source: U.S. Geological Survey (USGS)
Updated USGS Aftershock Advisory for the Magnitude 7.8 Gorkha earthquake in Nepal April 25, 2015 (as of May 20, 2015)

Aftershocks are earthquakes that occur following a large earthquake,in the same general area as the earthquake and during the following days-to-years. Both the magnitude 7.8 Gorkha mainshock and the subsequent May 12 magnitude 7.3 aftershock have triggered aftershocks. Aftershocks have the potential to create damage, just like other earthquakes.

The aftershocks are a normal occurrence after large earthquakes, and are expected to continue in Nepal but occur less often with time. The probability of future aftershocks is lower than in our forecast of May 13 due to the additional time that has passed since magnitude 7.3 earthquake on May 12, 2015, which was itself an aftershock of the Gorkha mainshock.

There is no way to predict the exact date or time of an earthquake or aftershock. The USGS produces a statistical analysis of the expected number or probability of aftershocks, in a given time period, based on past earthquakes and the aftershocks recorded in Nepal. As is normal,there will continue to be many felt aftershocks that do little or no damage. Some aftershocks may be strong enough to be felt widely throughout the area and may cause additional damage, particularly to vulnerable structures and those already weakened by the mainshock and the aftershocks. Although aftershocks may occur less often, people should remain aware of the possibility of aftershocks in the coming weeks and months, especially when working in or around vulnerable structures or in landslide - prone areas.

> Within the week of May 20 to May 26, the USGS estimates that the chance of at least one magnitude 5 to 6 aftershock is about 40% and upto 3 such events are likely to occur.
> A magnitude 6 to 7 earthquake has a probability of 6% or a 1-in-16 chance of occurring within this week. A magnitude 7 to 7.8 aftershock is possible, but less likely with about a 1-in-200 chance of occurring.
> The potential for an aftershock larger than the mainshock remains, but is even lower with about a 1-in-1000 chance within this week.

If an earthquake larger than the mainshock does occur, the USGS expects that it would most likely be about the size of the mainshock. The expected numbers and probabilities of aftershocks for the next month and year, and technical details of the analysis are available at http://earthquake.usgs.gov/earthquakes/eventproducts/us20002926/aftershock-statistics.pdf

Larger magnitude earthquakes have lower probabilities but larger consequences than smaller magnitude earthquakes. In making decisions based on the aftershock statistics, it is important to consider both the probability that an earthquake may happen and the potential consequences if it does happen.

In comparison to this forecast, prior to the recent M7.8 mainshock this region has experienced about 1 magnitude 5 or greater earthquake per year over the last 20 years; due to the aftershock sequence the rate of earthquakes is currently higher than that.

The expected location of the aftershocks will be in the zone of current activity and at its edges with a few located further away.

This information is preliminary and subject to change as more data becomes available. The USGS will update this analysis for future time periods on or before May 27, 2015.

original link source (pdf) below:
http://earthquake.usgs.gov/earthquakes/eventproducts/us20002926/aftershock-advisory.pdf