The Earth's largest earthquakes and tsunamis are usually caused by thrust-faulting earthquakes on the shallow part of the subduction interface between two tectonic plates, where stored elastic energy due to convergence between the plates is rapidly released. The tsunami that devastated the Samoan and northern Tongan islands on 29 September 2009 was preceded by a globally recorded magnitude-8 normal-faulting earthquake in the outer-rise region, where the Pacific plate bends before entering the subduction zone. Preliminary interpretation suggested that this earthquake was the source of the tsunami. Here we show that the outer-rise earthquake was accompanied by a nearly simultaneous rupture of the shallow subduction interface, equivalent to a magnitude-8 earthquake, that also contributed significantly to the tsunami. The subduction interface event was probably a slow earthquake with a rise time of several minutes that triggered the outer-rise event several minutes later. However, we cannot rule out the possibility that the normal fault ruptured first and dynamically triggered the subduction interface event. Our evidence comes from displacements of Global Positioning System stations and modelling of tsunami waves recorded by ocean-bottom pressure sensors, with support from seismic data and tsunami field observations. Evidence of the subduction earthquake in global seismic data is largely hidden because of the earthquake's slow rise time or because its ground motion is disguised by that of the normal-faulting event. Earthquake doublets where subduction interface events trigger large outer-rise earthquakes have been recorded previously, but this is the first well-documented example where the two events occur so closely in time and the triggering event might be a slow earthquake. As well as providing information on strain release mechanisms at subduction zones, earthquakes such as this provide a possible mechanism for the occasional large tsunamis generated at the Tonga subduction zone, where slip between the plates is predominantly aseismic.
The tsunamis on 26 December 2004 and 28 March 2005 killed only 7 people on Simeulue Island in Indonesia's Aceh province. At Langi, on the north end of Simeulue, which is 40 km south of the December earthquake's epicenter, maximum wave heights exceeded 10 m less than 10 minutes after the shaking ceased. In the more populous south, wave heights averaged 3 m and caused significant structural damage, destroying entire villages. Oral histories recount a massive 1907 tsunami and advise running to the hills after “significant” shaking (∼1 minute). All the interviewed Simeulue survivors knew of this event and of the necessary action. However, Jantang, on the Aceh mainland, suffered far more casualties. Simeulue's oral history provided an extraordinarily powerful mitigation tool that saved countless lives where even a high-tech warning system with a 15-minute response time would have been of no help.
An International Tsunami Survey Team ITST conducted field surveys of tsunami effects on the west coast of northern and central Sumatra and offshore islands 3–4 months after the 26 December 2004 tsunami. The study sites spanned 800 km of coastline from Breuh Island north of Banda Aceh to the Batu Islands, and included 22 sites in Aceh province in Sumatra and on Simeulue Island, Nias Island, the Banyak Islands, and the Batu Islands. Tsunami runup, elevation, flow depth, inundation distance, sedimentary characteristics of deposits, near-shore bathymetry, and vertical land movement subsidence and uplift were studied. The maximum tsunami elevations were greater than 16 m, and the maximum tsunami flow depths were greater than 13 m at all sites studied along 135 km of coastline in northwestern Sumatra. Tsunami flow depths were as much as 10 m at 1,500 m inland. Extensive tsunami deposits, primarily composed of sand and typically 5–20 cm thick, were observed in northwestern Sumatra. DOI: 10.1193/1.220772
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