Le Tsunami, informations techniques

26 janvier 2004

Voici des informations techniques abondantes, transmises par Pierre Meunier et Stéphane Levêque
( je reproduis ci-après leurs mails )

Le CalTech ( Californian Institure of Technology, USA ) a estimé la longueur de la ligne de glissement de faille à 400 km mais les données analysées sont limitées dans le temps. L'Institut des sciences de l'Univers au CNRS parle d'une rupture sur 600 km, les 600 autres kilomètres n'étant concernés que par les répliques. L'IISEE indique que la rupture initiale a bien concerné les 1200 km de la plaque birmane, mais selon deux modes successifs différents. Il reste donc un doute et il faudra sans doute attendre des analyses plus poussées pour avoir la réponse définitive.

Voici les différents sites :

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

Ce site (CalTech) indique une propagation de la rupture de 400 km vers le Nord, à une vitesse de 2 km/s, soit 7.200 km/h (je pense que c'est à peu près la vitesse de propagation d'une onde sismique de surface). Une propagation plus lointaine n'est pas exclue car l'analyse mentionnée est limitée aux 220 premières secondes de données.

http://www.insu.cnrs.fr/web/article/rub.php?rub=298 ( très intéressant, renvoie à des erticles sur la modification de l'axe de la Terre et sur le suivi du Tsunami par satellite )

C'est le site de l'INSU au CNRS. Il indique une propagation de la rupture à partir de l'épicentre sur plus de 600 km, sur une durée de 3 minutes au moins (soit une vitesse d'environ 12.000 km/h, valeur un peu plus élevé que celle donnée par le site précédent).

Ce site comporte également une intéressante carte qui montre la succession des répliques dans le temps.

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

Le site de l'IISEE (Japon) indique ceci :

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

D'autres informations :

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

Ce site donne une liste de séismes récents ayant des conséquences importantes. En regardant chacun des événements, on peut constater que la propagation de la rupture de faille dépasse rarement 50 km (100 km en 1 occasion).

Le tremblement de terre du Chili en 1960 (magnitude supérieure à 9) aurait aussi concerné 1.300 km le long de la zone de subduction andine. Mais il semble que le tremblement de terre initial soit relativement limité en étendue et que son extension soit seulement le résultat des répliques. Je n'ai pas trouvé d'infos sur le tremblement de terre de l'Alaska de 1964.

Enfin voici une liste de "FAQ" sur le séisme, récupérée sur un site et qui fournit beaucoup de réponses :

Question: What is a magnitude of an earthquake?

Answer: The earthquake size is usually measured by a magnitude scale.There are several types of magnitude scales among which the most well-known is the Richter’s scale proposed by Charles Richter in 1935 for Californian earthquakes. Most of these magnitude scales including the Richter’s scale are based on measuring the amplitude of various seismic waves recorded on 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. Seismic moment is calculated by the total area of fault rupture multiplied by the friction coefficient and slip along the fault plane. The moment magnitude based on these physical properties of fault rupture process is a better measure reflecting the actual size of the earthquake rather than measuring the amplitude of the seismic waves at some distance. However, because of its popularity the Richter’s scale is still used.

Question: How big is a magnitude 9 earthquake?

Answer : The size of the earthquake and the energy released is proportional to the size of the fault rupture area. In the case of the Sumatra earthquake with magnitude 9, the total fault area is estimated to be 1200 to 1300 km long and approximately 100 km wide, based on the aftershock distribution. However most of the slip during the mainshock occurred on approximately 400km long segment 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 10 times change in the size. However, the energy change between each magnitude unit is approximately 32 times. This would mean, even if the magnitude difference between a magnitude 6 and a magnitude 9 earthquake is 1000 times, the corresponding energy difference is around 31622 times.

If we consider roughly the energy released by a magnitude 6 earthquake is equivalent to an atom bomb similar to the one used in Hiroshima during the second world war, the energy released during the Sumatra earthquake of 26 Dec. 2004 corresponds to 31622 atom bombs.

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

( ( 10^1.5  )⁹ / ( (10^1.5  )⁶ )= 31622

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 1200 -- 1300 km parallel to the Sunda trench and a width of over 100 km perpendicular to the earthquake source. An early estimate from the study of elastic waves show the majority of slip was concentrated in the southernmost 400 km of the rupture.

The fault rupture during the Sumatra earthquake has propagated with a speed of approximately 2km/sec. The entire length of the fault as estimated by the aftershock distribution corresponds to an equivalent distance from Bergen to 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 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 to have 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 earthquake location can be determined fairly rapidly, earthquake size 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 megathrust 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 paralell strike slip fault system onland Sumatra).

( transmis par Pierre Meunier le 17 janvier 2005 )          

26 janvier 2005. Transmis par Stéphane Levêque :

Provenance : un mail transmis le 5 janvier 05, 2005
Sujet: Effet du tsunami : menace nucléaire dans le sud de l'Inde (le complexe nucléaire de Kalpakkam submergé)

Chennai, Inde. - Cette ville du sud de l'Inde vient de survivre à un double péril - le désastre du tsunami et une menace nucléaire.

Le raz-de-marée qui a atteint Chennai le dimanche 26 décembre ne s'est pas contenté de détruire des villages de pêcheurs, de submerger des milliers de maisons et d'emporter des vies. Le tsunami a aussi inondé une partie de la centrale nucléaire située dans les faubourgs de la ville, en bord de mer...
Si vous voulez en savoir plus, lisez la suite en anglais...en fin de mail Et n'oubliez pas : les 27-28 décembre 1999, la centrale du Blayais (Blaye et Braud Saint Louis, sur la Gironde) a subi un sort voisin : 105 000 m3 d'eau ont pénétré dans les bâtiments de deux des quatre réacteurs, inondé les parties basses jusqu'à deux mètres de hauteur, provoqué des courts-circuits, mis hors d'état des pompes de refroidissement, et placé la centrale à deux doigts d'un Tchernobyl à la française. Point n'est besoin de vivre en Inde ni de subir un tsunami pour vivre sous la menace constante d'une catastrophe nucléaire.
La presse et les médias français, qui couvrent si bien la catastrophe d'Asie, vont-ils en informer le public ?
acdn.france@wanadoo.fr <mailto:acdn.france@wanadoo.fr>
Michel Serre rappelait lundi sur France 2 qu'en 1775 un Tsunami avait fait 60 000 morts au Portugal et en Europe de l'Ouest. Le dernier Tsunami de l'Atlantique date de 1960 (Maroc).

Stéphane Lévêque       

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