AMSTERDAM - An earthquake shook the north of the Netherlands early on Tuesday morning. Measuring 3.5 on the Richter Scale, it was equal in strength to the strongest earthquake on record in the northern Netherlands. This took place in Alkmaar five years ago.

Tuesday's quake in Groningen at around 7am was registered by all seismic stations in the country. It was centred on the town of Middelstum which lies in the middle of the gas fields in the province.

The town has experienced several earthquake in recent years. Weather and seismic agency KNMI and local broadcaster RTV Noord were bombarded with telephone calls and emails from concerned residents after the latestquake.

Hein Haak said the earthquake was the largest ever measured in Groningen and was felt across the entire province.

There have been 10 quakes roughly equivalent to this magnitude since the first earthquake caused by the gas drilling took place in 1986. There have been several dozen smaller shooks during this period too.

The earthquakes result from tension in the earth's crust caused by the extraction of gas. The tension increases in correlation to the amount of gas extraction. The first earthquake took place 27 years after drilling began in Groningen in 1959.