Powerful earthquakes activate tsunami warnings

12 April 2012

Image showing two large earthquakes and multiple aftershocks off the coast of Indonesia. Copyright Geoscience Australia.

Image showing two large earthquakes
and multiple aftershocks off the coast
of Indonesia.
© Geoscience Australia

Two powerful earthquakes were recorded off the coast of Indonesia on 11 April 2012, prompting tsunami warnings to be issued throughout the Indian Ocean.

The initial earthquake measuring 8.5 in magnitude occurred at 6:38pm AEST and was followed by a magnitude 8.2 earthquake at 8:43pm AEST. The earthquakes occurred approximately 490km and 600km south-west of Banda Aceh, Indonesia.

"The second earthquake is a 'triggered' event, and not an aftershock, meaning it was most likely a new earthquake along a different area of the fault that ruptured from the power of the initial earthquake", said Clive Collins, Senior Seismologist at Geoscience Australia.

Tsunami warnings were issued in areas of Indonesia, Thailand, Malaysia and the Andaman and Nicobar Islands of India, but were all retracted following tide gauge readings that confirmed only a small tsunami was generated.

"These earthquakes were due to predominantly horizontal motion along the fault, which is much less likely to generate a large tsunami. By comparison, the devastating 9.2 magnitude earthquake that caused an ocean wide tsunami in 2004 was due to the Indo-Australia plate being pushed, or subducted, under Sumatra", said Mr Collins.

"There have been a number of aftershocks following these two earthquakes and we can expect them to continue in the coming weeks", he said.

The Joint Australian Tsunami Warning Centre (JATWC) is operated by Geoscience Australia - located in Canberra, and the Australian Bureau of Meteorology (BoM) - located in Melbourne. The Centre monitors, detects, verifies and warns the Australian community of potential tsunami impacts on Australia's coastline and external territories.

Seismogram recordings from the Christmas Island seismic station

Topic contact: media@ga.gov.au Last updated: October 4, 2013