The 2005 Nicobar-Island Earthquake Swarm from Teleseismic Double-Difference Locations: Post-2004 Megathrust Back-Arc Transtension along the Sumatra Fault
Abstract
The Nicobar-Island earthquake swarm of 2005 occurred in the back-arc region of the Andaman Sea about a month after the devastating Mw=9.3 Sumatra-Andaman megathrust earthquake of December 2004. One of the most energetic earthquake swarms ever observed globally, it had more than 600 teleseismically recorded earthquakes (3.7≤mb≤5.6) in the first five days of activity. We relocate a total of 2055 earthquakes by applying teleseismic cross-correlation and double-difference algorithms to global bulletin and waveform data. The swarm is characterized by a large number of events with similar waveforms and a detailed view of the seismogenic structures active during the swarm emerges after relocation. It is located along the Seulimeum strand (SEU) of the Sumatra Fault. This fault system is thought to absorb most of the dextral motion between forearc and the Sunda overriding plate in the oblique and slip-partitioned Nicobar convergence-subduction boundary. CMT solutions for earthquakes prior to the 2004 megathrust event indicate dextral motion along the forearc-back-arc boundary, particularly along the SEU fault, and a gap of seismicity is observed at the location of the future swarm. In the month between the 2004 event and the initiation of the swarm, the area switched to almost exclusively E-W extensional faulting mechanisms, indicating a substantial stress change in the back-arc of the 2004 megathrust rupture. The center of the swarm is dominated by NE-SW striking normal-fault mechanisms, whereas the NE and SW tips are dominated by strike-slip mechanisms. The observed pattern suggests a crustal pull-apart associated with a 15-20 km right step or bend along the dextral SEU fault. Further evidence for the proposed discontinuity of the SEU fault is provided by ≈20° difference in strike of dextral focal mechanisms north and south of the swarm region. We suggest that the step-over presents a barrier for right-lateral slip along the SEU fault, indicated by the absence of significant strike-slip events prior to the swarm in this region. The Sumatra-Andaman fault system in the area of the swarm is accommodating both dextral motion and back-arc extension. These motions may be locally partitioned to different parts of the fault system. They are not only partitioned in space, they are also partitioned in time, according to stress cycles associated with megathrust ruptures. Dextral motion prevailed before the rupture, while extensional deformation prevailed afterwards. This change was associated with the mainshock and happened abrupt, but the return from the current extension regime back to dextral motion may be gradual, reflecting interseismic loading on the megathrust. A cone-shaped bathymetric high just NW of the step-over may indicate the presence of magmatic processes. However, since the 2005 swarm originated from a transtensional segment of the Sumatra fault, we conclude that it was driven by the back-arc extensional stress induced by the 2004 megathrust rupture.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.T31F..01D
- Keywords:
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- 7230 SEISMOLOGY / Seismicity and tectonics;
- 8106 TECTONOPHYSICS / Continental margins: transform;
- 8111 TECTONOPHYSICS / Continental tectonics: strike-slip and transform