Earthquake risk in the Himalaya

By Victoria Stevens

Earthquakes have not been releasing energy as fast as the energy has been building up along the Himalayan arc. Meaning that there could be a giant earthquake in the region placing millions at risk.

A new study of the 2000 km long Main Himalaya Thrust, the largest earthquake generating fault in the Himalaya, has revealed that large quakes could occur in any location along the Himalayan arc.

Unlike in subduction zones, where some patches of the fault are moving, or ‘creeping’ at a constant speed, in the Himalaya we don’t see any creeping patches. This means that the fault is fully ‘locked’, i.e. strain energy is building up most of the time. This energy is released suddenly during earthquakes. Because there are no creeping patches, there are no barriers to rupture, which means once an earthquake has started, it could rupture a very long way along the fault without anything to limit its size.

The degree of 'locking' on the Main Himalayan Thrust. Where the fault is red, its fully locked and white where its not locked. Source: Stevens and Avouac 2015
The degree of ‘locking’ on the Main Himalayan Thrust. Where the fault is red, its fully locked and white where its not locked. Source: Stevens and Avouac 2015

The study shows that the pattern of coupling, i.e. the degree of fault locking, has been stationary with time. From the coupling pattern, the rate of moment build-up can be found. This is how much energy is building up each year, and is also the amount that needs to be released in earthquakes if all the energy is released seismically.

Earthquakes have not been releasing energy as fast as the energy has been building up, so we may expect very large earthquakes in this region in the future. Studies of ancient earthquakes have shown that quakes approaching magnitude 9 have occurred previously in both the western and eastern halves of the Himalayas. It is not impossible that these giant earthquakes could occur again.

night_lights
Night lights show large population densities living in the shadows of the Himalaya.

This has important implications for seismic hazard in the region. The population living in the Himalayas has increased dramatically in the past few decades, and most buildings are not resistant to large shaking caused by earthquakes. As we saw with the recent devastating April Gorkha-Nepal earthquake, the Himalayan countries prone to earthquakes are not yet prepared to meet all the challenges this natural hazards present.

Read the full journal article titled: Interseismic coupling on the main Himalayan thrust

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Victoria Stevens is a PhD graduate student at the California Institute of Technology (Caltech).

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