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GETTING TO THE “DEPTH” OF THE PROBLEM: NEW INSIGHT ON MAY 21, 2014 BAY OF BENGAL EARTHQUAKE

The Indian subcontinent is among the world’s most disaster-prone areas. It is also very seismically active and hence vulnerable to earthquakes. India has faced many major earthquakes in the past that have caused mass destruction and loss of lives. Earlier, India was vulnerable to earthquakes occurring in the active crust movement in the Himalayan mountains. However, now the crustal movements of the Andaman Sea, the Northern Arabian Sea, and a short segment of Sri-Lanka, along with the resulting tsunamis, also pose a risk to India.

One such event took place on May 21, 2014 in Bay of Bengal, Indian Ocean, which was jolted by an earthquake of scale Mw 6.1. Researchers from the Indian Institute of Science, Bangalore, Rishav Mallick and Prof. Kusala Rajendran, studied the event to understand its mechanism. They found that the 2014 earthquake seems to have an association with a northwest-southeast-oriented fracture, also called a planar fault. This fracture is located on the eastern side of the 85° E ridge, which has been interpreted as a hotspot trail in previous studies. On the basis of this, they have speculated that oceanic intraplate seismicity can also occur due to reactivation of remnant hotspot trails. The origin of oceanic intraplate seismicity is within the interior of a tectonic plate, away from plate boundaries, and does not involve displacement between two plates. “This work started off to simply evaluate the pattern of faulting for this rather unusual event. The association with a submerged bathymetric feature was a eureka moment that appeared much later while we were trying to reason why would such an event occur where it did,” says Rishav Mallick, the lead author of the paper and currently working under Prof. Kusala Rajendran, the co-author.

They have reported the source depth for the event at 54 km, which is more or less in agreement with previous studies. A focal depth of 54 km is uncommon but not uncertain, given the age of the ocean floor that is about 90 million years. They also reported the epicenter of the earthquake to be about 100 km to the east of the 85° E ridge and at about same distance from the northern extent of the 90°E ridge with no surface features or faults found to be exposed here. The rupture, which was brittle in nature, appears to have originated at the center of the fault plane, which is oriented northwest–southeast, at the source depth and spread bilaterally.

Though this study does not have enough evidence to prove 85° E ridge being the causative fault, the spatial association of past earthquakes with the presence of this structure cannot be ignored. Rishav imparts, “If you look at raw seismicity data, there is nothing spectacular about earthquakes in the Bay of Bengal. It's only when you look at relocated catalogs, which have much more precise locations, that you see the pattern of faulting with the 85°E ridge.” An earthquake catalog is a list of earthquake locations in terms of latitude, longitude, depth and magnitude. With time as more data  is obtained along with more robust models, these locations can be made precise and hence, “relocated”. This 85°E ridge has always been considered as seismically inactive until now. Also, it is quite hard to locate, being buried under some 20 km thick sediment. If a clear-cut association between the structure and the previous earthquakes mapped on its flanks is established, then this research work could be the first documented evidence of a hot-spot trail showing oceanic intraplate seismicity.

On probing into practical implications of the study, Rishav conveys, “This study only hints at the risk to coastal areas from intra-plate events in the Bay of Bengal. It does not directly help in early warning systems, although knowing that such events can occur and pose a risk is a step that will help in hazard models.”

About the authors

Kusala Rajendran is a Professor at the Centre for Earth Sciences (CEaS), Indian Institute of Science.
Rishav Mallick is a research assistant in Prof. Kusala’s lab.

Website:http://ceas.iisc.ernet.in/~kusala/

Email: kusala@ceas.iisc.ernet.in, mallickrishg@ceas.iisc.ernet.in

About the study

The paper was published in the journal Bulletin of the Seismological Society of America.
http://www.bssaonline.org/content/106/2/408.abstract