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Why Nepal Is Still Being Hit by Earthquakes

3 minute read

A second major earthquake struck eastern Nepal on Tuesday, less than three weeks after the country was devastated by a quake on April 25 that claimed over 8,000 lives. According to the U.S. Geological Survey, Tuesday’s quake measured 7.3 on the Moment Magnitude Scale, which succeeded the Richter scale in the 1970s, and struck 50 miles east of the capital Kathmandu, close to Mount Everest. Tremors were felt in several Indian states, Bangladesh and China.

At least 42 people have been killed in and more than 1,000 others have been injured. Here’s why Nepal has been struck again and what to expect next.

What is an aftershock?

Large-magnitude earthquakes tend to be followed by several smaller magnitude earthquakes known as aftershocks.

Aftershocks look and act exactly the same as earthquakes, except that they are smaller than the mainshock and defined in their relation to the main event. The only difference is that an aftershock occurs in the same basic location, within an ‘aftershock zone’ and should occur before the rate of seismic activity returns to its pre-mainshock level.

Is Tuesday’s Nepal earthquake an aftershock?

Some aftershocks can occur on nearby faults outside the normal ‘aftershock zone’, but Tuesday’s quake could be seen as separate quake. Whereas the April quake’s epicenter was west of Kathmandu, the May 12 one struck east of the capital, near the Chinese border.

Tuesday’s quake has also sparked off another round of seismic jolts in its wake. Just 30 minutes after the 7.3 quake, an aftershock of 6.3 magnitude hit. There have been another five major aftershocks of magnitude 5.0 or higher.

Scenes From the Aftermath of Another Massive Quake in Nepal

Nepalese search for open space as a strong earthquake hits Kathmandu on May 12, 2015.
Nepalese search for open space as a strong earthquake hits Kathmandu on May 12, 2015. Mast Irham—EPA
Nepalese armed police force search for victims after a house collapsed in strong earthquake hits Kathmandu on May 12 2015.
Nepalese armed police force search for victims after a house collapsed in strong earthquake hits Kathmandu on May 12 2015. Narendra Shrestha—EPA
Nepalese run for open space as a strong earthquake hits Kathmandu on May 12, 2015.
Nepalese run for open space as a strong earthquake hits Kathmandu on May 12, 2015. Mast Irham—EPA
Nepalese military personnel and International rescue check on a collapsed building after an earthquake in the centre of Kathmandu on May 12, 2015.
Nepalese military personnel and International rescue check on a collapsed building after an earthquake in the centre of Kathmandu on May 12, 2015. Athit Perawongmetha—Reuters
Nepalese rescue workers with a dog search for victims after a house collapsed during a strong earthquake, in Kathmandu on May 12, 2015.
Nepalese rescue workers with a dog search for victims after a house collapsed during a strong earthquake, in Kathmandu on May 12, 2015. Narendra Shrestha—EPA
Nepalese military personnel remove debris in search of survivors after a fresh 7.3 earthquake struck, in Kathmandu on May 12, 2015.
Nepalese military personnel remove debris in search of survivors after a fresh 7.3 earthquake struck, in Kathmandu on May 12, 2015. Athit Perawongmetha—Reuters
A patient is carried by Nepalese military personnel and volunteers back to the hospital building after she had been evacuated when an earthquake hit, in Kathmandu on May 12, 2015.
A patient is carried by Nepalese military personnel and volunteers back to the hospital building after she had been evacuated when an earthquake hit, in Kathmandu on May 12, 2015. Mast Irham—EPA
People gather in the safety of open space following a further major earthquake in Kathmandu on May 12, 2015.
People gather in the safety of open space following a further major earthquake in Kathmandu on May 12, 2015. Jonas Gratzer—Getty Images
People bring their injured to a Police Hospital in Kathmandu following a further major earthquake in Kathmandu on May 12, 2015.
People bring their injured to a Police Hospital in Kathmandu following a further major earthquake in Kathmandu on May 12, 2015.Jonas Gratzer—Getty Images
A woman stands on the debris of collapsed houses after a fresh 7.3-magnitude earthquake struck Nepal, in Sankhu May 12, 2015.
A woman stands on the debris of collapsed houses after a fresh 7.3-magnitude earthquake struck Nepal, in Sankhu on May 12, 2015. Navesh Chitrakar—Reuters

So are the two unrelated?

There is evidence to suggest that earthquakes can trigger other earthquakes, even outside of their ‘aftershock zones.’ Nepal sits on a continental collision zone between the Indian Plate and the Eurasian Plate and the stress changes caused by the first April quake likely triggered the second earthquake on Tuesday. In fact, the U.S. Geological Survey (USGS) forecast an aftershock in that general area: its modelling suggested a 0.5% chance of a quake between 7 and 7.8 on the scale occurring this week.

Have there been lots of aftershocks since the Nepal quake on April 25?

A steady stream of aftershocks rattled Nepal in the aftermath of the April quake, initially at 15-20 minute intervals. One day after the quake, a shock of magnitude 6.7 struck the same region, causing fresh avalanches on Mount Everest, a landslide on a major highway and was felt in several parts of northern India. By May 1, more than 100 aftershocks with more than a 4.0 magnitude had occurred.

