Indonesia Earthquake: USGS Says Less Chance of Tsunami; No Casualties Reported


A powerful earthquake of 8.7 magnitude rocked Indonesia's northern Aceh province Wednesday, triggering a tsunami alert across the Indian Ocean.

According to the US Geological Survey, the quake struck 308 miles southwest of the city of Banda Aceh, on the northern tip of Sumatra Island, at a depth of 20.5 miles. With a population of over 220,000, the island is located some 310 miles away from the epicenter.

The Pacific Tsunami Warning Center in Hawaii said that a tsunami watch was in effect for Indonesia, India, Sri Lanka, Australia, Myanmar, Thailand, the Maldives and other Indian Ocean islands, Malaysia, Pakistan, Somalia, Oman, Iran, Bangladesh, Kenya, South Africa and Singapore.

However, Bruce Pressgrave, a geophysicist with the US Geological Survey, told BBC that it was unlikely that a tsunami would be generated given that this type of earthquake, which largely involves the horizontal movement of the earth's blocks, does not generate a column of water that could cause a tsunami.

There's always a hazard but with this kind of quake (a strike slip mechanism) it reduces the risk of a tsunami, BBC quoted Pressgrave as saying.

The Tsunami Warning Center had earlier said a quake of this magnitude could potentially cause a destructive tsunami. If the powerful quake triggers a tsunami, the giant waves could affect coastlines across the entire Indian Ocean basin.

People in Banda Aceh screamed God is great! as they jumped into cars and the backs of motorcycles, clogging streets as they fled to high ground, the Associated Press reported.

Indonesia's president Susilo Bambang Yudhoyono said there were no reports of casualties or damage from the quake, and no threat from a tsunami, although an alert for a giant wave remained in place, Reuters reported.

There is no tsunami threat although we are on alert, said Yudhoyono at a joint news conference with visting British Prime Minister David Cameron, who said Britain was standing ready to help if needed.

In India, tremors were felt in cities like Mumbai, Kolkata, Bangalore and the southern part of Chennai. The tremors lasted for a few seconds. A tsunami alert has reportedly been issued for the Eastern coast of India, the Andaman Islands, Andhra Pradesh and Tamil Nadu.

People have also taken to social media platfroms and posted insightful comments. This appears to have been a strike-slip earthquake (focal mechanism). It is unlikely that there will be a devastating tsunami, commented a user in a growing thread in Reddit. He explained:

Tsunamis are generated by displacement of water. In the case of the 2004 Indian Ocean earthquake, it was a megathrust which displaced a lot of water vertically, like so. This earthquake was almost purely strike-slip. Strike-slip earthquakes are completely horizontal--there is no vertical displacement. Since the vertical displacement of this earthquake will be relatively limited, the amount of water displaced and therefore tsunami is also likely to be limited.

I should further note that a devastating tsunami is not likely, at least not like the 2004 one, but I do not know exactly how large a tsunami this earthquake may generate. To generate a tsunami, you must have vertical uplift of water. It is not impossible for strike-slip earthquakes to generate vertical uplift of water, however. If the strike-slip movement is towards the continental slope, there may still be some vertical offset. Imagine that the earthquake occurs in an area with 100m depth water, and the strike-slip motion is towards shallower water and land. The huge 100m stack of that water lying above the epicenter then moves towards land, too, where the water may only be 80m deep. Suddenly you've just produced 20m of vertical offset because you moved a 100m deep stack of water to an area that's only 80m deep. The strike-slip motion appears to be in ESE oblique against Indonesia, though, so I still doubt a devastating tsunami.

On Dec. 26, 2004, a giant 9.1-magnitude quake off the country caused a tsunami in the Indian Ocean, killing 230,000 people in 13 Indian Ocean countries, including Thailand, Sri Lanka and India, with nearly three quarter of them in Aceh.

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