Unbroken Andaman plate could be dangerous

A segment of the Indian plate left unruptured after the deadly 2004 Andaman earthquake is a potential source of high magnitude earthquakes, a leading Indian geologist has cautioned.

Vineet Gahalaut of the National Geophysical Research Institute (NGRI) in Hyderabad has reached this conclusion after analysing two major quakes that took place in the Andaman region since 2004 — the 7.5 magnitude quake of August 11, 2009 near Coco Island and the 6.6 magnitude quake on June 27, 2008 near Little Andaman (LA).

"These two earthquakes are actually delayed aftershocks of the 2004 earthquake and very similar to each other," Gahalaut told Nature India.

The 9.2 magnitude Sumatra-Andaman earthquake that produced the deadly tsunami occurred in the Andaman Sumatra Subduction zone formed by the subducting India plate beneath the Sunda Plate. Gahalaut said simulation studies suggested that both the LA and Coco aftershocks were strongly promoted by the deformation that took place due to the 2004 earthquake.

"Both the aftershocks appear to have occurred in the subducting Indian plate under the unruptured segment of plate boundary fault," Gahalaut said. This, he said, "has got implications for future seismic hazard in the Andaman region."

The 2004 Sumatra-Andaman earthquake (Mw 9.2) ruptured about 1400 km long frontal arc of the Sumatra-Andaman subduction zone between northwest of Sumatra and the Andaman region. According to the NGRI scientist, the rupture did not reach up to the trench (formed by the subduction process), thus leaving unruptured segment of about 400-500 km between the trench and the western limit of the rupture. Both the 2009 Coco and 2008 LA aftershocks occurred under this unruptured portion.

"It implies that similar strong aftershocks may occur under the 400-500 km long unruptured segment that lies between the Coco and LA aftershocks," Gahalaut and co-workers claim¹.

"The unruptured region is loaded by stresses caused by deformation due to the 2004 earthquake and it has already produced two powerful aftershocks," Gahalaut said. "My worry is, it can potentially produce more earthquakes. It may not cause great earthquakes but magnitude 7 to 7.5 can occur, capable of triggering local tsunami."

The team proposed that the Indian plate obliquely subducts under the Burmese plate, along with the 'ridge' or an elongated sea mount of about three km high and 100 km wide. The so called 90°E ridge, made up of material from the Earth's mantle, "acted as a structural barrier influencing rupture characteristics of the 2004 earthquake," Gahalaut said.

He said such a barrier in the Andaman region retarded the rupture speed of the 2004 earthquake at its northern end and for some reason a portion of the plate boundary segment remained unruptured.

"The occurrence of 2009 Coco aftershock further supports our hypothesis about the process and mechanism of occurrence of 2008 LA aftershock and implies that the unruptured segment of the Andaman region, that lies between the trench and the clusters of 2004 Sumatra-Andaman aftershocks, may experience strong aftershocks on the reactivated steep planes of the subducting 90°E ridge," the paper says.

Deep seismic reflection surveys across the Andaman region using ocean bottom seismographs can provide additional and direct evidence for the presence of the 90° E ridge under the Andaman region and its possible extension further north, Gahalaut said.