Monday 27 July 2020

Magnitude 4.8 Earthquake in Son La Province, northern Vietnam.

The United States Geological Survey recorded a Magnitude 4.8 Earthquake at a depth of 10 km, in Son La Province, northern Vietnam, close to the border with Laos, slightly after 12.15 pm local time (slightly after 5.15 am GMT) on Monday 27 July 2020. There have been no reports of any casualties associated with this event, but it causes minor damage to several buildings in Thailand and was felt across northern Vietnam. 

The approximate location of the 27 July 2020 Son La Earthquake. USGS.

Although Vietnam is located far from any active tectonic plate boundaries, it is still effected by the collision between the Indo-Australian and Eurasian Plates, and the northern part of the country is a recognised area of Earthquake hazards. The area is located on the northern part of the a breakaway part of the Eurasian Plate which is being pushed southward by the impact of India into Eurasia. This impact is causing uplift in the Himalayas and pushing the Tibetan block to the east, which in turn forces the Sunda Plate and South China Block to the southeast.

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.

To the south  Indo-Australian Plate, which underlies the Indian Ocean to the west of Sumatra, is being subducted beneath the Sunda Plate along the Sunda Trench, passing under Sumatra and Java, where friction between the two plates can cause Earthquakes. As the Indo-Australian Plate sinks further into the Earth it is partially melted and some of the melted material rises through the overlying Sunda Plate as magma, fuelling the volcanoes of Sumatra and Java.
The Subduction zone beneath Sumatra. NASA/Earth Observatory.
The two plates are not directly impacting one-another here, as occurs in the subduction zones along the western margins of North and South America, but at a steeply oblique angle. This means that as well as the subduction of the Indo-Australian plate beneath the Sunda, the two plates are also moving past one-another. This causes rifting within the plates, as parts of each plate become stuck to the other, and are dragged along in the opposing plate's direction. The most obvious example of this is the Sumatran Fault, which runs the length of Sumatra, with the two halves of the island moving independently of one-another. This fault is the cause of most of the quakes on the island, and most of the island's volcanoes lie on it.
The movement of the tectonic plates around Sumatra. NASA/Earth Observatory.
To the east and northeast the Burma (or Burmese) Plate, a small tectonic plate which underlies part of the eastern Indian Ocean and the western part of Sumatra,  is being pushed northward relative to the Eurasia and the Sunda Plate by the northward movement of the Indian Plate. As these larger plates move together the Burma Plate is being squeezed and fractured, with a major fault line, the Kabaw Fault, having formed across much of the north of the country, along which the Burma Plate is slowly splitting. Most Earthquakes in the region are caused by movement on this fault.
 The movement of the Burma and surrounding plates. Sheth et al. (2011).

Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. The international non-profit organisation Earthquake Report is interested in hearing from people who may have felt this event; if you felt this quake then you can report it to Earthquake Report here.

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