Will there be more to come after Tuesday’s earthquake?

Yes, in general, the greater the size of the earthquake, the higher the number of aftershocks. With any major earthquake, there will always be aftershocks. These can continue for weeks, months and even years. They tend to decrease in number but it’s very difficult to say when or where they’ll happen.

Could the next aftershock be worse?

Sometimes, aftershocks can be larger than the original earthquake, in fact making the original quake a foreshock, tremors that occur before and in the same area as the mainshock. But foreshocks can only be identified once the mainshock has occurred.

James Nachtwey's Dispatches from Nepal

Nepal earthquake. Hindu funeral cremation rituals at Pashupatinath Temple on the Bagmati River in Katmandu. by James Nachtwey
Bodies are prepared for cremation during a Hindu ritual at the Pashupatinath Temple in Kathmandu on April 28, 2015, three days after a 7.8 magnitude earthquake devastated Nepal, killing at least 7,000 people and causing untold damage. James Nachtwey for TIME
Nepal earthquake. Hindu funeral cremation rituals at Pashupatinath Temple on the Bagmati River in Katmandu. by James Nachtwey
During funeral rites in Kathmandu on April 28, 2015, women mourn for loved ones killed in the earthquake. James Nachtwey for TIME
Nepal earthquake. Hindu funeral cremation rituals at Pashupatinath Temple on the Bagmati River in Katmandu. by James Nachtwey
A man performs a Hindu cremation ritual at the Pashupatinath Temple in Kathmandu, April 28, 2015. James Nachtwey for TIME
Nepal earthquake. Bhaktapur. A city near Katmandu. People reclaiming their possessions from the wreckage. A man who had been buried and died being pulled out of the wreckage by there Nepali Army rescue team. Sculpture of elephant in ruins of an ancient, sacred temple.by James Nachtwey
A man sorts through the wreckage of destroyed homes in the ancient city of Bhaktapur, near Kathmandu, April 28, 2015. James Nachtwey for TIME
Nepal earthquake. Bhaktapur. A city near Katmandu. People reclaiming their possessions from the wreckage. A man who had been buried and died being pulled out of the wreckage by there Nepali Army rescue team. Sculpture of elephant in ruins of an ancient, sacred temple.by James Nachtwey
Residents search through the debris in Bhaktapur, April 28, 2015. James Nachtwey for TIME
Nepal earthquake. Bhaktapur. A city near Katmandu. People reclaiming their possessions from the wreckage. A man who had been buried and died being pulled out of the wreckage by there Nepali Army rescue team. Sculpture of elephant in ruins of an ancient, sacred temple.by James Nachtwey
A body is found in the wreckage in Bhaktapur, April 29, 2015. James Nachtwey for TIME
Nepal earthquake. Bhaktapur. A city near Katmandu. People reclaiming their possessions from the wreckage. A man who had been buried and died being pulled out of the wreckage by there Nepali Army rescue team. Sculpture of elephant in ruins of an ancient, sacred temple.by James Nachtwey
Nepal's 7.8 magnitude earthquake has caused massive destruction of historical temples and palaces like this ancient and sacred site in Bhaktapur, April 29, 2015. James Nachtwey for TIME
Nepal earthquake. Town of Sankhu. Mush of town was destroyed. People digging out their possessions. Aerial images of Nuwakot district, hit by the earthquake and not able to be reached by aid and rescue teams. Farming villages built atop steep hillsides with cascading, terraced fields.by James Nachtwey
Destroyed farming villages are seen from a helicopter during an Indian Army relief mission to Nepal’s remote Nuwakot District, April 30, 2015.James Nachtwey for TIME
Nepal earthquake. Small village in foothills with people gathering near helipad to watch Nepal Army helicopter land with supplies. Cremation pyres in Katmandu at Pashupatinath Temple on Bagmati River.by James Nachtwey
After being airlifted from a remote village to Kathmandu, an injured woman is carried to receive care, April 30, 2015. James Nachtwey for TIME
Nepal earthquake. Town of Sankhu. Mush of town was destroyed. People digging out their possessions. Aerial images of Nuwakot district, hit by the earthquake and not able to be reached by aid and rescue teams. Farming villages built atop steep hillsides with cascading, terraced fields.by James Nachtwey
Villagers sort through rubble in Sankhu, on the outskirts of Kathmandu, May 1, 2015.James Nachtwey for TIME
Nepal earthquake. Town of Sankhu. Mush of town was destroyed. People digging out their possessions. by James Nachtwey
Residents look through wreckage in the town of Sankhu, which was largely destroyed in the earthquake, May 1, 2015. James Nachtwey for TIME
Nepal earthquake. Town of Sankhu. Mush of town was destroyed. People digging out their possessions. by James Nachtwey
A shadow is reflected on the remains of a collapsed house in the largely destroyed village of Sankhu on the outskirts of Katmandu, May 1, 2015. James Nachtwey for TIME

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Write to Naina Bajekal at naina.bajekal@time.com