The Indian Ocean earthquakes of April 11, 2012: yet another surprise?
Kusala Rajendran, Centre for Earth Sciences
The April 11, 2012 sequence of Indian Ocean earthquakes has once again taken scientists by surprise. At magnitude 8.6 and 8.2, these earthquakes separated by about two hours were surprisingly big. Given their great size, how did the April 11 earthquakes fail to generate a significant tsunami? Neither of this pair resulted from shifting one tectonic plate beneath another near the deep-sea trench. Instead, they resulted from sideways motion on faults farther offshore, that follow the structural grain of the oceanic plate. Such so-called strike-slip faulting earthquakes were analyzed in a study published by IISc researchers in 2011 in the Bulletin of the Seismological Society of America. The study suggests that subducting plate off the Sumatra and Nicobar segments deforms in response to a generally northwest-southeast oriented compression, contrasting the general NE-directed motion that dominates the India-Eurasia plate convergence. Ninety east ridge, a prominent feature impinges the Andaman-Sumatra trench close to 10¡ N, marking a transition in morphology, physical properties, and the style of deformation in the northern and southern segments of the Andaman and Nicobar arc. The study by IISc team concluded that the oceanic plate off the Sumatra and Nicobar segments of the plate boundary behaves as a chip of the India-Australia plate, with its NW-directed plate motion. Thus, while most earthquakes on the plate boundary occur in response to NE-directed compression leading to thrust type earthquakes with vertical component of slip, the earthquakes on the deforming oceanic plate occur in response to NW-oriented compression, with little or no vertical component of slip. Although the earthquake sources are not too far separated, the lack of vertical component explains the near absence of tsunami. What are the implications of such stress partitioning on the GPS and other deformation models of the subduction processes? This is an important question to be resolved, with implications on the earthquake cycle.
Fault plane solutions of significant earthquakes post 2004.
Referrence: Kusala Rajendran, V. Andrade, and C. P. Rajendran (2011), The June 2010 Nicobar Earthquake: Fault Reactivation on the Subducting Oceanic Plate, Bulletin of the Seismological Society of America, Vol. 101, No. 5, pp. 2568Ð2577, doi: 10.1785/0120110002