Earthquakes
are usually caused when rock underground suddenly breaks along a fault. This
sudden release of energy causes the seismic waves that make the ground shake.
When two blocks of rock or two plates are rubbing against each other, they
stick a little. They don't just slide smoothly; the rocks catch on each other.
The rocks are still pushing against each other, but not moving. After a while,
the rocks break because of all the pressure that's built up. When the rocks
break, the earthquake occurs. During the earthquake and afterward, the plates
or blocks of rock start moving, and they continue to move until they get stuck
again. The spot underground where the rock breaks is called the focus of the earthquake. The place right
above the focus (on top of the ground) is called the epicenter of the earthquake.
On April -25,2015 in Nepal, a chunk of rock about 9 miles below the earth’s
surface shifted, unleashing a shock wave described as being as powerful as the
explosion of more than 20 thermonuclear weapons that ripped through the Katmandu
Valley. In geological terms, the tremor occurred like clockwork, 81
years after the
“The collision between India and Eurasia is a
showcase for geology,” said Lung
S. Chan, a geophysicist at the University of Hong Kong. The so-called India
plate is pushing its way north toward Asia at a rate of about 5 centimeters, or
2 inches, a year, he said. “Geologically speaking, that’s very fast.” As the
plates push against each other, friction generates stress and energy that builds until the crust ruptures,
said Dr. Chan, who compared the quake to a thermonuclear weapons explosion. In
the case of Saturday’s quake, the plate jumped forward about 2 meters, or 6.5 feet,
said Hongfeng Yang, an earthquake expert at the Chinese University of Hong
Kong. Saturday’s quake was also relatively shallow, according
to the U.S. Geological Survey. Such quakes tend to cause more damage and more
aftershocks than those that occur deeper below the earth’s surface. After an
earthquake, the plates resume moving and the clock resets. “Earthquakes
dissipate energy, like lifting the lid off a pot of boiling water,” said Dr.
Chan. “But it builds back up after you put the lid back on.”
Nepal
is prone to destructive earthquakes, not only because of the massive forces
involved in the tectonic collision, but also because of the type of fault line
the country sits on. Normal faults create space when the ground cracks and
separates. Nepal lies on a so-called thrust fault, where one tectonic plate
forces itself on top of another. The most visible result of this is the
Himalayan mountain range. The fault runs along the 1,400-mile range, and the
constant collision of the India and Eurasia plates pushes up the height of the
peaks by about a centimeter each year.
Despite the seeming regularity of severe earthquakes
in Nepal, it isn’t possible to predict when one will happen. Historic records
and modern measurements of tectonic plate movement show that if the pressure
builds in the region in a way that is “generally consistent and homogenous,”
the region should expect a severe earthquake every four to five decades, Dr.
Yang said. The complexity of the forces applying pressure at the fault means
scientists are incapable of predicting more than an average number of
earthquakes that a region will experience in a century, experts say. The
earth’s tectonics plates are constantly in motion. Some faults release built-up
stress in the form of earthquakes. Others release that energy quietly. “Some
areas, like Nepal, release energy as a large earthquake, once in a while,” said
Dr. Chan. “These regions all have different natures for reasons geologists
don’t really know.”
Written by: Indra L. Sapkota .
With the Special consult of :
Dr . Michio kaku
Dr. S. Chan
Dr. Shem Dharmapual
Dr. Hongfeng Yang
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