Geodynamics of the Sumatran Earthquakes

Gordon Lister

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

eQuakes allows the analysis of 3D orientation data from earthquakes, either in respect to their location (latitude, longitude, depth) or orientation (using data as provided by the centroid moment tensors for individual earthquakes).

We used eQuakes to produce a map (of aftershocks from the Great Sumatran Earthquakes of 2004 (Fig. 1). eQuakes has automatically classified the earthquake type: blue dots are normal faults, red dots are thrusts, and gold dots are strike slip faults).  The map shows thrusts dominating in the south, while normal faults are prominent in the north.  This means there were rather different orientations of the bulk deviatoric stress axes in the lithosphere in the two regions during the aftershock events, and different movement pictures. 

We used eQuakes to analyse orientation data from a vigorous cluster of aftershocks from near the Nicobar Islands, in the vicinity of the boundary separating these two domains.  Perhaps the vigour of the aftershock swarm is related to geometric incompatibility between these two competing movement pictures? To test this hypothesis we derived a lower hemisphere stereographic projection of slip lines from the Nicobar aftershock cluster, using data from centroid moment tensor (CMT) solutions for earthquakes available through the Global CMT project.  These data could have been plotted as beachballs, but more often than not, one beachball obscures another, and in any case, such diagrams help little if we really want to understand what the patterns mean.

Ambiguity in the centroid moment tensor data was eliminated by considering only solutions that are compatible with local fault traces discernible in SeaSat data.  What can be seen in the resultant stereoplot (Fig. 2) is a definition of five orientation groups: here we show the C1 and C4 slip lines for north-south striking right-lateral strike-slip faults, and the C2 and C3 slip lines for normal oblique-slip left-lateral strike-slip faults. The latter faults operated parallel to what are normally interpreted as sea floor spreading ridges in the Andaman Sea.

 

The state of stress (red dots) can be inferred from this classic Mohr-Coulomb failure geometry.  NE-SW directed compressive stress can be inferred, paralle to the direction of relative plate motion, but with the axis of maximum compression inclined ~ 30° to the plane of the Sumatran megathrust.  This is an attitude that suggests continued aseismic slip on the underlying basal Sumatran megathrust during the aftershock sequence, rotating the stress axes away from parallelism with the surface of the Earth.