2009 Sumatra earthquakes
The Sumatra earthquake of September 2009 was a strong undersea earthquake off the coast of the Indonesian island of Sumatra. It occurred on September 30 at 17:16:10 clock local time ( 10:16:10 UTC clock ). The hypocenter of the quake was at a depth of 80-90 km, 60 km west-northwest of Padang, around 475 km south- south-west of Kuala Lumpur and 960 km north- west of Jakarta. It reached an intensity of 7.6 MW on the moment magnitude scale and is thus of the strength comparable to the San Francisco earthquake of 1906 or the earthquake in Kashmir in 2005.
The earthquake several hundred people were killed, as many houses collapsed. The relief work was complicated by another earthquake of magnitude 6.6 Mw with an epicenter 250 km to the southeast about 15 hours later.
Tectonic overview
The Sumatra earthquake of September 30, 2009 occurred in connection with the oblique thrust fault at the nearby plate boundary between the Australian plate and the Eurasian plate at the Sunda Trench. At the epicenter of the earthquake, the Australian plate moves with respect to the Sunda plate at a velocity of about 65 millimeters per year to the northeast.
The subduction zone surrounding the immediate region of the island of Siberut off the coast Padangs experienced in the recent past, no strong earthquakes. She tore at least 1797 an earthquake measuring 8.5 M or more. At that time the Australian plate had slipped in this area about ten meters under the Sunda plate and thus has led to a stress relief. This relief is given the speed at which two plates move against each other, now more than used up.
About 250 km further south slipped a 250 km long section of the plate boundary during the Sumatra earthquake series in September 2007 (up to Mw = 8.5 ), while some 300 km further north, a 350 km long section of the Sumatra earthquake of March 2005 (Mw = 8.7 ) slipped. At the site of the earthquake of 30 September, the plate boundary in the beginning was a shallower depth in 2008 the scene of a sequence of several earthquakes of magnitude Mw = 5-6. It is not clear whether these former earthquakes, are related to the earthquake of 30 September, which occurred in much greater depth.
Due to the location of the hypocenter at a depth of 80 km, it is likely that the earthquakes within the subducting Australian plate was formed. The earthquake hypocenter was lower than in typical Subduktionserdbeben, which generally occur in less than 50 km depth. It is therefore likely that in this deep earthquake, the jacket of the Australian plate is cracked.
The earthquake of 30 September, therefore, not lead to a rupture of the subduction zone in the area of Mentawai islands, so that the risk of a large earthquake with a moment magnitude of about Mw = 8.5 was not reduced.