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Nepal earthquake: latest tremor an aftershock, experts say

05/12/2015 03:14 EDT | Updated 05/12/2016 05:59 EDT
Just weeks after a deadly, 7.8 magnitude earthquake rocked the Himalayan city of Kathmandu, burying many beneath the rubble, another huge tremor struck about 150 kilometres away near the town of Namche Bazaar. 

Dozens in Nepal, Bangladesh and Tibet were reported dead and over 1,000 injured on Tuesday, according to Nepal's home ministry. The U.S. Geological Survey is calling the latest tremor a major aftershock of the April 25 earthquake, and warned more could be felt throughout the region for years to come. 

The death toll from the April 25 temblor — known as the Ghorka earthquake after an affected district in Nepal — has risen to over 8,000. Tuesday's event is the result of shifting tectonic plates, which form the Earth's outer layer. The motion of the plates, of which there are more than a dozen major and smaller ones, not only created the world's mountain ranges but causes earthquakes, volcanoes and other seismic activity. 

Why is Nepal such a hotspot?

Basically, the forces that produce earthquakes in the region are the same forces that created the famous Himalayas that draw travellers from all over the world and were also the site of an avalanche triggered by the earthquake, killing 18 climbers at base camp on Mount Everest.

Between 40 and 50 million years ago, the Indian and Eurasian plates collided to form the famous mountain range in one of the most dramatic examples of plate tectonics visible today.

"That's why the Himalayas are where they are. It's really the boundaries between these two plates," said Natural Resources Canada seismologist Allison Bent. 

Where the April earthquake occurred, the Indian plate is still being pushed beneath the Eurasian plate at a rate of about 45 millimetres per year, just slightly faster than fingernails grow.

Why did the latest earthquake happen?

Earthquakes can lead to more earthquakes, whether in the form of an aftershock or increased pressure that builds up along the fault line. They change the dynamics of the plate tectonics that cause them by releasing pressure in some areas and increasing pressure at other points, Bent said.

"When an earthquake happens, it changes the stresses around it," she said. "Generally, in the areas where it's increased, it's more likely the next earthquake will happen there."

Aftershocks have already been recorded since Tuesday's 7.3 magnitude quake, said Julie Dutton, a geophysicist at the U.S. Geological Survey, adding it's even possible an aftershock could be stronger than the original quake.

Can we predict when the next earthquake will strike?

Because the forces that created the Himalayas are still at work, the region will remain a seismically active zone. An educated guess is the best experts can offer. 

"We have areas where we have a higher incidence of earthquakes and by statistics, we can figure out that they could expect to have a sizable earthquake in a certain amount of time. But to actually predict accurately when and where an earthquake is to occur is impossible," Dutton said. 

Could such a thing ever happen in Canada?

The Pacific and North American plates are sliding past each other on Canada's West Coast. A horizontal slip along the fault beneath Haida Gwaii caused a major earthquake in 2012, but an expected, so-called subduction or "megathrust" earthquake between the Juan de Fuca and North American plates is likely to cause the "Big One" within 500 years.

What are the chances of this happening twice in the same area?

"It's hard to predict because it depends on how much of the fault ruptures. Over long time periods, they can happen in the same places they happened before," Bent said, noting cities like San Francisco, located near the San Andreas Fault, have endured major earthquakes in the past and remain at risk for more. ​

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