Mild tremors measuring 5 on the Richter scale were felt in Sikkim on Sunday evening, MeT office said in Gangtok on Monday.

The low intensity quake which lasted just a few seconds had its epicentre 23 km north-west of Gangtok in the north district, the sources said adding, these tremors must have been felt in parts of north Bengal and Assam too.

Sikkim falls in seismic zone 4, the second highest quake prone zone after Gujarat and Meghalaya, which fall in zone 5.

On an average at least two very low intensity tremors occur in Sikkim every week which is normal considering its high risk seismic zone.

At least 17 shocks too mild to be recorded on the Richter scale have been recorded in Sikkim so far in May itself.

In April the number of such shocks recorded in the state were 14, MeT officials said.

There are two theories regarding mild intensity quakes. While one says that smaller shocks can be a warning bell for a larger shock the other one says that smaller shocks help reduce the possibility of a larger shock by releasing the energy underground and preventing its accumulation, which may heighten the chances of a quake.