Mt. Everest got shorter last Saturday
These colors show some really cool, if clearly very sad, science. This image shows the rupture of last weekend’s earthquake in Nepal as measured by the European Space Agency’s Sentinel 1-A spacecraft.
Sentinel 1 carries a radar system that works by sending waves of energy at the Earth’s surface and detecting them when they bounce back. This tool can be used to image surfaces and textures, or it can be used like this – as an interferogram.
An interferogram is built by overlaying 2 radar images. If nothing changes between the images, the radar images should perfectly match, but if the planet’s surface moves, the radar waves will not line up. Instead, the 2 images will interfere – some of the waves will add, others will be canceled out, just like if you add 2 sine waves that are out of phase with each other. These 2 images were taken by Sentinel 1-A just before and just after the quake.
There’s a ton of information in this plot. The tight contours between colored lines show the area on Earth’s surface that moved the most during the quake. Every color cycle represents 10 centimeters of deformation, so the tightly wrapped color bands at the center suggest that the largest ground deformation during this quake was 1.2 meters. Counting exactly how many bands there are in an area can give an exact amount of displacement for each spot.
Second, we can also tell that there seems to have been no surface rupture from this quake. When a fault breaks the surface, it creates a sharp step in radar patterns because one side moved up by a lot relative to the other side. The fact that those color contours aren’t broken suggests that all of the displacement happened underground– what geologists would call a “blind” thrust fault.
Third, we can see the areas that were hardest hit, because the areas with the most tightly-spaced color contours are areas that sit directly above the fault. You can of course see that the capital of Kathmandu sits directly above the ruptured fault, leading directly to the devastation in that city. For rescuers wanting to predict what rural areas were hardest hit and are in greatest need of aid, a map like this literally outlines the parts of the fault that moved the most.
Finally, we can make geologic predictions about the area. Mt. Everest, for example, sits to the northeast of Kathmandu. As the stress built on this fault over the last several hundred years, Mt. Everest’s peak would have gradually been pushed upwards. This earthquake released some of the stress on that fault that had been pushing Everest upward in one instant.
You can simulate this pretty easily – take a ream of paper or a notepad and push it against a hard surface. As you push against it, the notepad will fold – make sure it folds concave downwards since that’s how the earth’s rocks have to fold. That folding is similar to how the stress on this fault pushed Everest’s peak higher. Now, release the notepad – suddenly it snaps back to the orientation it had earlier. That release was the earthquake in this example – the extra folding went away and the elevation of the peak decreased.
Similar behavior is observed at faults around the world. Recently we featured this image (http://tmblr.co/Zyv2Js1dbFv2t) from Oregon showing trees that are popping out of the ocean as stress on the Cascadia fault builds and this photo (http://tmblr.co/Zyv2Js1d1NVcI) taken after the 2004 Sumatra earthquake that shows islands that popped out of the ocean when the quake happened.
Everest has been built in part by many cycles of earthquakes over 50 million years since India and Eurasia collided. In this quake we got a small look at how it changes during that time – a long period of slow deformation followed by a rapid, co-seismic change in the other direction; the growth of the mountain is caused by whatever motion isn’t released in this cycle.
Everest is outside the range shown in this picture, created by scientist John Elliott by overlaying Satellite data on top of a 3-D topographic model and imagery from Google Earth. It probably only moved by a few centimeters since its outside the area where the fault actually slipped, but based on the mechanism of this quake, it’s very likely that Mt. Everest is a few centimeters closer to sea level today than it was last week.
Image credit: John Elliat, NERC/COMET, Centre for the Observation and Modelling of Earthquakes, Volcanoes and Tectonics. Image shared with permission. https://twitter.com/jrelliott82
Read more about coseismic displacement: http://www.tulane.edu/~sanelson/eens1110/earthint.htm
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