Here we study the evolution of stress and its rotations in the Sumatra–Andaman Subduction Zone (SASZ) after the 2004 Mw 9.2 megathrust earthquake, using moment tensor stress inversions. Models indicate returning of North Andaman stress... more
Here we study the evolution of stress and its rotations in the Sumatra–Andaman Subduction Zone (SASZ) after the 2004 Mw 9.2 megathrust earthquake, using moment tensor stress inversions. Models indicate returning of North Andaman stress regime to the inter-seismic oblique-compression, after a short stint of extension. Likewise, Little Andamans experience extension indicating a trench-ward guidance of extensional stresses. Similarly, Nicobar regime is presently normal-oblique, from early post-seismic reverse-oblique, and North Sumatra experiences pure-reverse regime. The influence of the Wharton Basin stress field and 2012 earthquakes on the SASZ fore-arc deformation is also probed. A near-complete co-seismic stress-relief is observed at Nicobar, followed by North Sumatra and Little Andaman. Co-seismic and post-seismic model comparisons indicate strong correlation with zones of co-seismic stress relief and regions of rapid post-seismic reloading. The North Andaman shows a northward compressive shear than the margin-normal component of subduction, as evident from prominent oblique stress regime with lower co-seismic stress relief and lack of post-seismic back rotations. At Andaman Spreading Ridge (ASR), transition from oblique to extensional stress indicates lateral shear to back-arc spreading enhancement.
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Abstract The Indian Ocean earthquake of 26 December 2004 led to significant ground deformation in the Andaman and Nicobar region, accounting for !800 km of the rupture. Part of this article deals with coseismic changes along these... more
Abstract The Indian Ocean earthquake of 26 December 2004 led to significant
ground deformation in the Andaman and Nicobar region, accounting for !800 km
of the rupture. Part of this article deals with coseismic changes along these islands,
observable from coastal morphology, biological indicators, and Global Positioning
System (GPS) data. Our studies indicate that the islands south of 10" N latitude
coseismically subsided by 1–1.5 m, both on their eastern and western margins,
whereas those to the north showed a mixed response. The western margin of the
Middle Andaman emerged by #1 m, and the eastern margin submerged by the same
amount. In the North Andaman, both western and eastern margins emerged by #1 m.
We also assess the pattern of long-term deformation (uplift/subsidence) and attempt
to reconstruct earthquake/tsunami history, with the available data. Geological evidence
for past submergence includes dead mangrove vegetation dating to 740 !
100 yr B.P., near Port Blair and peat layers at 2–4 m and 10–15 m depths observed
in core samples from nearby locations. Preliminary paleoseismological/tsunami evidence
from the Andaman and Nicobar region and from the east coast of India, suggest
at least one predecessor for the 2004 earthquake 900–1000 years ago. The history of
earthquakes, although incomplete at this stage, seems to imply that the 2004-type
earthquakes are infrequent and follow variable intervals.
ground deformation in the Andaman and Nicobar region, accounting for !800 km
of the rupture. Part of this article deals with coseismic changes along these islands,
observable from coastal morphology, biological indicators, and Global Positioning
System (GPS) data. Our studies indicate that the islands south of 10" N latitude
coseismically subsided by 1–1.5 m, both on their eastern and western margins,
whereas those to the north showed a mixed response. The western margin of the
Middle Andaman emerged by #1 m, and the eastern margin submerged by the same
amount. In the North Andaman, both western and eastern margins emerged by #1 m.
We also assess the pattern of long-term deformation (uplift/subsidence) and attempt
to reconstruct earthquake/tsunami history, with the available data. Geological evidence
for past submergence includes dead mangrove vegetation dating to 740 !
100 yr B.P., near Port Blair and peat layers at 2–4 m and 10–15 m depths observed
in core samples from nearby locations. Preliminary paleoseismological/tsunami evidence
from the Andaman and Nicobar region and from the east coast of India, suggest
at least one predecessor for the 2004 earthquake 900–1000 years ago. The history of
earthquakes, although incomplete at this stage, seems to imply that the 2004-type
earthquakes are infrequent and follow variable intervals.
