On 29 September 2009, a tsunami devastated the Samoan and northern Tongan islands. It is shown here that an unusual earthquake sequence preceded this tsunami. A magnitude-8 earthquake in the outer-rise intraplate region occurred almost simultaneously with rupture of the shallow subduction interplate interface, equivalent to a magnitude-8 earthquake. The findings provide information on strain release mechanisms at subduction zones, and a possible mechanism for the occasional large tsunamis generated at the Tonga subduction zone.
- J. Beavan
- X. Wang
- R. Kautoke