September 30, 2009 Padang, Sumatra Quake

Updated 10/17/2009

A 7.9 Magnitude quake shook southern Sumatra on September 30, 2009 at 10:16 GMT, 5:16 PM local time. The epicenter was 31 miles west of the city of Padang, Sumatra, Indonesia. Padang is the capital of West Sumatra and has a population of 900,000. the quake was felt as far away as Singapore, 200 miles to the NE. 1,110 people have been reported dead, with 300 still unaccounted for. More than 2,000 are reported injured and over 100,000 building are reported destroyed, with another 100,000 damaged. Unoffical casualty reports have given a higher number of deaths, up to 4,000, but there is no indication how reliable that figure is. Early estimates of fatalities in Padang were reduced while the figures in outlying areas increased. Rescue efforts were hampered because many roads, bridges and telephone communications systems were damaged in the quake. Among the damaged buildings are schools, apartments, offices and hospitals. In places patients were treated in parking lots due to damage from the quake. Most of the casualties are the result of people caught in collapsing buildings.

While early rescue efforts concentrated on the city of Padang, outlying villages in the area were also badly damaged. Over 600 people are thought to be dead in this area. Many buildings in the Padang Pariman district were damaged when their metal roofs collapsed. The quake caused landslides that either undermined houses, sending them crashing down the hillside or buried them under tons of earth. A wedding celebration attended by 400 guests was buried by a massive landslide.

The international press gave this quake extensive coverage for a few days before moving on to other things, with few follow up stories.

The southern coast of Sumatra is prone to very large earthquakes. In this area, the Australian
Plate is subducting under the Sunda Plate to the northeast. As the plate is forced down under Sumatra into the Earth's mantle, it heats up releasing magma, which rises to form the island of Sumatra and the volcanoes which comprise it. There have been a series of megathrust quakes of over 8.0 M on different sections of the subduction zone. The 2004 tsunami was caused by a 9.0 megathrust quake at the northwestern end of Sumatra, about 500 miles from Padang. This earthquake was deeper, at 50 miles underground, than these quakes and the USGS says that it most likely occurred within the Australian plate below the plate boundary. At 7.6 M it was also not as strong than those others. This section of the fault has not had a large quake, over 8.0M, since 1797. The USGS says that it is not clear how this quake relates to the series of stronger, shallower quakes on sections of the fault zone on either side of this area. It is likely that this quake was not large enough to relieve the pressure that has been building up on this section of the subduction zone.

This quake is unrelated to the Somoan quake of the day before, which was centered 6,000 miles to the east in the Pacific Ocean.

A second large earthquake hit the following day 135 miles SE of Padang, closer to the city of Benkulu. This earthquake, with a Magnitude of 6.6, was strong enough to cause some damage, but there have been no reports of casualties. This quake was much shallower and was on the Sumatra fault in the overlying Sunda plate.

(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 quake deaths pass 1,000 (BBC 10/1/09)
* In Pictures: Indonesian rescue effort (BBC 10/2/09)
*Huge aid push in Indonesia, quake levels villages (Reuters (10/2/09)
*Padang lives with quake stress (BBC 10/1/09)
*Village deaths to lift Indonesia death toll (AP 10/3/09)
*Sumatra quake 'levelled villages'(BBC 10/3/09)
*West Sumatra loses entire hamlets under landslides (The Jakarta Post 10/3/09)
*Indonesian quake toll soars to 1,115(Brisbane Times 10/13/09)
*Diggers to help rebuild quake-hit Padang(Brisbane Times 10/17/09)
*USU to send team to overcome quake victims` trauma(Antara News 10/17/09)

September 29, 2009 Somoa Quake and Tsunami

Updated 10/1/2009

The Samoa Islands were hit by a 7.9 M earthquake at 17:48 UTC (6:48 AM local time) on September 29, 2009. The epicenter was 139 miles SW of Pago Pago, American Somoa. The resulting tsunami caused considerable damage in both Somoa and in American Somoa. Over 150 people are estimated to have been killed as the coastal areas of all the Samoan islands were inundated. Casualties were reported from Somoa, American Somoa and Tonga. American Somoa is a US territory and has been declared a disaster area by President Obama.

Reports from the scene were that the quake was strong enough to rock buildings and cause rockslides in the mountains. It was distinguished more by its long duration of over a minute than by the strength of the shaking. This is indicative of a very strong quake that is distant enough to dilute its strength. Within 5 minutes the sea receded as the tsunami approached then swept ashore 10-15 feet deep. Boats were swept inland and cars were swept out to sea. Most people rushed to higher ground. A number of people were killed when they went down to the shore to gather stranded fish after the first wave receded, only to be caught by the second wave. Altogether there were 4 waves in the course of 20 minutes. There was extensive damage throughout the region. Although Somoa has a tsunami warning system, the tsunami hit too soon after the quake to allow the warning to take effect.

The earthquake occurred on the boundary between the Pacific and Australian Plates. The Pacific Plate subducts under the Australian Plate at the Tonga Trench. The epicenter was located near a bend in the plate boundary, at the northern end of the subduction zone. The magnitude has been reported as anywhere from 7.9 M to 8.3M. An earthquake of this size is catagorized as a "Great" earthquake and is capable of extensive damage. This is about the size of the 1906 earthquake that devastated San Francisco. In this case the fact that the quake occurred at sea over 100 miles from any populated area greatly reduced the damage from the quake itself. However, an earthquake of this size underwater does have the potential to create a tsunami, as this one did. The tsunami can travel great distances and in this case, the tsunami appears to have caused the most damage.

(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:

* Deadly Tsunami Strikes in Pacific (BBC 9/30/09)
*Somoa tsunami: survivors tell of giant walls of water ( 10/1/09)
* Somoa tsunami lesson: Early warning system too slow (Christian Science Monitor 9/30/09)