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Field Survey of the Samoa Tsunami of 29 September 2009

On 29 September 2009, a strong earthquake took place south of the Samoa Islands in the southcentral Pacific. It triggered a local tsunami, which caused considerable damage and 189 fatalities on the Samoa Islands and in the northern Tonga archipelago. We present here the results of a tsunami survey conducted by an International Tsunami Survey Team in the Samoa Islands on 4-10 October 2009 and in northern Tonga on 25–27 November 2009.

### The Earthquake of 29 September 2009: Geographical Background

The earthquake occurred at 17:48:10 GMT (local time 06:48 on the 29th in Samoa; on the 30th in Tonga), with a source located at 15.51°S and 172.03°W and a focal depth estimated at 18 km by the U.S. Geological Survey (USGS). The epicenter is thus 200 km south of the Samoa Islands and 350 km NNE of the principal groups of Tonga (Figure 1). Note however, the presence of a small island, Niuatoputapu, only 200 km WSW of the epicenter.

The Samoa Islands comprise the territory of American Samoa, which regroups the island of Tutuila (142 km2; capital: Pago Pago), and the islets of Ofu, Olosega, Ta'u, Rose, and Swains, and the independent country of Samoa (formerly Western Samoa), comprised of the islands of Upolu (1,125 km2; capital: Apia), Savai'i (1,708 km2), and a few islets including Manono. The island of Niuatoputapu, the nearby islet of Tafahi, and the more distant island of Niuafo'ou belong to the Kingdom of Tonga.

### Plate Tectonics Background

The Samoa Islands are located 200 km north of the bend in the boundary of the Pacific plate marking the termination of the Kermadec-Tonga subduction zone. The convergent boundary expressing the subduction of the Pacific plate under the Australian one gives way to a strike-slip regime along a transform fault running north of the Fiji Islands, and linked across a spreading center in …