Office workers and shoppers gasped just before 1:30 pm (1230 GMT) as the latest tremor -- with a preliminary rating of 2.5 on the Richter scale, and lasting three seconds -- made itself felt.

"It was pretty hair-raising. I thought it was a bomb, a plane crashing or an earthquake," said Alan Birchall, senior security office at the Royal Exchange in the heart of Manchester.

No serious injuries or property damage were reported.

The first of the Manchester tremors, on Monday, woke up rush-hour commuters at 8:45 am, and scored 3.2 on the Richter scale. A bigger earthquake -- measuring 3.9 -- followed at lunch.

Yet more tremors followed on Monday evening and Tuesday morning, before the latest lunch-hour rumble.

"It looks like this earthquake is slowly releasing its energy," said Julian Bukits of the British Geological Survey, who added that the epicenter stretched for 10 kilometers (six miles) across the city center.

Catastrophic earthquakes are unknown in Britain, though 200 to 300 milder ones are recorded every year. On September 23 a temblor measuring 5.0 on the Richter scale in the West Midlands was felt as far away as London.

SPACE.WIRE