Sciency Thoughts: Bajhang District

Wednesday, 4 October 2023

Magnitude 5.7 Earthquake in Bajhang District, Nepal.

The United States Geological Survey recorded a Magnitude 5.7 Earthquake at a depth of 20.1 km, in Bajhang District, Nepal slightly after 2.20 pm local time, (slightly after 9.20 am GMT) on Tuesday 3 October 2023. The event was felt as far away as New Delhi in India, and is reported to have injured at least seventeen people, with one more missing and presumed dead after being engulphed by a rockfall. A number of homes are reported to have collapsed in the town of Chainpur, and a major highway is reported to have been blocked by a landslide triggered by the event. The initial event has been followed by at least two significant aftershocks.

The approximate location of the 3 October 2023 Bajhang District Earthquake. USGS.

Earthquake activity in Nepal is caused by the uplift of the Tibetan Plateau, due to the impact of India into Eurasia to the south. The Indian Plate is moving northwards at a rate of 5 cm per year, causing it to impact into Eurasia, which is also moving northward, but only at a rate of 2 cm per year. The collision of the Indian and Eurasian plates has led to the formation of the Himalayan Mountains, the Tibetan Plateau, and the mountains of southwest China, Central Asia and the Hindu Kush.

Block diagram showing how the impact of the Indian Plate into Eurasia is causing uplift on the Tibetan Plateau. Jayne Doucette/Woods Hole Oceanographic Institution.

Much of northern India and neighbouring areas of Central Asia and the Himalayas, are prone to Earthquakes, caused by the impact of the Indian Plate into Eurasia from the south. When two tectonic plates collide in this way and one or both are oceanic then one will be subducted beneath the other (if one of the plates is continental then the other will be subducted), but if both plates are continental then subduction will not fully occur, but instead the plates will crumple, leading to folding and uplift (and quite a lot of Earthquakes). The collision of the Indian and Eurasian plates has led to the formation of the Himalayan Mountains, the Tibetan Plateau, and the mountains of southwest China, Central Asia and the Hindu Kush.

The movement of India into Eurasia over the last 71 million years. USGS.

Lansdslide kills at least six in Bajhang District, Nepal.

Six people have now been confirmed dead and another one is missing following a landslide in the Bajhang District of Nepal on Saturday 4 July 2020. The incident hit the village of Mallesi at about 2.00 am local time, sweeping away seventeen homes, and it was initially feared that as many as 28 people had died. though most of these have now been accounted for. Landslides are a common problem in rural Nepal at this time of year, , associated with the annual monsoon season. Landslides are a common problem after severe weather events, as excess pore water pressure can overcome cohesion in soil and sediments, allowing them to flow like liquids. Approximately 90% of all landslides are caused by heavy rainfall. The confirmed victims of the landslide have been named as Ramti Devi Jethara, 43, Shanti Kumari Jethara, 16, Prayag Jethara, 13, Ashmita Jethara, 7, Santu Jethara, 15, and Dale Jethara, 81, while the missing person has been identified as Giri Jethara, 20.

Damage caused by a landslide in the village of Mallesi in Bajhang District, Nepal, on 4 July 2020. Lokendra BK/Kathmandu Post.

Monsoons are tropical sea breezes triggered by heating of the land during the warmer part of the year (summer). Both the land and sea are warmed by the Sun, but the land has a lower ability to absorb heat, radiating it back so that the air above landmasses becomes significantly warmer than that over the sea, causing the air above the land to rise and drawing in water from over the sea; since this has also been warmed it carries a high evaporated water content, and brings with it heavy rainfall. In the tropical dry season the situation is reversed, as the air over the land cools more rapidly with the seasons, leading to warmer air over the sea, and thus breezes moving from the shore to the sea (where air is rising more rapidly) and a drying of the climate. This situation is particularly intense in South Asia, due to the presence of the Himalayas. High mountain ranges tend to force winds hitting them upwards, which amplifies the South Asian Summer Monsoon, with higher winds leading to more upward air movement, thus drawing in further air from the sea.