The First Heavier Quake
February 19th, 1997
2nd big quake, in Roswinkel, 3.4 Richter scale / 3 km depth
August 8th, 2006
3rd big quake: triangle Middelstum, Stedum, Westeremden, 3.5 Richter scale / 3 km depth
Luckily, the seismologists at the KNMI aren't worried (sarcasm) (Video-fragment: RTV Noord.)
Audio Transcript: “The KNMI expects slightly heavier earthquakes to occur, with a maximum magnitude of 3.8 on the Richter scale. But the seismologists aren't worried.”
August 16th, 2012
(Huizinge, 3.6 Richter scale / 3 km)
Also, the frequency of smaller quakes is increasing.
Earthquakes as a result of gas extraction occur just below the surface of the earth. A shallow “gas quake” has a lot more impact than a natural earthquake of the same magnitude 20 to 100 km deep.