A
sign showing the way to Everest Base Camp high in the Khumb Glacier
stands among rocks on April 26, 2015, one day after an
earthquake-triggered avalanche swept through parts of the base camp.
ROBERTO SCHMIDT/AFP/Getty Images
The first good view of the aftermath of Nepal's deadly
earthquake from a satellite reveals that a broad swath of ground near
Kathmandu lifted vertically, by about 3 feet (1 meter), which could
explain why damage in the city was so severe. The data also indicate the
tallest mountain in the world, Mount Everest, got a wee bit shorter.
The
new information comes from Europe's Sentinel-1A radar satellite.
Scientists are racing to interpret the Sentinel data, which were made available April 29
just hours after the satellite passed over Nepal. The preliminary data
can help guide relief efforts on the ground by identifying areas that
were damaged or hit by landslides.
Researchers detected the
vertical shift in the ground by comparing before-and-after radar images
from the satellite using a technique that produces an image called an
interferogram. The resulting images have rainbow-colored areas that
represent the movement of the ground between the times each radar image
was taken. Each colorful fringe on the European Space Agency's Nepal
interferogram reflects about 1 inch (2.8 centimeters) of vertical
movement. The results will be refined in the coming weeks, with as
scientists further analysize the images and additional data from
satellites become available. [See Images of the Kathmandu Uplift & Other Nepal Quake Effects]
According
to the early analysis, a region 75 miles (120 kilometers) long by 30
miles (50 km) wide lifted upward by as much as 3 feet during the
earthquake, said Tim Wright, a geophysicist at the University of Leeds
in the United Kingdom. This uplift peaked only 10 miles (17 km) from
Kathmandu, even though the city was relatively far from the earthquake's
epicenter.
"That's one of the reasons why Kathmandu has so much damage," Wright told Live Science.
The radar images reveal that some of the world's tallest peaks -- including Mount Everest
-- dropped by about 1 inch (2.5 cm), according to the nonprofit UNAVCO,
a geoscience research consortium. That's because the Earth's crust
relaxed in the areas north of the Kathmandu, after the earthquake
released pent-up strain.
Satellite radar image of the ground changes due to the magnitude-7.8 Nepal earthquake on April 25, 2015.
ESA SEOM InSARap Study - Norut/PPO.labs/Univ Leeds
Still,
on the whole, the Himalayas continue to grow to stupendous heights,
studies show. Some parts of the Himalayas are rising about 0.4 inches (1
cm) every year, due to the ongoing collision between the Indian and
Eurasian tectonic plates.
"This is only one earthquake, and the overall tectonics give you uplift of the mountains," Wright said.
The
new data from the satellite also confirm what researchers had detected
from seismometers: The fault involved in the earthquake ruptured
eastward, out from the earthquake epicenter, Wright said. "Presumably, much of the damage will be to the east of the epicenter," he said.
The
April 25 earthquake struck on a shallowly-dipping thrust fault that
angled only 10 degrees from the surface. The structure of this fault
meant the damage was spread over an area of more than 5,600 square miles
(more than 14,000 square km).
In size and structure, the
magnitude-7.8 earthquake compares most closely to temblors on subduction
zones, said Rich Briggs, a geologist with the U.S. Geological Survey in
Golden, Colorado. "We don't often see a big, broad bulge at the surface
like we see with this one," Briggs said.
Scientists plan to
continue monitoring ground changes in Nepal. For instance, the fault did
not break all the way up to the Earth's surface, which may mean that
some strain that built up prior to the earthquake still needs to be
unleashed. The fault could release this energy with more earthquakes
or by slowly shifting without triggering major temblors -- a phenomenon
called creep. Further studies will also help researchers understand how
the earthquake stressed other faults on either side of the rupture.
"I
think this will give us our clearest insight into the workings of the
faults along the Himalayan front," said Stephen Hicks, a seismologist at
the University of Liverpool in the United Kingdom. Follow Becky Oskin @beckyoskin. Follow Live Science @livescience, Facebook & Google+. Originally published on Live Science.
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