A series of severe earthquakes in the Sunda Trench offshore of the Indonesian island of Sumatra has caused a bit of a stir among the 460 students on our new Volcanoes, Earthquakes and Tsunamis course. 'Nothing if not topical' was one comment. The quakes, beginning on 12 September, brought down buildings but mercifully seem to have killed very few, even though the largest was magnitude 8.4 on the famous Richter scale. This was the biggest in the region since the tragic 26 December 2004 quake, which happened only a few hundred kilometres further along the trench - that caused the devastating Indian Ocean tsunami. These quakes are not a great surprise; many scientists had predicted that the slippage on the plate boundary between the Indian Ocean floor and south-east Asia would propagate along the trench. One now wonders if the zone of slippage will spread northwest as well, giving a major quake off the coast of Thailand or Burma.
The initial jerk associated with a large undersea earthquake can displace the water, triggering a series of tsunami waves. The trouble is that although earthquakes are easy to detect and to locate, and you may think it likely that a tsunami has been triggered, you can never be sure of this, or of its height, unless you can measure it directly. In 2004 there was no tsunami detection system in the Indian Ocean, nor any system in place for transmitting warnings to regional authorities. Now there is one tsunami detection buoy installed in the Indian Ocean (between India and Thailand) - fairly pathetic compared to the 20+ ringing the Pacific, but a lot better than nothing. This did in fact record the passage of tsunami waves triggered by this week's earthquakes, but in this case they were small. Tsunami warnings were issued, and then later withdrawn. All's well that ends well, they say, and let's hope that the system is still working well when another big tsunami happens.
Here is a map of the recent earthquake location, taken from the USGS Earthquake Center today. Yellow = in the past week. Orange = in the past 24 hours. Red = in the past 1 hour. The bigger the symbol, the larger the earthquake. The earthquakes are becoming weaker, developing into what are described as 'aftershocks'. A further big quake is still possible, but unlikely.
Map showing 45 earthquakes, from 13:24 on Friday 24th September 2007.