|
|
|
Chapter 2 of "Earthquake and Tsunami":
Earthquake: the first Disaster
|
Table of Contents
2.1. The Big One: the Richter scale 9.0 even of 26 December 2004 2.2. Sequence of events on 26 December 2004 2.3. Aftershocks 2.4. The Richter scale 8.7 event of 28 March 2005 (Simeulue/Nias islands) 2.5. The Richter scale 6.8 event of 10 April 2005 (Siberut island) 2.6. The Richter scale 6.7 event of 5 July 2005 (Nias island) 2.7. The Richter scale 7.2 event of 24 July 2005 (Nicobar islands) 2.8. The Richter scale 4.4 event of 26th October 2005 (Simeulue island) 2.9. The Richter scale 6.5 event of 19 November 2005 (Simeulue island) |
2.1. The Big One: the Richter scale 9.0 event of 26 December 2004
|
|
Rupture zones and slip mechanism for the earthquakes of 26 December 2005 and 28th March 2005 are shown below. The total length of the two ruptures exceeds 1,600 km. (Graphic courtesy Bilham R. 2005. "A Flying Start, Then a Slow Slip". Science 308:1126-1127, 20 May 2005). white outlines: historical earthquakes with year and magnitude red line: Andaman fault system terminated by the Andaman spreading center (sawtooth portion)
The tsunami was generated by the southern 650 km of the quake of 26 December 2004. Seismic estimates of slip are quite heterogeneous both along strike and down dip, but they agree well with estimates of mean slip (white arrows left) and black ractangles (center) derived from GPS analysis. Representative magnitudes are calculated for various segments assuming uniform sli (center). Right: the panel illustrates the northward-increasing delay between the fast rupture front (sine wave), subsequent downdip slip (square wave) and aftershocks (smoothline). |
Rupture progression with indication of energy radiation from 30 to 420 seconds after earthquake initiation 26 December 2004. The entire rupture event lasted 8 mins (480 sec). long.The epicenter is marked by a red star off Simeulue island near Sumatra. (ref. Ishii M., Shearer P.M., Houston H., and Vidale J.E. 2005. "Extent, duration and speed of the 2004 Sumatra-Andaman earthquake imaged by the Hi-Net array." Nature 435:933-936).
|
|
Cumulative radiated seismic energy over 600 secs after initiation on 26 December 2004 black star: epicentre green dots: aftershocks during the first month after the quake contour lines: plotted at increments of 0.1 and starting at 0.5. The image is plotted over the entire map but amplitudes are very weak in the area coloured blue outside the contoured(red-brown and yellow) region. (ref. Ishii M., Shearer P.M., Houston H., and Vidale J.E. 2005. "Extent, duration and speed of the 2004 Sumatra-Andaman earthquake imaged by the Hi-Net array." Nature 435:933-936). |
2.2. Sequence of events on 26 December 2004
07:58 (local time at Aceh, northwest Sumatra, Indonesia). 60 km off the west coast of northern Sumatra, at 10 km depth, the Indo-Australian plate slides further under the Eurasian plate which is instantly lifted by about 10 m. The energy released is equivalent to around 10,000 atomic bombs. A gigantic volume of water is set in motion in the form of a monstrous wave that moves off in all directions. Initially the speed of this wave is 700 km/h and its height 37 m.
07:59. The official geophysicist on duty at Padang, Indonesia, notes that his seismopraph has recorded an earthquake ofintensity 8 on the Richter scale. He tries to notify the National Earthquake Centre in Jakarta but cannot get through.
08:14. The Pacific Tsunami Warning Centre registers an earthquake. "I know at once that this was no puny thing, perhaps a 7", said someone at the Center. Together with a colleague, he does some detailed calculations and reaches the conclusion that the quake must have been an 8.0 on the Richter scale. This news is distributed in a Bulletin that goes to many countries, among them Indonesia and Thailand. Other coastal regions of the Indian Ocean do not get the news since they are not connected to the warning system for the Pacific Ocean. In Thailand there are discussions if perhaps the quake had started a Tsunami. On the interest of tourism it is decided not to give out a warning. "Five years ago we gave such a warning after an earthquake in Papua-Newguinea, but the tourism authority complained and told us that this was damaging the tourist industry," a seismologist said in Bangkok.
08:20. The Tsunami breaks over the Sumatran city of Banda Aceh.
