Natural and induced seismicity

Natural seismicity in the Netherlands is mainly associated with large faults in the south of the country. These faults are part of the Roer Valley Rift System, the northwestern extension of the Rhine rift system. Their style of faulting, as concluded from the analysis of seismicity, is dominantly normal with a small strike-slip component and is in accordance with the current stress pattern. South of the Roer Valley Rift System, smaller earthquakes suggest the reactivation of strike-slip faults and thrust faults along the Brabant Massif. The effects of natural seismicity have been moderate on the whole and include structural damage to buildings in epicentral areas. Hazard analysis based on historical seismicity suggests a maximum expected magnitude of ML = 6.3. Paleoseismological investigations indicate that events that may reach one magnitude higher, occur at a recurrence interval of at least 2 to 3 kyr along the major fault zones. In the north of the Netherlands no significant natural activity has been detected, but since 1986 induced events, most of which coincide with producing gas fields have been recorded. Initially, small gas fields started to show seismicity and from 2003 onward seismicity in the large Groningen gas field became dominant. The largest recorded event near Huizinge in 2012 (ML = 3.6) caused considerable damage to buildings in the region, due to its shallow depth of 3 km. After the Huizinge event, monitoring in Groningen was intensified and a large research programme was started, leading to a significant increase in knowledge on the origin and effects of seismicity in the region. Induced seismicity was also recorded in the vicinity of salt mining, geothermal activities and possibly water injection.