The US Geological Survey has said aftershocks are normal occurrences after large earthquakes, and are expected to continue in Nepal, but will occur less frequently with time.

“The probability of aftershocks is lower than it was in our April 27 and May 1 analysis because the expected number of aftershocks per day decreases as time passes and because the number of aftershocks is decreasing faster than expected,” an aftershock advisory updated by the USGS yesterday read.

It further stressed there was no way to predict the exact date or time of an earthquake or aftershock. The USGS produces a statistical analysis of 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 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 main shock. Although aftershocks may occur less often, people should remain aware of the possibility of aftershocks in the coming weeks and months, specially when working in or around vulnerable structures or in landslide-prone areas,” the USGS said.

For the week of May 8 to May 14, USGS estimates that aftershocks will continue, but at a decreased rate. The chance of a magnitude 5 to 6 aftershock is about one-in-three or 37 per cent and fewer such events are expected. A magnitude 6 to 7 earthquake is less likely with a 5 per cent or one-in-20 chance during this week. A magnitude 7 to 7.8 aftershock is even less likely with about a 1-in-200 chance. This means there is over a 99 per cent chance that a magnitude 7 aftershock will not occur during the coming week.

“The potential for an aftershock larger than the main shock remains, but is even lower with about a 1-in-1,000 chance. This means there is over a 99.9 per cent chance that an aftershock larger than the main shock will not occur during the coming week. If an earthquake larger than the main shock does occur, the USGS expects it to be most likely about the size of the main shock,” it said.

source: the himalayan times,10 may 2015