More than a year after the earthquake that set off a tsunami in the Indian Ocean, which killed nearly 250,000 people. Now, new data and ideas about the earthquake mechanism are emerging. It is becoming clear that it was not one earthquake, but indeed two that triggered the devastating tsunami.

The giant earthquake of 26 December 2004 of Banda Aceh ruptured a very long and complex segment of the thrust fault accommodating the Indo-Australian/Sundaland convergence. The southern part of the rupture occurred on the Australia-Sumatran sliver boundary, where the relative plate motion is 53 mm/yr oriented roughly North, and the northern part occurred on the Indian-Burma micro plate boundary, where the relative motion is only 35 mm/yr and the subduction becomes much more oblique.

Data collected at ~60 Global Positioning System (GPS) sites in southeast Asia are used to determine the bed displacements (uplift) that occurred in the Indian Ocean during the 26 December 2004 Sumatra-Andaman earthquake. This data are then used as the initial surface displacement fields in a numerical model of the tsunami. Analysis of these GPS measurements suggests that far from being one single rupture propagating from one end to the other, the process actually corresponds to two different earthquakes. The first one on the southern segment triggered the second one on the northern segment within less than 1 minute, through static stress increase.

This finding has important consequences on the characteristics of the devastating Tsunami. The ensuing waves interfered with each other and built up in the directions of the Islands of Phuket and Sri Lanka.

Christophe Vigny: Status of GPS based Investigations on the 26 December 2004 Mega-thrust Earthquake
Julie Pietrzak: Co-seismic deformation along the Sumatra Andaman fault derived from GPS data and tsunami models abstract  
"... slip and vertical bed displacements derived from ... GPS data ... could be available within 30 minutes. Hence, GPS data should be included as an important component of future tsunami warning systems."

Session: G9/GD16 Geodynamics and Deformation of Southeast Asia | >>programme

Background information:

animation of the rupture and displacement field in SE Asia that has not been published in the joint Nature article
article "Insight into the 2004 Sumatra–Andaman earthquake from GPS measurements in southeast Asia"
animation of Tsunami by WLDelft
Tsunami propagation and flooding simulation with Delft3D link
more photos and animations from TU Delft

Christophe Vigny vigny@geologie.ens.fr
Julie Pietrzak  j.pietrzak@citg.tudelft.nl