Near-simultaneous great earthquakes at Tongan megathrust and outer rise in September 2009

Abstract

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.

El terremoto de 1647 de Chile central como un evento intraplaca: ¿otra amenaza para Chile metropolitano?

“…Así al menos ocurrió con los terremotos out-rise de Clouard et al, 2007, que no causaron daños en la costa y apenas fueron percibidos en el valle central. A pesar de lo anterior, los terremotos out-rise sí pueden generar tsunamis, como ocurrió durante el desastroso tsunami del noreste de Japón de 1896 (Satake & Tanioka, 1999) y el tsunami de Samoa de 2009 (Beavan et al, 2010 falla cortical asociada a los Andes o una falla de profundidad intermedia en la subductada placa de Nazca. La sugerencia de una fuente intraplaca cortical para el evento de 1647 no es nueva.…”

Developing fragility functions for the areas affected by the 2009 Samoa earthquake and tsunami

“…To understand the mechanism and impacts of the 2009 Samoa earthquake and tsunami, some researchers have conducted numerical simulations of the tsunami (Beavan et al, 2010;Didenkulova, 2013;Fritz et al, 2011;Roeber et al, 2010). Several hypotheses related to a series of earthquake doublets have been proposed; however, no consensus has been reached regarding the earthquake seismology.…”

“…In contrast to most earthquakes, the 2009 Samoa earthquake involved the nearly simultaneous rupture of distinct faults with different geometries (Beavan et al, 2010;Lay et al, 2010). To understand the mechanism and impacts of the 2009 Samoa earthquake and tsunami, some researchers have conducted numerical simulations of the tsunami (Beavan et al, 2010;Didenkulova, 2013;Fritz et al, 2011;Roeber et al, 2010).…”

“…On 29 September 2009 (UTC), an earthquake doublet of magnitudes 8.0 and 7.9 and a subsequent tsunami struck the Samoan Islands and Tonga (Beavan et al, 2010;Lay et al, 2010). After the event, an International Tsunami Survey Team (ITST) was deployed, and the mechanism and impacts of this earthquake and tsunami have been studied in terms of geology, geophysics, seismology, sociology and engineering (Apotsos et al, 2011;Dudley et al, 2011;Lamarche, 2010;Okal et al, 2011;Roeber et al, 2010;vanZijlldeJong et al, 2011;Wilson et al, 2011).…”

Evidence of a previously unrecorded local tsunami, 13 April 2010, Cook Islands: implications for Pacific Island countries

“…The tsunami was originally thought to have been generated by a single earthquake, but subsequently it was shown to be associated with an earthquake doublet, a M w = 8.1 normal fault rupture on the outer trench-slope was followed within minutes by a M w = 8.0 subduction interface thrust event. Both ruptures contributed to tsunami generation (Beavan et al, 2010). This recent event indicates that while simple generic subduction modelling gives a general idea of what could happen, they can reasonably be assumed to provide limited estimates of the maximum size of tsunamis from such sources since they are only approximations of the generating mechanism (e.g., Govers and Wortel, 2005;Fritz et al, 2011).…”