08:31. In Australia, an alarmed official sends a Tsunami warning to all Australian embassies. Endangered countries are not warned, apparently out of fear that diplomatic protocol could be broken. Worldwide the most highly equipped earthquake observation center with 300 monitoring stations is the organsiation to monitor observation of the trreaty prohibiting nuclear test explosions, based in Vienna/Austria. This organisation, which ould be best qualified to evaluate the incoming signals and issue warnings, remains quiet. The organisation's computers note the situation without human help. It is a public holiday and no staff are present.
08:45. The two scientists at the Pacific Tsunami Warnuing Centre refine their analyses. They now find that the quake had a force of 8.5. Now seriously worried, they call their boss. He sugggests that a new warning is issued but he does not think that a quake of this magnitude could cause a destructive Tsunami posing a threat.
08:54. The meteorological service of India sends a warning per fax - unfortunately not to the responsible minister of science but to his predecessor who has been out of office since the change of government in May 2004. One hour later, he sends another fax, this time to the responsible control centre at the Ministry of the Interior. Another hour passes before the cabinet is informed - at that time many thousands are already dead. The country "leading worldwide in call centres, did not manage to use telephones in such a case", one commentator said.
08:59. Agence France Press reports the earthquake but not the tsunami.
09:04. The Pacific Tsunami Warning Center on Hawaii corrects the strength of the earthquake in its bulletin to 8.5 and mentions the possibility of a local tsunami. The connection between the strength of an earthquake and the tsunami is not clear-cut: not every earthquake lifts the sea floor and causes a tsunami.
09:50. The tsunami reaches the Thai holiday resort of Phuket. A little later it reaches the coastsof southern India and Sri Lanka.
10:31. The Pacific Tsunami Warning Centre first sees news bulletins of a news agency on the internet where deaths in Sri Lanka are reported. For them, this is the first hint at the power of the tsunami. They cocnluded that "more will die." They try to send more warnings to the region but find that they "do not have the telephone number of a single person in that area".
10:40. News agencies report firth deaths in Phuket.
11:00. The British BBC reports 150 dead in Sri Lanka. Thai officials confirm 4 dead in Phuket.
11:05. The tsunami reaches Male, the capital of the Maldive islands. "The islands are only about 1 m above sea level and a wave of 1.5 m went right over us," says a government official.
12:25. The Tsunami spreads further in the Indian Ocean. Scientists from the American Harvard University send out an E-mail in which they state that they have calculated a force 8.9 on the Richterscale for the earthquake. That means that the quake was ten times as strong as originally assumed. Now it becomes clear why the quake could cause such a monstrous tsunami. The last quake with comparable strength took place 40 years ago in Alaska.
13:01. The Pacific Tsunami Warning Centre issues a warning that tsunamis could form in the western parts of the Indian Ocean.
14:00-15:00. The tsunami reaches the coasts of Somalia and Kenya in eastern Africa.
15:00. The news agencies now report at least 150 along the coast of southern India, 400 dead in Sri Lanka and 21 dead in Phuket, among them some tourists.
Evening. There is talk of 11,000 dead.
For a frequently updated analysis of aftershocks see http://www.gsi.gov.in/suma_eq.htm
|
|
Seismic activity before, during and after the great 26 December 2004 quake, including seismic events beneath the Andaman Sea. Each circle represents an earthquake of a strength proportional to the size of the circle The two red stars represent the two main quakes: Day 90 event: 28 March 2005 More than 5,000 aftershocks have been located and activity continues across the region Chart from Ch. J. Amman "Megathrust Investigation", Nature, 2 March 2006, 440:31-32 |
|
|
The main- and the after-shocks between 26 and 29 December
2004 Yellow star: main shock of 26 December 2004. |
|
|
The main- and the aftershocks for the 13 weeks (26 December 2004 to 26 March 2005) following the earthquake and tsunami of 26 December 2005 Yellow dots: recorded aftershocks, radii proportinal to seismic magnitude Star in large yellow dot: main event of 26 Dec 2005 White/pink circles: moment tensor solutions (shown enlarged for the two main events of 26 December 2004 and 28 March 2005) Map courtesy Lay T., Kanamori H., Ammon Ch.J., Nettles M., Ward S.N., Aster R.C., Beck S.L. Bilek S.L., Brudzinski M.R., Butler R., DeShon H.R., Ekström G., Satake K., and Supkin S. 2005. "The Great Sumatra-Andaman Earthquake of 26 December 2004". Science 308:1127-1133, 20 May 2005 |
2.4. The Richter scale 8.7 event of 28 March 2005 (Simeulue/Nias islands)
|
|
28 March 2005 11:09:36 local time at epicenter Magnitude 8.7 (other estimates: 8.2, 8.5 and 8.9) Epicenter off southwestern coast of Sumatra, Indonesia
2.074°N, 97.013°E, off the west coast of Sumatra
between the islands of Simeulue and Nias. |
At least 290 people killed, 100 injured and 300 houses destroyed on Nias island by the earthquake and fears 500 to 2,000 dead on Nias have been voiced. There are also likely to be some hundreds of dead on neighbouring Simeulue island (already hit hard be the first tsunami in December 2004) and in the Batu islands, with extensive damage to housing and infrastructure.
