The 2009 Samoa Earthquake and Tsunami
Introduction:
An earthquake is 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. A tsunami is defined as a
very large wave caused by an underwater earthquake or volcanic eruption. In addition both of
these terms have a very similar definition in which they share some common causes. Both of
these definitions help explain the reason to why the September 2009 Samoa earthquake and
tsunami occurred.
Background Information:
On Tuesday September 29, 2009 at 6:48 A.M 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. There were at least three aftershocks that reached at
least 5.6 magnitude. There were 15 foot waves during the tsunami. 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 (Figure 1). American
Samoa was hit by the tsunami about 25 minutes after the earthquake.
Figure 1. Graph showing the measurement of the wavelengths at Pago Pago.
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 (Figure 2). There were power outages in 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 2. Cars being taken away by the huge water masses in American Samoa.
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 broadscale 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 is caused by the Australian plate at a velocity of 86 mm/yr which is a long term average,
not during earthquake. The most active earthquake areas in the world are Australian/Pacific plate
boundary (Figure 3). 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 and outer-rise earthquake
occurred which are earthquakes that occur within the subducting or down-going plate before it
enters the subduction zone. Moreover, they are caused by tension in the subducting, oceanic plate
induced by bending as the plate enters the trench (Figure 4). The few outer-rise earthquakes that
have occurred in the past have been very devastating.
Figure 3. The earthquake occurred close to the subduction zone in the Pacific Ring of Fire.
Figure 4. Subduction zone, which shows the outer rise and tensional pressure within
the subducting plate.
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 5. Crews working on the damages that were left behind caused by the earthquake
and tsunami at the National Park of American Samoa.
Conclusion:
In conclusion the September 2009 Samoa earthquake and tsunami had an unusual type of faulting
associated with the earthquake and 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 causes these events and how
we can be prepared for them. Although, there were only a few of the same type of earthquakes
and tsunamis to have occurred in the past they have caused huge amount of damages and have all
been very devastating.
Sources:
Sources:
http://tidesandcurrents.noaa.gov/press/September09Tsunami.shtml
http://www.npr.org/templates/story/story.php?storyId=113311304
http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/us2009mdbi.php#summary
http://walrus.wr.usgs.gov/tsunami/samoa09/
http://www.ask.com/web?q=what+was+unusual+about+the+type+of+faulting+associated
+with+the+2009+samoa+earthquake+and+tsunami&search=&qsrc=0&o=0&l=dir
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