Tsunami earthquake fault rupture magnitude

Tsunami, technical information

January 26, 2004

Here are abundant technical informations, sent by Pierre Meunier and Stéphane Levêque
( I reproduce their emails below )

The CalTech (California Institute of Technology, USA) estimated the length of the fault slip to be 400 km, but the analyzed data is limited in time. The Institute of Earth Sciences at CNRS mentions a rupture over 600 km, the other 600 km being only affected by aftershocks. The IISEE indicates that the initial rupture affected the 1200 km of the Burmese plate, but according to two different successive modes. Therefore, there remains some uncertainty, and it will probably be necessary to wait for more detailed analyses to obtain the definitive answer.

Here are the different sites :

http://www.geo.uib.no/seismo/quakes_world/Sumatra-2004/Rupture/SEQ-rupture.html

This site (CalTech) indicates a rupture propagation of 400 km to the North, at a speed of 2 km/s, i.e. 7,200 km/h (I think that is approximately the speed of propagation of a surface seismic wave). A more distant propagation is not excluded because the mentioned analysis is limited to the first 220 seconds of data.

http://www.insu.cnrs.fr/web/article/rub.php?rub=298 ( very interesting, refers to articles on the modification of the Earth's axis and on the tracking of the tsunami by satellite )

This is the site of the INSU at CNRS. It indicates a rupture propagation from the epicenter over more than 600 km, over a duration of at least 3 minutes (i.e. a speed of about 12,000 km/h, a value slightly higher than that given by the previous site).

This site also contains an interesting map showing the sequence of aftershocks over time.

http://iisee.kenken.go.jp/staff/yagi/eq/Sumatra2004/Sumatra2004.html

The IISEE (Japan) site indicates the following:

"From a broadband seismic wave, we can divide the giant earthquake into two stages. In the first stage, the rupture mainly propagated to the northwest from the hypocenter during the initial 100 seconds. The second rupture started about 100 seconds after the initial break. The second rupture generated an ultra long period seismic wave. This may imply that a slow and large dislocation occurred in the second stage."

Other information :

http://iisee.kenken.go.jp/cgi-bin/large_quakes/recent.cgi

This site provides a list of recent earthquakes with significant consequences. Looking at each event, it can be seen that the propagation of the fault rupture rarely exceeds 50 km (100 km on one occasion).

The earthquake in Chile in 1960 (magnitude greater than 9) would also have affected 1,300 km along the Andean subduction zone. However, it seems that the initial earthquake was relatively limited in extent and that its extension was only the result of the aftershocks. I have not found any information on the Alaska earthquake of 1964.

Finally, here is a list of "FAQ" on the earthquake, retrieved from a site and providing many answers :

Question: What is the magnitude of an earthquake?

Answer: The size of an earthquake is usually measured by a magnitude scale. There are several types of magnitude scales, among which the most well-known is the Richter scale, proposed by Charles Richter in 1935 for earthquakes in California. Most of these magnitude scales, including the Richter scale, are based on measuring the amplitude of various seismic waves recorded by seismographs, and therefore do not reflect the real size of the earthquake. Seismologists prefer the Moment Magnitude (denoted as Mw or M), which is based on the seismic moment. The seismic moment is calculated by multiplying the total area of the fault rupture by the friction coefficient and the slip along the fault plane. The moment magnitude, based on these physical properties of the fault rupture process, is a better measure reflecting the actual size of the earthquake than measuring the amplitude of the seismic waves at a certain distance. However, because of its popularity, the Richter scale is still used.

Question: How big is a magnitude 9 earthquake?

Answer: The size of the earthquake and the energy released are proportional to the size of the fault rupture area. In the case of the magnitude 9 earthquake in Sumatra, the total fault area is estimated to be 1,200 to 1,300 km long and approximately 100 km wide, based on the distribution of aftershocks. However, most of the slip during the main earthquake occurred on a segment of about 400 km of the fault around the epicentral area offshore west of Northern Sumatra.

The magnitude scale is logarithmic. In other words, between each unit, there is a factor of 10 change in size. However, the change in energy between each magnitude unit is approximately 32 times. This means that even if the magnitude difference between an earthquake of magnitude 6 and an earthquake of magnitude 9 is 1,000 times, the corresponding energy difference is about 31,622 times.

If we consider that the energy released by an earthquake of magnitude 6 is equivalent to that of an atomic bomb similar to the one used in Hiroshima during World War II, the energy released during the Sumatra earthquake of December 26, 2004 corresponds to 31,622 atomic bombs.

This is calculated using 10^1.5 as the actual energy change (corresponding to approximately 32 times):

( (10^1.5)^9 ) / ( (10^1.5)^6 ) = 31,622

Question: What was the size of the fault that produced the earthquake?

Answer: An initial estimate of the size of the rupture that caused the earthquake is obtained from the length of the aftershock zone, the dimensions of historical earthquakes, and a study of the elastic waves generated by the earthquake. The aftershocks suggest that the earthquake rupture had a maximum length of 1,200 to 1,300 km parallel to the Sunda trench, and a width exceeding 100 km perpendicular to the earthquake source. An early estimate based on the study of elastic waves shows that most of the slip was concentrated in the southernmost 400 km of the rupture.

