Commentaire d'experts | le 11 avril 2012 PDF Imprimer Envoyer
Un tremblement de terre de magnitude 8,7 a frappé l’Indonésie, suivi de fortes répliques. Nos collègues du Royaume-Uni et d’Australie ont recueilli des commentaires d’experts que vous trouverez plus bas. D’autres commentaires suivront sous peu.

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Du UK-Science Media Centre:

Expert reaction to Indonesian earthquake and tsunami warning

Dr Andy Gibson, Director of the Centre for Applied Geoscience at the University of Portsmouth, said:

"From our experience of the tsunami in 2004, we know that the worst affected regions are those low lying areas immediately adjacent to the coast. Broadly speaking, the shallower the slope of the shore, the greater the susceptibility to wave run-up.  Areas farther inland could be affected if the tsunami was able to move along a water inlet. Lessons learned from that tsunami go far beyond early warning systems, residents are encouraged to take part in emergency drills which may include understanding escape routes, identifying safe havens and knowing when it is safe to return to the coast.

"Early indications are that this earthquake and tsunami risk are actually much smaller than that of 2004, but it will be some time before the pattern of any damage is known. It is always difficult to estimate damage as any tsunami wave depends upon many factors such as the volume of water displaced, the angle it approaches the coast, tidal conditions, the gradient of land offshore and onshore and of course, what is found on the shoreline."


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Dr Susanne Sargeant, Seismologist & NERC Knowledge Exchange Fellow, British Geological Survey, said:

“Critical information that is required to assess the potential for a tsunami is the location, magnitude, depth and faulting mechanism. Tsunamis are caused when vertical displacement of the seafloor occurs. In the case of the 11 April earthquake, an earthquake of this magnitude (8.7 Mw) has the potential to generate an ocean-wide tsunami. However, although the earthquake is relatively shallow and offshore, the data indicate that the earthquake was the result of movement on a strike-slip fault. Strike slip earthquakes are caused when two blocks move horizontally past each other. Such an earthquake would not lead to the vertical displacement of the sea floor that would be required to generate a tsunami. Consequently, the potential for a large tsunami from this earthquake is likely to be low.

“The alert from the Pacific Tsunami Warning Centre issued after the earthquake indicates that an Indian Ocean wide tsunami watch is in place. See http://ptwc.weather.gov/ptwc/text.php?id=indian.TSUIOX.2012.04.11.1054 for info. Arrival time estimates issued by PTWC indicate that the initial wave (had one been generated) would have reached Banda Aceh a little under an hour after the earthquake. Whether any wave has been observed in this region has yet to be confirmed. The PTWC alert gives estimated arrival times for other locations in the Indian Ocean.

“The Sunda trench region is highly active. Earthquakes here are related to subduction of the Indian plate beneath Eurasia. Today's earthquake occurred on a structure related to the subduction that is occurring here. The tectonics of the region are complex and large earthquakes are relatively frequent. The aftershock sequence has started and this includes an earthquake of magnitude 8.3.

“Although large, the 08:38 UTC earthquake is located approximately 400 km from the coast of Banda Aceh. As such, the potential for significant damage caused by ground shaking is likely to be relatively low although the actual impact of the earthquake in this region has yet to be confirmed.”


Q&A ON AFTERSHOCKS Dr Bruce D. Malamud, Reader of Natural and Environmental Hazards, Department of Geography, King's College London:

How many and what size aftershocks might we expect?
“When an earthquake occurs, it releases stress that has built up over time, along a fault. However, in addition to releasing stress, it redistributes the stress along that fault, and sometimes these will be redistributed to other nearby faults. In the case of the 11 April 2012 earthquake that occurred off the west coast of Northern Sumatra, the preliminary estimate of magnitude by the USGS is M8.6, and hundreds of km of fault may have been affected. With the redistribution of stress, aftershocks occur, for weeks, to months (and sometimes years) after the main shock. The magnitude 8.6 earthquake will result in aftershocks occurring all along the fault on which the original earthquake occurred. Some scientists say that one can expect aftershocks as much as 1 unit less than the original shock. So in this case, we might expect aftershocks of all sizes, but as big as a magnitude of about 7.6 (which would be in itself a concern of potentially triggering a tsunami).

How long might aftershocks continue for?
“After an earthquake occurs along a fault, stress is released in parts. But then, part of this stress is redistributed to other parts of the fault. This means that they are now more likely to become unstable, with many subsequent earthquakes. Aftershocks can continue for weeks and months after the main shock (the biggest earthquake in the sequence), sometimes even years.

How frequent have earthquakes been over the last century and are they increasing?
“One of the questions that has been asked by many is whether there have been more frequent large earthquakes in the last few years. Let’s take as a ‘large’ earthquake one with moment magnitude 7. The number of earthquakes per year with moment magnitude greater than or equal to 7 varies certainly, year to year, but the average from 1900 to present is about 17 magnitude 7 or greater earthquakes per year (compared to about 1 magnitude 8 or greater earthquake). If we just look at 1990 to 2010, then the average was about 15 magnitude 7 or greater earthquakes per year. And if we look at the last three years, then the average is also 15 of this size earthquake per year. So, no, the actual number of very large earthquakes is not increasing over time. It fluctuates year to year, with some years less and some years more.