Tremors were felt in Indonesia, Malaysia, Singapore and as far north as Bangkok, Thailand.
No major tsunami has been triggered by this quake and what waves were generated seem to have been focused towards the south and southwest. The near-absence of a tsunami in this quite powerful quake was thought to have been the dominance of horizontal and the near absence of vertical movement. This may still hold true, up to a point, but computer modelling done on the 28 March 2005 earthquake by US scientist C. Synolakis (ref. Kerr R.A. 2005. "Model Shows Islands Muted Tsunami after latest Indonesian Quake", Science 308:341, 15 April 2005) shows that the quake did generate a tsunami but that this was damped into near-insignificance by the presence of islands (above all Simeulue and Nias). This simulation reflected he reality of the actual quake. When he removed the islands from his model, the tsunami generated was much larger and spread much more widely (map to the right below (from Kerr's article in Science 308:341). The results mean that tsunami intensity and reach will be much harder to forecast than had been hoped. The magnitude of a quake alone will not suffice for a reliable forecast, the exact position of the epicenter and its geographical surroundings have to be taken into consideration, too. As Kerr notes "only a dense network of tsunami detectors on the ocan floor will do."
|
|
Map courtesy US Geological Survey |
2.5. The Richter scale 6.8 event of 10 April 2005 (Siberut island)
|
|
10 April 2005 at 10:29:13 local local time at epicenter Depth 30 km (18.6 miles) Epicenter off southwestern coast of Sumatra, Indonesia
3.9°S, 99.551°E, off the southeastern eastern tip
of Siberut island (the largest of the Mentawai group of
islands). Tremors were felt in Indonesia, Malaysia and Singapore. No tsunami was triggered and no casualties caused by this quake. |
2.6. The Richter scale 6.7 event of 5 July 2005 (Nias island)
|
|
5 July 2005 at 01:52:02local time at epicenter Location 1.836°N, 97.034°E Depth 21 km (13.0 miles) Damage to building and roads in Gunungsitoli area. Felt at Sibolga, Sumatra. Felt at Medan and Pematangsiantar, Sumatra. Also felt at Kelang, Kuala Lumpur, Petaling Jaya, Shah Alam, and Sungai Ara, Malaysia. No tsunami was triggered and no casualties caused by this quake. |
2.7. The Richter scale 7.2 event of 24 July 2005 (Nicobar islands)
|
|
24 July 2005 at 15:42:05 local time at epicenter Magnitude 7.2 Location 7.926°N, 92.146°E Depth 10 km (6.2 miles) The US Geological Survey comments as follows: This earthquake occurred near the western boundary of the aftershock zone of the devastating 26 December 2004 earthquake. The aftershock zone of the 26 December earthquake continues to be active and forms a crescent-shaped region extending about 1200 km north of Sumatra, Indonesia. The recent event was a strike-slip earthquake in the Burma plate near its boundary with the India plate. Strike-slip earthquakes generate horizontal motion across a fault. This is unlike the motion caused by the 26 December earthquake that was the result of thrust-faulting on the interface of the India plate and the Burma plate, in which the India plate thrust beneath the overriding Burma plate. Given the different mechanisms, the recent event did not occur on the same fault that caused the Great Sumatra-Andaman Islands earthquake. |
2.8. The Richter scale 4.6 event of 26th October 2005 (Simeulue island)
|
|
26 October 2005 at 19:41:01 local time at epicenter Location 7.926°N, 92.146°E Depth 57 km (35.5 miles) No tsunami was triggered and no casualties caused by this quake. |
2.9. The Richter scale 6.5 event of 19th November 2005 (Simeulue island)
19 November 19, 2005 at 21:10:15 local time at
epicenter Location 2.232° N, 96.769° E Depth 30 km (18.6 miles) No tsunami was triggered and no
casualties caused by this quake.
[ Go to HOME
] [ Go to HEAD
OF THIS CHAPTER ]
Last changed 27 August 2006
