North Wales Police received several calls from members of the public from Anglesey, Caernarfon and the Llyn Peninsula when the incident occurred just after 6am on Wednesday morning.

The British Geological Survey in Edinburgh, which monitors seismological incidents, has also received enquiries and its scientists will analyse data to assess if it was a tremor.

BBC Wales's Barry Michael Jones reports on the tremor

This morning's incident lasted just a few seconds and witnesses said it was like a distant sonic boom.

Scientists said that if it was a tremor, the area could experience after-shocks over the coming days or weeks.

The Menai Strait area has a known geological fault under it and the shifting plates are thought to be responsible for separating Anglesey for the mainland.

Dr Russ Evans: "It can be quite alarming"

Dr Russ Evans, from the British Geological Survey, said the north Wales area had experienced one of Britain's largest-ever tremors in 1984.

"These are, by international standards, quite small earthquakes," Dr Evans explained.

Tremors are caused by faults a long distance underground, but the 1984 incident - which touched five on the Richter scale - produced so many telephone calls that police switchboards were jammed.

"Even at these kinds of levels, it can be quite alarming," said Dr Evans.

"Fortunately, houses in this part of the world are well-built."