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Nepal Earthquake: Not in Pacific Ring of Fire But Prone to Dangerous Quakes, Why?

Today’s 7.8

Kathmandu: A portion of a building collapses in Kathmandu as a 7.5 magnitude earthquake hits Nepal on April 25, 2015. (Photo: Sabina Lama/IANS)

Kathmandu: A portion of a building collapses in Kathmandu as a 7.5 magnitude earthquake hits Nepal on April 25, 2015. (Photo: Sabina Lama/IANS)

urvey, due to frontal thrust between the subducting India plate and the overriding Eurasia plate to the north. The India plate is converging with Eurasia at a rate of 45 mm per year towards the north-northeast, driving the uplift of the Himalayan mountain range, it said.

Though Nepal or India do not fall in the Pacific Ring of Fire that is prone to most earthquakes on earth, Nepal and the Himalayan range is prone to more dangerous earthquakes, according to USGS.

The location of the earthquake is known for its past earthquakes but large earthquakes on the Himalayan thrust are rare in history. Just four earthquakes of M6 or larger have occurred within 250 km of the April 25, 2015 earthquake over the past century. In recent history, a M 6.9 earthquake in August 1988, 240 km from today’s epicentre, caused close to 1,500 deaths. On record, the largest one of an M 8.0 quake known as the 1934 Nepal-Bihar earthquake, occurred in the same location as was the 1988 event. It had damaged Kathmandu and led to 10,600 fatalities.

Otherwise, seismicity in the Himalaya generates numerous earthquakes, making the region one of the most seismically hazardous regions on Earth. The tectonic plate boundary movement is marked in the foothills of the north-south trending Sulaiman Range in the west, the Indo-Burmese Arc in the east and the east-west trending Himalaya Front in the north of India.

The India-Eurasia plate boundary is rather diffuse in the region near the north of India, lies within the limits of the Indus-Tsangpo (also called the Yarlung-Zangbo) Suture to the north and the Main Frontal Thrust to the south. The Indus-Tsangpo Suture Zone is 200 km north of the Himalaya Front.

The narrow, less than 200km, stretch of Himalaya Front has the highest rates of seismicity and largest earthquakes in the Himalaya region, caused mainly by movement on thrust faults resulting in major earthquakes such as the 1934 M8.0 Bihar quake, the 1905 M7.5 Kangra quake and the 2005 M7.6 Kashmir earthquake.

The Kangra and Kashmir quakes resulted in the highest death tolls for Himalaya earthquakes, together killing over 100,000 people and leaving millions homeless. The largest ever recorded Himalayan earthquake occurred on 15 August 1950 in Assam with a magnitude of 8.6 causing extensive damage to villages around the epicentre.

The Tibetan Plateau north of the Himalayas, stretching in 1,000-km north-south and 2,500-km east-west, is also tectonically complex with several sutures which are hundreds of kilometer-long and generally trend east-west. The Tibetan Plateau is cut by a number of large (>1000km) east-west trending, left-lateral, strike-slip faults, including the long Kunlun, Haiyuan, and the Altyn Tagh.

Along the western margin of the Tibetan Plateau, in the vicinity of south-eastern Afghanistan and western Pakistan, the India plate translates obliquely relative to the Eurasia plate, resulting in a complex fold-and-thrust belt known as the Sulaiman Range often see highly destructive earthquakes. The active, left-lateral, strike-slip Chaman fault is the fastest moving fault in the region, according to USGS.

In 1505, a segment of the Chaman fault near Kabul, Afghanistan, ruptured causing widespread destruction, while the more recent 30 May 1935, M7.6 Quetta earthquake, in Pakistan, killed between 30,000 and 60,000 people.

On the north-western side of the Tibetan Plateau, beneath the Pamir-Hindu Kush Mountains of northern Afghanistan, earthquakes in a range of 200 km occur due to remnant lithospheric subduction. The curved arc of deep earthquakes found in the Hindu Kush Pamir region indicates the presence of a lithospheric body at depth, says USGS.

In the Indo-Burmese Arc, a combination of strike-slip and reverse faults such as Sagaing, Kabaw and Dauki faults caused quakes and between 1930 and 1956, six M7.0+ earthquakes occurred near the right-lateral Sagaing Fault, causing severe damage in Myanmar and taking away 610 lives.

Deep earthquakes (200km) have also been known to occur in this region due to the subduction of the eastwards dipping, India plate, though whether subduction is currently active is debatable, according to USGS. In recent recorded history, the large Shillong earthquake occurred on 12 June 1897, causing widespread destruction.

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