Tsunami Historical Series: Samoa - 2009

Tsunami Historical Series: Samoa - 2009 thumbnail


At 6:48 on the morning of September 29, 2009 (17:48 UTC), an 8.1 moment magnitude earthquake struck near the Samoan Islands in the southwest Pacific Ocean. The Pacific Tsunami Warning Center (PTWC) quickly determined that the large magnitude of this earthquake, its location under the sea floor, its relatively shallow depth within the earth, and a history of tsunami-causing earthquakes in the region meant that it could have moved the seafloor and thus posed a significant tsunami risk. PTWC issued its first tsunami warning several minutes later for Samoa, American Samoa, Tonga, and other nearby island groups. The earthquake did in fact cause a dangerous tsunami, and over the following hours PTWC tracked it through the Pacific Ocean and updated its alerts with measured tsunami wave heights and recommended that additional areas consider coastal evacuation. PTWC canceled all tsunami alerts about four hours after the earthquake.

Destructive waves only struck islands near the earthquake’s epicenter, where casualties were significant. The tsunami reached over 12 m or nearly 40 ft. high in Samoa, killing 149 people there. In nearby American Samoa the waves were even higher at over 17 m or 55 ft, killing 34, and topped 22 m or 72 ft in Tonga, killing 9 more people there. Smaller waves traveled throughout the Pacific Ocean but caused no more deaths or damage.

The Pacific Tsunami Warning Center (PTWC) can create an animation of a historical tsunami like this one with the same tool that it uses to determine tsunami hazards in real time for any tsunami today: the Real-Time Forecasting of Tsunamis (RIFT) forecast model. The RIFT model takes earthquake information as input and calculates how the waves move through the world’s oceans, predicting their speed, wavelength, and amplitude. This animation shows these values through the simulated motion of the waves and as they travel through the world’s oceans one can also see the distance between successive wave crests (wavelength) as well as their height (half-amplitude) indicated by their color. More importantly, the model also shows what happens when these tsunami waves strike land, the very information that PTWC needs to issue tsunami hazard guidance for impacted coastlines. From the beginning the animation shows all coastlines covered by colored points. These are initially a blue color like the undisturbed ocean to indicate normal sea level, but as the tsunami waves reach them they will change color to represent the height of the waves coming ashore, and often these values are higher than they were in the deeper waters offshore. The color scheme is based on PTWC’s warning criteria, with blue-to-green representing no hazard (less than 30 cm or ~1 ft.), yellow-to-orange indicating low hazard with a stay-off-the-beach recommendation (30 to 100 cm or ~1 to 3 ft.), light red-to-bright red indicating significant hazard requiring evacuation (1 to 3 m or ~3 to 10 ft.), and dark red indicating a severe hazard possibly requiring a second-tier evacuation (greater than 3 m or ~10 ft.).

Toward the end of this simulated 48 hours of activity the wave animation will transition to the “energy map” of a mathematical surface representing the maximum rise in sea-level on the open ocean caused by the tsunami, a pattern that indicates that the kinetic energy of the tsunami was not distributed evenly across the oceans but instead forms a highly directional “beam” such that the tsunami was far more severe in the middle of the “beam” of energy than on its sides. This pattern also generally correlates to the coastal impacts; note how those coastlines directly in the “beam” are hit by larger waves than those to either side of it.

You can view a YouTube version of this animation here.

Earthquake source used: USGS Finite Fault Model

Notable Features

  • This tsunami was generated by an 8.1 magnitude earthquake near the Samoan Islands on September 29, 2009.
  • Though this tsunami was not a Pacific-wide hazard, it was very destructive near the earthquake and killed 192 people in Samoa, American Samoa, and Tonga.
  • A tsunami is a series of waves, not just a single wave.
  • In the open ocean, tsunami waves can travel at speeds up to 800 km per hour or 500 mi. per hour, as fast as a jet plane.
  • Tsunami waves may be small in the open ocean, but wave heights can increase substantially as they approach the shore (indicated by the colored dots).
  • The wave "energy map" of maximum wave heights show that those coastlines directly in the energy "beam" of red/yellow had a much higher impact than those to either side of it.

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