. 2009. Web.
Western Coastal & Marine Geology. U.S. department of interior, 6 Nov. 2009. Web. 5 Nov. 2009.