Earthquake FAQ

History of Earthquakes in Nepal

Why do earthquakes occur in Nepal?

As the Indian plate slowly glides underneath the Tibetan plate, energy is stored in the plates (just like compressing a spring) because the friction between the two plates will try to resist their movement. The accumulated energy keeps on increasing, until the friction can no longer resist the movement. Then a massive slip between the plates occurs releasing energy in the form of a major earthquake. The longer the period between two major earthquakes, the larger will be the expected magnitude of the earthquake because more energy will have stored in the plates. In Nepal, the typical period between two major earthquakes is about 80-90 years.

Now, when the plates move to a new position after the main earthquake, there will be subsequent movements of the plates as they adjust in a new position, thus releasing subsequent aftershocks which are of smaller magnitudes.

Figure: Collision of Indian plate with Tibetan plate (Source: USGS, 1999 and Dahal, 2006)

What are aftershocks and how long will they continue to occur?

An aftershock is a smaller earthquake that occurs after a previous large earthquake, in the same area of the main shock. Aftershocks are formed as the crust around the displaced fault plane adjusts to the effects of the main shock. 61 aftershocks of magnitude greater than 2.5 has been recorded by USGS as of May 1, 2015.

Aftershocks can keep on occurring weeks or even months after the main shock, but they generally become smaller and less frequent. Expected number number of aftershocks issued by USGS are: 3 to 13 aftershocks of magnitude greater than 5M and 0 to 3 aftershocks of magnitude greater than 6M is expected to occur within the next one month period (May 2015) [Ref: USGS]. However, please note that exact date, time and magnitude cannot be predicted.

aftershocks

Figure: Yellow dots are aftershocks that were triggered within one week period after the mainshock (red dot) (Source: USGS)

Can earthquakes be predicted?

No. The occurrence,  non-occurrence, time or magnitude of future earthquakes cannot be predicted. However, aftershocks are expected weeks and sometimes even months after the main shock, but they generally become smaller and less frequent.

What is an Epicenter?

In the case of earthquakes, the epicenter is the point on the surface that is directly above the point where the fault begins to rupture, and in most cases, it is the area of greatest damage. However, in larger events, the length of the fault rupture is much longer, and damage can be spread across the rupture zone.

Why were Kathmandu and Sindhupalchowk hit so badly even though they were so far from the epicenter?

The epicenter of the main shock of 2015 Nepal earthquake was 80km away from the Kathmandu valley and more than 100km away from Sindhupalchowk. Despite of the large distances, Kathmandu and Sindhupalchowk were hit badly and this can be attributed to the following reasons:

  1. An epicenter is a point on the surface that is above the place where the fault begins to rupture. However, in larger events, the length of the rupture is much longer, and damage can be spread across the rupture zone. In the case of the 2015 Nepal earthquake, the rupture started from the epicenter at Barpak Gorkha district, but the block that ultimately moved extended from Gorkha to Sindhupalchowk (as shown in the shake map below). Kathmandu was on the block that actually moved, and so was Sindhupalchowk. Thus both of these places sustained high amount of shaking.
  2. In the case of Kathmandu, the shaking was further intensified because of the soil property. Kathmandu consists of large depths of lake deposit soil. As the earthquake wave travels from the underneath rock to the soil, the shaking is amplified. Furthermore, portion of the wave that travels from the soil to surrounding rocks will be reflected back to the soil, thus further increasing the intensity of shaking. This amplification effect depends on various factors like the depth of soil, type of soil, saturation, etc. Hence, different part of Kathmandu experienced different amount of shaking (esp. central and northern portion sustained more shaking). For analogy, Kathmandu can be imagined as a bowl of yogurt. If you shake the bowl, the center of the yogurt will shake even more.

intensity

Figure: Shake intensity map. The rectangle shows the approximate location of the block that actually moved during the 2015 Nepal earthquake. (Source: USGS)

More questions about Earthquakes?

If you have more questions regarding earthquakes, you may email us at: faq@nepalquake.org

References

 


 

Position paper on earthquakes in Nepal,  by the American Society of Nepali Engineers is here.