'No tsunami' doesn't surprise scientists
doi:10.1038/nindia.2012.51 Published online 13 April 2012
Researchers at the Centre for Earth Sciences of the Indian Institute of Science (IISc) in Bangalore admit that the sequence of two surprisingly big Indian Ocean earthquakes — of magnitude 8.6 and 8.2 separated by about two hours on April 11, 2012 — took them by surprise because both did not occur on the plate boundary but on the subducting oceanic plate. However, the fact that these massive earthquakes failed to trigger a tsunami doesn't surprise them.
The quakes were located approximately 100 km to the southwest of the major subduction zone that defines the plate boundary between the Indo-Australia and Sunda plates, offshore Sumatra. Along this plate boundary — which had sourced the 2004 great tsunamigenic earthquake — the Indo-Australia plate moves north-northeast with respect to the Sunda plate at a velocity of approximately 52mm per year.
But researchers Kusala Rajendran and her team at IISc are not taken unawares aback by the fact these earthquakes, despite their great size, failed to generate a tsunami. Their earlier study of earthquakes in that region had shown the reason why the April 11 events did not produce a tsunami such as the one that followed the 9.1 quake on 26 December 2004.
"Neither of the two April 11 earthquakes resulted from shifting one tectonic plate beneath another near the deep-sea trench," Rajendran of IISc told Nature India. "Instead, these earthquakes resulted from sideways motion on faults farther offshore within the subducting Indian plate and not at the plate boundary itself as in the case of the 2004 earthquake."
Rajendran said that the earlier study by her team had in fact analysed the so-called 'strike-slip faulting earthquakes' that had taken place since 2004. She said the study "suggested that subducting plate off the Sumatra and Nicobar segments deforms in response to a generally northwest-southeast oriented compression, contrasting the general northeast directed motion that dominates the India-Eurasia plate." Although rare, large strike-slip earthquakes (where motion is horizontal), are not uncommon here, she said.
Thus, she said, 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." In other words, "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."
"Although the earthquake sources within the trench and those on the subducting plate are not too far separated, the lack of vertical component explains the near absence of tsunami," Rajendran said.
"The implication of such stress partitioning on deformation models of the subduction processes is an important question that needs to be resolved," she said.
- Rajendran, K. et al. The June 2010 Nicobar Earthquake: Fault Reactivation on the Subducting Oceanic Plate. B. Seismol. Soc. Am. 101, 2568-2577 (2011) | Article |