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Nepal aftershock: Sorting out the shocks before, during and after seismic events

After every earthquake, experts warn of aftershocks. But why does the earth quake more than once? DW has the answers to some important questions around seismic events.

What is the difference between an earthquake and an aftershock?

You may assume from the name itself that aftershocks are simply continued reverberations of the main seismic shock, the earthquake. But this is not the case. In fact, this question isn't even a question at all. Aftershocks are the same exact phenomenon as the actual earthquake. They simply happen afterwards. With that said, seismologists do differentiate between foreshocks - quakes that happen before the earthquake - the earthquake itself, and the shocks that follow. These can occur days or even months after the actual earthquake, which is also known as the mainshock.

How strong are aftershocks?

When it comes to seismic intensity, foreshocks are very light, oftentimes immeasurably so, but aftershocks can register a magnitude just as high as the mainshock. On April 25, Nepal was utterly devastated by a 7.8 magnitude earthquake. The aftershock on Tuesday, according to the USGS, measured 7.3 on the Richter magnitude scale.

A number of aftershocks have been recorded in Nepal over the past two weeks. Only one of those was greater than 6.0 on the Richter scale.

While foreshocks often go unnoticed, aftershocks can be as intense as the mainshock

While foreshocks often go unnoticed, aftershocks can be as intense as the mainshock

Why do they shocks happen?

In regions threatened by earthquakes, tensions in the Earth often build up over hundreds of years before a seismic event occurs. The source of the tension is the movement of tectonic plates on the Earth's crust, and the tension is discharged not in one burst, but over time, in shearing. Long phases of inactivity can lead to short periods with many seismic shocks, until the tension has been discharged. And then the process starts anew.

The plate boundary in Nepal is one of the most seismically active regions in the world. The Indian plate can move as fast as four centimeters under the Eurasian plate per year. For continental plates, this is a very high velocity.

Can aftershocks be predicted?

In Nepal, the last major earthquake took place around 80 years ago. From a geological perspective, another quake could have certainly been predicted. Whether the next one happens today, tomorrow or in a year, however, cannot be predicted with any certainty.

As with any other seismic event, aftershocks are not predictable. Neither when nor where a quake will happen can be determined. However, this does not preclude predicting that generally aftershocks can - and most likely will - happen. So a population near the mainshock can and should be warned.

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