Indochina (Viet Nam ,Cambodia, and Laos) lies in the “Extrusion Tectonic” model site. Under this theory, India detached and moved northward – from what used to be Gondwanaland in ancient times – around 50 million years ago and collided with present-day Eurasia.

This collision gave rise to the Himalayas and fault systems – defined as a displacement of rock layers in the Earth’s crust because of stress –both to the west and east.

The fault systems that developed in southeast Asia as a result were the Wang Chao system, Ailao Shan/Red River system and Altin Tagh system.

Tremendous stress produced by the collision pushed away geological blocks in southeast Asia along new fault systems over time. The Wang Chao fault system formed around 35 million years ago along what is now the Mekong river. The geological block in the southern part of this fault system has extruded southeastward along the fault line.

However, the Wang Chao system ceased to be active around 17 million years ago as India moved further northward and a new fault system developed north of the Wang Chao, called the Ailao Shan/Red River fault system.

Activities along this new system began 17 million years ago but were previously believed to have ceased 5 million years ago. However, studies by Allen (1984) proved that the Ailao Shan/Red River system is still active and earthquakes are regularly reported along the northern part of this fault in Yunnan province (China).

The earthquake in Dien Bien Phu (2001) and other places in northern Vietnam are, therefore, attributed to this system.

As India continues to move northward, the main Himalayan orogeny – the process of mountain formation – is concentrated along the Altin Tagh fault system in China.

The recent earthquakes off southern Viet Nam can be attributed either to the Qui Nhon fault zone – which continues from the Red River fault as it enters the north Vietnam bay and turns south along central Viet Nam – or to the extended branches of the Wang Chao system in the south.

Earthquake epicenters were also reported in 1983 and 1989 along the Wang Chao fault in Cambodia.

This strengthens a suggestion that the recent earthquakes off the coast of southern Vietnam could be caused by the reactivation of the Wang Chao fault system or possibly fault activities within ongoing continental margin basins.

More geological studies must be done to see if the Wang Chao fault system is still active or if other regional tectonic activities are causing earthquakes in southern Vietnam. 

Written by Tuan Nguyen

San Jose State University, California, USA.