The Geological Setting of Nepal and Why it is Earthquake Prone

As a geoscientist, the occurrence of a significant earthquake is always of interest to me.  Since many people really don't understand why "the earth moves", I will use this posting to explain why the recent earthquake in Nepal that has killed thousands of people took place and why such an event is likely to be repeated in the future, just as it has in the past.

In the late 1960s and early 1970s, geologists and geophysicists discovered that the earth's crust, the outermost and thinnest layer of the globe, was formed of a number of plates that "float" on the earth's mantle.  These plates have moved throughout the hundreds of millions of years of geological time, creating and destroying continents and oceans.  When two or more of these plates collide, they can form a subduction zone where one plate is thrust over top of another plate.    There are two main types of subduction zone; the first occurs when a plate of oceanic crust is subjected under a plate of continental crust (oceanic - continental convergence) and the second occurs when two plates of continental crust collide (continental - continental convergence).  Here is a schematic section showing what an oceanic - continental convergence looks like:

The down-sliding plate will melt as it is subducted into the earth's mantle; as the melted continental crust rises, it forms a chain of volcanoes and a series of along the margins of the over-riding plate as it is crushed by the plate that is being subducted.  A prime example of this geological phenomenon can be found along the western margin of North America and is the driving force behind the most recent significant volcanic eruption of our lifetime, Mount St. Helens.

In the case of the Himalayan tectonic region, Nepal's major mountain terrain is being formed as two continental plates collide; in this case, the Tibetan/Eurasian plate is colliding with the Indian plate as the Indian plate moves northward as shown on this diagram:

The geological framework of the Himalayas can be simply thought of as two continents colliding with each other, resulting in one continent pushing up another.   As the stresses of the collision build, they are released as earthquakes along the multitude of faults that cross the region.  In many ways this process is similar to the process that formed the Cordillera of North America which most of us know as the Rocky Mountains.  In that case, a series of smaller island arcs located on the Pacific plate collided with the western margin of the North American plate, pushing the earth's crust up into a long mountain chain that runs the length of North America.

As the Indian plate is subducted under the Eurasian plate, it pushes the continental crust upwards, forming the formidable Himalayan plate as shown on this block diagram:

This collision results in significant shortening of the earth's crust as the earth's crust is thrusted upwards creating the Himalayan mountain chain; in the case of the Himalayan fold belt, a 2010 paper by Long et al suggests that there has been at least 344 to 405 kilometers of crustal shortening across the Himalayas.  This amounts to between 70 and 75 percent shortening.

Geologists believe that the Himalayan Range is one of the younger mountain systems in the world with the collision first taking place around 55 million years ago.  The process that formed the mountains is still occurring with the Indian plate moving northward at a rate of about 5 centimetres per year.  It is this forward drift and ongoing collision that creates the seismic activity which is manifested as earthquakes.  

The 2400 kilometer-long Himalayas can be divided into several geological or tectonic zones that are each separated by major thrust faults as shown on these diagrams:

Approximately one-third of the Himalayan range lies within the boundaries of Nepal.

Here is a cross section showing the physiography of the Himalayas:

As I noted above, the Himalayan mountains are crossed by a series of faults of the thrust (faults where one side moves vertically with respect to the other) and slip-strike faults (faults where one side moves laterally with respect to the other).  Some regions of the Himalayas are far more heavily faulted than others, particularly the central Himalayas found in Nepal.  In the Western and Central parts of Nepal, there are mainly thrust faults and in Eastern Nepal, there are mainly strike-slip and thrust faults. 

Let's close this brief posting with a map showing the seismic regions of Nepal that also shows the numerous earthquakes that have taken place since the early 1800s:

Nepal and the Himalayas in general are one of the world's most seismically active areas.  The complex tectonic setting of the region has made it a focus study area for the world's seismologists who are trying to gain a better understanding of the tectonic framework of the region so that, in the future, they may be able to predict the occurrence of significant earthquakes through changes in seismicity patterns.