29/04/2015
How The Deadly Nepal Earthquake Happened [Infographic] ­ Scientific American
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Saturday's terrible earthquake was the latest result of an ongoing collision of giant pieces of our
planet, a slow-moving disaster that started about 50 million years ago.
April 27, 2015 | By Josh Fischman
Between 55 million and 40 million years
ago, the northern edge of what is now India
began to slam into the giant slab of Earth's
crust that today carries Nepal and Tibet.
This ancient collision had a terrible after­
effect this past Saturday: The deadly
earthquake, centered in Nepal, which had
an estimated death toll of nearly 4,000
people as of Monday evening.
India bulled its way under Nepal those
many millions of years ago, shoving the
northern land skyward. That move began to
create the towering Himalaya, including Mt.
Everest. The collision is still going on, as
India moves several centimeters north each
year, and this has created an unstable
fissure in the planet's crust, known as the
Himalayan frontal thrust fault. This
boundary zone, shown below, continues to
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release enormous earthquakes. Saturday's
magnitude 7.8 disaster appears to overlap a
segment that released a 8.1 magnitude quake in 1934, according to Susan Hough, a
seismologist with the U.S. Geological Survey in Pasadena, California. That quake
How The Deadly Nepal
Earthquake Happened
[Infographic]
killed an estimated 10,700 people.
Here are illustrations that show, first, how the initial collision occured, then how the
thrust fault is continuing to fracture the crust in the area, and finally where the frontal
thrust fault lies in relation to other cracks in this very quake­prone zone.
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How The Deadly Nepal Earthquake Happened [Infographic] ­ Scientific American
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HIMALAYAS WERE FORMED when the Indian lithospheric plate drifted northward
and collided with the Eurasian plate. The collision is shown here in simplified,
vertically exaggerated diagrams. Some 60 million years ago the oceanic lithosphere at
the leading edge of the Indian plate was being subducted under southern Tibet (1).
Magma rising above the Indian plate erupted from volcanoes and formed granite
intrusions. Sediments and oceanic crust scraped off the descending plate piled up in
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an accretionary wedge, which created a forearc basin that trapped sediments eroded
from Tibet. Sometime between 55 and 40 million years ago the two landmasses
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collided (2). Presumably the Indian crust was too buoyant to plunge far under Tibet;
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as a result a new fault, the Main Central Thrust, broke through the Indian crust.
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Subsequently motion continued along the fault (3). A slice of Indian crust, topped by
Paleozoic and Mesozoic sediments that had been deposited on the continental shelf,
was thrust up onto the oncoming subcontinent. The accretionary wedge and the
forearc sediments were thrust northward onto Tibet. (Much of this material has since
been eroded away.) About 20 to 10 million years ago the Main Central Thrust became
inactive. Since then India has slid northward along a second fault, the Main Boundary
Fault (4). A second slice of crust has been thrust up onto the subcontinent, lifting up
the first slice. The two uplifted slices make up the bulk of the Himalayas; many of the
peaks are capped by Paleozoic sediments. The Indian plate bends slightly under the
weight of the mountains, and the resulting trough, now filled with sediments, can be
detected under the Ganges plain. [Originally produced for "The Structure of Mountain
Ranges," by Peter Molnar, in Scientific American, July 1986; Illustration by Ian
Worpole]
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29/04/2015
How The Deadly Nepal Earthquake Happened [Infographic] ­ Scientific American
SHINGLING EFFECTS occur when tectonic plates collide and create thrust faults.
Such shingling—the result of the India­Asia plate collision—has occurred in the
Himalaya. Faults of a second type are found near the crest of the Himalaya, dipping
northward below the Tibetan Plateau. Constituting what is known as the South
Tibetan fault system, these faults share geometric similarities with the thrust faults,
but rocks slip along this system in the opposite direction. This fault system may also
mark the top of the fluid lower crustal channel below Tibet. New evidence suggests
that northward slip along the South Tibetan fault system and simultaneous southward
slip along the southern faults permit the southward extrusion of this channel toward
the Himalayan range front. (Tan regions are moving north. Purple and gray regions
are moving south.) [Originally produced for "Climate and the Evolution of
Mountains," By Kip Hodges, in Scientific American, August 2006; Graphic by Jen
Christiansen; Source: “Southward Extrusion of Tibetan Crust and its Effect on
Himalayan Tectonics," By K. V. Hodges, J. M. Hurtado and K. X. Whipple, in
Tectonics, VOL . 20, NO. 6 , pages 799–809; 2001].
PRINCIPAL TECTONIC FEATURES that are thought to be associated with continuing
northward push of the India plate againstthe Eurasia plate have been plotted by the
authors, partly on the basis of the analysis of ERTS photographs and partly on the
basis of studies of major earthquakes (colored dots), which reveal how the crust has
moved along faults. The straight lines without arrowheads through dots indicate
thrust faults. The double­headed arrows indicate normal faults. The pairs of anti
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29/04/2015
How The Deadly Nepal Earthquake Happened [Infographic] ­ Scientific American
parallel arrows indicate movement along strikeslip faults. The areas in color appear to
be zones of recent uplift resulting from crustal shortening. The overall impression is
that the large Eurasian land mass that lies to the west of 70 degrees east longitude has
remained more or less undeformed as China bas been pushed to east. [Originally
produced for "The Collision between India and Eurasia," By Peter Molnar and Paul
Tapponnier, Scientific American, April 1977; Graphic by Andrew Tomko].
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