8.7 Earthquake Prompts Tsunami Warnings For Indian Ocean Coastlines
April 11, 2012

8.7 Earthquake Prompts Tsunami Warnings For Indian Ocean Coastlines

Lawrence LeBlond for RedOrbit.com

A tsunami warning has been issued for the entire Indian Ocean on Wednesday April 11 after a massive 8.7 magnitude earthquake off the coast of Indonesia at 2:38 p.m. local time (3:38 a.m. EST) sent residents around the region out of their homes and offices in fear.

The US Geological Survey (USGS) said the temblor was centered 20 miles beneath the ocean floor, a little more than 300 miles from the provincial capital of Banda Aceh. The quake put emergency management specialists on high alert, fearing a similar tragedy could unfold like the December 26, 2004 9.1 magnitude earthquake that triggered a deadly tsunami killing more than 230,000 people, many from the Aceh province.

The Pacific Tsunami Warning Center (PTWC) in Hawaii said a tsunami watch was in effect for the entire Indian Ocean and individual countries, including Malaysia, Pakistan, South Africa, Thailand, Indonesia, Sri Lanka and India, issued tsunami warnings as well.

People living near the coast in six Thai provinces were ordered to higher ground.

While it was unclear at the time if a tsunami was likely, the PTWC wasn´t taking any chances. However, a tsunami watch only means there is a potential for a tsunami, not that one is imminent.

It is hard to predict tsunamis, but they are usually seen within 30 minutes of the earthquake. It reported scenes of panic across the region, with residents fleeing buildings.

The powerful quake shook the region for what seemed like an eternity, but lasted roughly four minutes.

“The quake was felt very strongly. Electricity is down, there´s traffic jams to access higher ground. Sirens and Koran recitals from mosques are everywhere,” a spokesman for Indonesia's disaster mitigation agency told Reuters.

The temblor was felt as far away as Bangkok, Thailand and southern India. Hundreds of office workers in Bangalore, India left their buildings during the quake, while the Indian port of Chennai closed down because of the danger of a tsunami, said the port.

“There was a tremor felt by all of us working in the building,” a Calcutta, India worker, told the BBC. “All just ran out of the building and people were asked not to use the elevator. There was a minute of chaos where all started ringing up to their family and asking about their well-being.”

Despite a high level of fear of a possible devastating tsunami across the region, quake expert Bruce Presgrave with the USGS told BBC News that Wednesday´s quake was a “strike-slip” event, meaning a more horizontal shift of the ground under the sea as opposed to a sudden vertical shift, giving a smaller risk of large displacements of water, which triggers a tsunami.

It is possible for a tsunami to occur in a “strike-slip” event, depending on the movement of plates, giving some vertical offset. However, that scenario is far less likely, said Presgrave. “We can't rule out the possibility, but horizontal motion is less likely to produce a destructive tsunami,”

Simon Boxall, a UK oceanographer, told Sky News (reprinted by MSNBC) that the danger would not necessarily be over even if the quake does not produce a tsunami.

“The initial earthquake may not cause a tsunami “¦ [but] there´s no reason why an aftershock, which could still reach up to 8 in magnitude, cannot still cause a tsunami,” he added.

Boxall said that not all offshore earthquakes produced tsunamis and issuing evacuation orders every time there is one could start to get “very messy.”

Amy Vaughn, a spokeswoman for the USGS, said a tsunami similar in scale to the one in 2004 could occur. “Of course, being this close to the west coast of Sumatra, it could be very devastating to this coast again,” she said.

MSNBC reported that Wednesday´s quake was rated a ℠5´ on the USGS MMI scale, which measures the physical intensity of an earthquake as felt on the ground. A 5-strength quake is defined as: “Felt inside by most, may not be felt by some outside in non-favorable conditions. Dishes and windows may break and large bells will ring. Vibrations like large train passing close to house.”

Indonesia´s earthquake monitoring agency said the tsunami warning would remain in place for a few hours, although there was no initial sign of a low tide, which would indicate the water was receding before building into a tsunami.

The most severe earthquake since 1900 was of 9.5 magnitude and struck Santiago and Concepcion in Chile on May 22, 1960, triggering tidal waves and volcanic eruptions. Some 5,000 people were killed and 2 million made homeless.