A 7.7M Undersea earthquake triggered a 10 foot tsunami on Monday October 25, 2010 at 2:42 PM UTC (9:42 PM local time). 400 people have been confirmed dead and another 300 missing. It appears unlikely that any of these have survived. A government official said that they most likely had been swept out to sea or buried in the sand. The epicenter of the quake was off the southwest coast of Sumatra, 150 miles west of the city of Benkulu and 175 miles south of Padang. This area is 500 miles from Jakarta. The quake was felt as far away as Singapore. The tsunami was the main cause of the damage and casualties in the Mentawai Islands, a string of islands about 100 miles off the Sumatra Coast. North Pagai Island is 50 miles from the epicenter of the quake and took the brunt of the tsunami. Whole villages were leveled as the tsunami came ashore without warning. Indonesia has been installing tsunami warning systems following the 2004 tsunami that swept across the Indian Ocean killing over 200,000 people, half of them in Indonesia. However this quake was located close to shore and the tsunami arrived within minutes, before people could evacuate. It also appears that a buoy crucial to the warning system was not working at the time.
Aid has been hampered by bad weather and rough seas, which is making it difficult for rescuers to reach all the affected areas. Access is generally by sea in this island chain. Three Indonesian ships loaded with supplies arrived within three days but have had difficulty reaching those in need.
The quake was caused by the Australian plate subducting under the neighboring Sunda Plate to the Northeast. It was only the most recent of a series of quakes along the southwest coast of Sumatra that makes this the most seismically active area in the world. The current series began with the huge Acheh earthquake and tsunami in 2004 and has been working its way down the coast with major quakes every few years in different sections of this fault zone. As one area it builds up pressure on the adjoining areas. Actually this is a continual process since the motivating factor is the pressure of the Australian plate moving towards and forcing its way under the overlying Sunda plate. This will continue to create large quakes as stick and then slip past eachother. An undersea quake can displace enough of the seafloor to cause a tsunami. In general the larger the quake, the larger the tsunami, but there are a lot of factors can affect it. Theses factors are not all that well understood, especially in the immediate aftermath of a quake when the need to know whether to expect a tsunami is the greatest.
(See the Plate Tectonics page for more information on these processes.) The US Geological Survey (USGS) is an invaluable resource in understanding and tracking earthquakes. See the USGS summary of this quake.
See also the following news stories:
* Indonesia tsunami relief slowed by bad weather(BBC 10/29/10)
* Indonesian tsunami zone welcomes aid shipments (BBC 10/28/10)
* In Pictures: Indonesian Tsunami (BBC 10/27/10)
*Animated Guide - Tsunamis(BBC 9/8/08)