How much energy is released in a magnitude 7 earthquake?
“Equivalent to the energy released in half a megaton nuclear bomb.

How much energy is released in a magnitude 9 earthquake?
“Equivalent to 1000 times the energy released in a magnitude 7 earthquake, or one thousand half-megaton nuclear bombs. If we converted this to energy, this would be roughly enough to power every home in the USA for 50 days.

How accurately can an earthquake like this be predicted? Why is it so difficult to predict the timing of earthquakes?
“For a complete prediction, we need to tell people when a disaster will occur, where, and how big. As scientists, we have a good idea of where large events might occur based on written and instrumental records of past events. So for instance, we know that Indonesia is near subduction zones, and that there is an extensive history of earthquakes in the past, so we know that Indonesia is likely to experience earthquakes. Based on these past records, we can also forecast the chance that a given size or larger earthquake might occur, in a given year. This is called probabilistic hazard forecasting, and has been very useful in telling us about how big we might expect, on average, each year. But true prediction is much more difficult, where we tell people that ‘next week there will be an earthquake of magnitude of 9’. Although scientists have been trying for many years to predict earthquakes (the when and how big), they so far have not succeeded, but are still working at it.”


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DU SCIENCE MEDIA CENTRE DE L’AUSTRALIE:

Kevin McCue is an adjunct professor at CQUniversity, President of the Australian Earthquake Engineering Society and Director of the Australian Seismological Centre.
“According to the USGS website the magnitude 8.7 earthquake occurred well offshore, at least 300 km west of Sumatra so the damage onshore on Sumatra is likely to be minimal. The magnitude may well be decrease to 8.5 or 8.4 after more analysis. The epicentre is well west of the plate boundary and in the Indian Ocean, a fracture along the hinge where the subducting slab of oceanic crust starts bending downward and under Sumatra. The mechanism seems to have been predominantly strike-slip i.e. no substantial vertical displacement of the sea floor so any tsunami would be small and local.”

Professor James Goff is Director of the Australia-Pacific Tsunami Research Centre and Natural Hazards Research Laboratory, University of New South Wales. Note that Professor Goff is currently in the Cook Islands and so has limited availability for interviews.
“Like everyone else who just heard this news, we are now waiting to see what comes to pass. BUT, the tsunami warning system has worked well, and a tsunami watch is in place. Many people have self-evacuated and fortunately, because of the incredible amount of tsunami-related work in the region, since the massive 2004 event authorities and the general public are considerably better prepared this time. Those of us who are not in the region at the moment will now be monitoring developments over the next few hours. It appears to be a strike-slip fault which means that it is unlikely to generate a large tsunami, but then we hear that at least small ones have been reported. Of course such a large shake could generate submarine landslides (like in Haiti) which can also generate tsunamis. We continue to watch.”

Professor Kevin Furlong from Pennsylvania State University is currently in the US and is working with the US Geological Survey on this disaster. He is available for interview and can be reached on his mobile (+1 814 880 5957) over the next 8 hours.
“The 11 April 2012, Mw 8.7 earthquake west of Banda Aceh, Sumatra, Indonesia is a very large earthquake within the Indo-Australian plate. Although it is within the plate, its occurrence is almost certainly linked to the plate interactions between Indo-Australian plate and Indonesia (part of the Sunda segment of the Eurasian plate). This earthquake reflects a style of faulting (strike-slip) which involves principally horizontal motion, and thus is unlikely to generate a significant tsunami; although very strong ground shaking would be felt on Sumatra. This is also an extremely large magnitude earthquake for this style of faulting, meaning that it likely involved substantial fault movement, and the fault likely extends for 200+ km. This earthquake is of the same style of faulting and in approximately the same location as a Mw 7.2 earthquake on January 10, 2012. Although this earthquake was within the Indo-Australian plate, any earthquake of this size will change the stress regimes acting on the nearby plate boundaries. The result is that stress conditions on the subduction plate boundary beneath Sumatra have changed, although the implications of that change are uncertain.”


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Dr David Rothery, who runs the Open University’s Volcanoes, Earthquakes and Tsunamis course, said:

“Today’s M8.7 quake in the Indian Ocean offshore of Sumatra has the potential to cause a tsunami that could cause devastation to coasts all around the Indian ocean. A tsunami warning from the Pacific Tsunami Warning Centre predicts tsunami wave arrival at Banda Aceh (Sumatra) 10:31 BST, and Sri  Lanka between 11.30 and 12.30 BST.
http://ptwc.weather.gov/ptwc/text.php?id=indian.TSUIOX.2012.04.11.0845

“A tsunami detection buoy (a DART station) in the Bay of Bengal http://www.ndbc.noaa.gov/station_page.php?station=23227
detected a wave with an amplitude of 30 cm in deep water (see figs below), which has the potential to become several times higher as it approaches the shore.

“Today’s quake was probably about ten times less energetic than the M9.1 quake that caused the 26 Dec 2004 Indian Ocean tsunami. In addition, the nature of the quake, on the bending Indian ocean floor as it approaches the Sunda Trench (rather than a megathrust quake in the trench itself) may mean that the displacement of water to trigger the tsunami is less bad; the motion of the sea bed is different to that which occurred in 2004.”