The fault rupture during the Sumatra earthquake propagated at a speed of approximately 2 km/sec. The total length of the fault, as estimated by the aftershock distribution, corresponds to a distance equivalent to that between Bergen and Bodø in Norway. These enormous dimensions help us to understand why this earthquake had catastrophic consequences.

Question: What was the maximum displacement on the rupture surface between the plates?

Answer: The maximum displacement estimated from a preliminary study of the seismic body waves is 20 meters.

Question: What was the maximum displacement of the sea bottom above the earthquake source?

Answer: The displacement of the ground surface will be related to, but somewhat less than, the displacement on the earthquake fault at depth. In some places, the block of crust beneath the sea floor and overlying the causative fault is likely to have moved on the order of 10 meters to the west-southwest and been uplifted by several meters.

Question: What is the angle of subduction of the India plate beneath the Burma plate?

Answer: At the source of the earthquake, the interface between the India plate and the Burma plate dips about 10 degrees to the east-northeast. The subducting plate dips more steeply at greater depths.

Question: Why did the magnitude of this earthquake change?

Answer: While the location of an earthquake can be determined fairly rapidly, the size of the earthquake is somewhat more problematic. This is because location is mainly based upon measurements of the time that seismic waves arrive at a station. Magnitude, on the other hand, is based upon the amplitude of those waves. The amplitude is much more variable than the arrival times, thus causing greater uncertainty in the magnitude estimate.

For larger earthquakes, the problem is compounded by the fact that the larger the earthquake, the lower the characteristic frequency of the seismic waves. This means that surface wave arrivals, which contain lower frequency energy than the body waves, must be used to determine the magnitude. For a great earthquake, several hours of data must be recorded in order to accurately determine the magnitude.

Thus, accurate estimates of the magnitude can follow an accurate estimate of the location by several hours. In the case of the M 9.0 Sumatra-Andaman Islands earthquake, the standard methods were inadequate for measuring the very low frequency energy produced and had to be modified. This delayed the final determination of the magnitude until the next day.

Question: Can we expect many aftershocks to this earthquake?

Answer: There have been numerous aftershocks detected following the recent magnitude 9 earthquake. As of January 1st 2005, more than 100 aftershocks with M>5.0 have been recorded. The largest occurred about three hours after the main shock and is now assigned a magnitude of 7.1. Thirteen of the aftershocks thus far cataloged have magnitudes of 6.0 or larger. There have been no reports of tsunamis being generated from the aftershocks. We know from past experience that the number of aftershocks will decrease with time. However, the number of aftershocks can be quite variable. There might be short episodes of higher activity as well as lulls in activity, but the overall trend will be for fewer aftershocks as time goes by. Seismologists are not able to predict the timing and sizes of individual aftershocks.

Question: How has the occurrence of this earthquake affected the probability of another great earthquake?

Answer: The occurrence of this earthquake will have produced a redistribution of tectonic stresses along and near the boundary between the India plate and the Burma plate. In some areas, this redistribution of stresses will be such as to shorten the time to the next big earthquake compared to what would have been the case if the earthquake had not happened. In other areas, the redistribution of stresses will be such as to increase the time to the next big earthquake. Once the distribution of slip along the earthquake fault has been mapped, it will be possible to estimate the areas that were moved closer to future failure and those that were moved farther from future failure. It is not yet possible, however, to reliably estimate when the future failure will occur in a given area or how large will be the resulting earthquake.

The slip partitioning due to oblique plate convergence in this area raises further questions regarding the stress conditions along the Great Sumatran Fault (a trench parallel strike slip fault system on land Sumatra).

**( sent by Pierre Meunier on January 17, 2005 ) ** ---

**January 26, 2005. Sent by Stéphane Levêque **:

Origin: a message sent on January 5, 2005
Subject: Tsunami effect: nuclear threat in southern India (the Kalpakkam nuclear complex submerged)

Chennai, India. - **This city in southern India has just survived a double danger: the tsunami disaster and a nuclear threat. **

The tsunami that reached Chennai on Sunday, December 26, did not only destroy fishing villages, flood thousands of homes and take lives. The tsunami also flooded part of the nuclear power plant located in the outskirts of the city, by the sea... If you want to know more, read the rest in English... at the end of the email. And don't forget: on December 27-28, 1999, the Blayais nuclear power plant (Blaye and Braud Saint Louis, on the Gironde) suffered a similar fate: 105,000 m³ of water entered the buildings of two of the four reactors, flooded the lower parts up to two meters in height, caused short circuits, disabled the cooling pumps, and put the plant on the brink of a French Chernobyl. It is not necessary to live in India or to suffer a tsunami to live under the constant threat of a nuclear catastrophe. The French press and media, which cover the Asian disaster so well, will they inform the public? acdn.france@wanadoo.fr mailto:acdn.france@wanadoo.fr Michel Serre reminded on France 2 that in 1775 a tsunami had caused 60,000 deaths in Portugal and Western Europe. The last Atlantic tsunami dates back to 1960 (Morocco).

**Stéphane Lévêque ** ---

Dossier Tsunami: information collected before January 15, 2005

Back to Guide Back to Home Page

**Number of visits to this page since January 26, 2005: **