Magnitude 8.1 - SAMOA ISLANDS REGION

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2009 September 29 17:48:10 UTC

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Earthquake Details

  • This event has been reviewed by a seismologist.
Magnitude8.1
Date-Time
Location15.509°S, 172.034°W
Depth18 km (11.2 miles) set by location program
RegionSAMOA ISLANDS REGION
Distances185 km (115 miles) ENE of Hihifo, Tonga
190 km (120 miles) S of APIA, Samoa
710 km (440 miles) NNE of NUKU'ALOFA, Tonga
2700 km (1680 miles) NNE of Auckland, New Zealand
Location Uncertaintyhorizontal +/- 5.2 km (3.2 miles); depth fixed by location program
ParametersNST=403, Nph=403, Dmin=>999 km, Rmss=0.97 sec, Gp= 22°,
M-type=teleseismic moment magnitude (Mw), Version=V
Source
  • USGS NEIC (WDCS-D)
Event IDus2009mdbi
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Earthquake Summary

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Felt Reports

At least 149 people killed in Samoa, 34 people in American Samoa and nine people killed and four injured on Niuatoputapu, Tonga. Widespread damage to infrastructure at Pago Pago, American Samoa, in many parts of Samoa and on Niuatoputapu, Tonga. Felt (V) at Apia, Samoa and (IV) at Ili`ili and Tafuna, American Samoa. Felt in much of American Samoa, Samoa and northern Tonga. Felt as far away as Wallis and Futuna Islands. A tsunami with wave heights (peak-to-trough) was recorded at: 314 cm at Pago Pago, American Samoa; 140 cm at Apia, Samoa; 47 cm at Rarotonga and 8 cm at Penrhyn, Cook Islands; 41 cm in the Chatham Islands, 37 cm at Raoul, 22 cm at East Cape, 21 cm at North Cape, 21 cm at Tauranga, 16 cm at Napier, 15 cm at Gisborne, 11 cm at Wellington and 3 cm at Waitemata Harbour, New Zealand; 14 cm at Nuku`alofa, Tonga; 11 cm at Papeete, French Polynesia.

Tectonic Summary

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The broad-scale tectonics of the Tonga region are dominated by the relative convergence of the Pacific and Australia plates, with the Pacific plate subducting westward beneath the Australia plate at the Tonga trench. At the latitude of the earthquake of September 29, 2009, the Pacific plate moves westward with respect to the interior of the Australia plate at a velocity of about 86 mm/year. The earthquake occurred near the northern end of a 3,000 km long segment of the Pacific/Australia plate boundary that trends north-northeast.; farther north of the earthquake’s source region, the plate boundary trends northwest and then west. The eastern edge of the broad Australia plate may be viewed as a collection of small plates or microplates that move with respect to each other and with respect to the Pacific plate and the Australia plate interior.

On the basis of currently available location and fault mechanism information, we infer that the September 29 earthquake occurred as a normal fault rupture on or near the outer rise of the subducting Pacific plate.

The broad-scale Australia/Pacific plate boundary is one of the most active earthquake regions in the world. Earthquakes occur on the thrust-fault boundary between the Australia and Pacific plates, within the Pacific plate on both sides of the trench, and within and on the boundaries of the small plates that compose the eastern edge of the overall Australia plate.

Tsunami Information

The earthquake locations and magnitudes cited in NOAA tsunami statements and bulletins are preliminary and are superseded by USGS locations and magnitudes computed using more extensive data sets.

General Tsunami Information