The north of the Netherlands contains a number of large on-shore gas fields that are in production since 1960. Starting in 1986 seismic activity has been recorded in the area, which was previously considered aseismic. The seismic events are associated with differential compaction due to gas extraction and reactivation of existing faults at reservoir level. The KNMI monitors the area with a network of seismic sensors in shallow (200m) boreholes. Although the seismic activity is limited, the sustained monitoring effort has led to a database with currently (summer 2008) more than 500 local events with magnitudes ranging from -0.8 to 3.5.
Whereas the surface of the Netherlands is well known to be flat, its subsurface is highly complex in places. This is especially true near the gas fields, where the stratification has been distorted severely due to salt tectonics. The complexity is a challenge for accurate earthquake locations. To improve our locations we have recently started to use a 3-D velocity model from the gas industry for the upper 5 kilometers. In combination with our maturing database this allows us to associate groups of induced earthquakes with specific reactivated fault systems.

Reference:
Kraaijpoel, D., B. Dost, R. Sleeman en F. Goutbeek, Location of induced earthquakes in the Dutch gas fields
Poster: ESC General Assembly 2008, September 7-12, 2008, Hersonissos