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Northern Groningen was rattled by an earthquake early on Friday morning, the strongest tremor in the region for months. The quake struck at 5.50am and, according to the national weather and seismology institute KNMI, had a preliminary magnitude of 2.8, its epicentre near the villages of Rottum and Zandeweer, in the municipality of Het Hogeland.
A strong tremor, felt across the area
The earthquake occurred at a depth of around three kilometres. Residents across a string of nearby villages, including Uithuizen, Kantens, Rottum and Middelstum, reported feeling it. In general, people described being woken by their houses shaking and creaking, followed by a loud bang and several seconds of trembling, with some reporting what felt like a heavy thud or a series of jolts.
A quake of this size is relatively unusual for the province. Since the 1980s, more than 2,000 tremors have been recorded in Groningen, but most stay below magnitude 2 and are not widely felt. Friday’s was only the 200th recorded at magnitude 2.0 or above, and the strongest since March, when a 3.0 quake struck near Assen, at the Eleveld gas field in neighbouring Drenthe, and prompted close to 2,000 damage reports.
Caused by gas extraction, not nature
The KNMI described Friday’s event as an “induced” earthquake. Unlike natural earthquakes, which are caused by the movement of tectonic plates, induced quakes here are the result of decades of natural gas extraction from the Groningen field. Pumping gas out of the ground over many years changed the pressure in the deep rock layers and caused the ground to compact, and it is the resulting stresses that shake loose as tremors.
One reason these quakes are felt so strongly, despite their relatively modest magnitude, is that they happen at shallow depth compared with natural earthquakes. That places the shaking closer to the surface, and to the homes above.
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Why the quakes will keep happening
Perhaps the most important point for the region is that the tremors are not over, even though the drilling is. Gas extraction from the Groningen field was brought to an end and the field was permanently closed, and the number of earthquakes has fallen markedly as a result. Yet quakes still occur, and are expected to continue for years to come.
The reason is that the ground does not immediately settle once extraction stops. The deep layers continue to compact and the pressure differences built up over decades keep equalising long after the last gas is pumped, a process that plays out gradually rather than switching off with the wells. As a result, the after-effects of the extraction, including earthquakes, will be felt for a long time yet.
The downward trend is nonetheless clear. Since the start of this year, the KNMI has recorded around 20 tremors in the province. Last year there were 31 induced earthquakes at the Groningen field, of which six reached magnitude 1.5 or above, the level from which they generally become noticeable. That compares with roughly 100 quakes a year between 2011 and 2019, and a record 133 in 2013. The strongest in recent times came in November last year, a 3.4 magnitude quake near Zeerijp, one of the heaviest ever recorded in the province.
A long aftermath
For the people of Groningen, then, Friday’s quake is a reminder that the closure of the gas field drew a line under the extraction, but not under its consequences. The shaking has grown less frequent, but the ground beneath the province is still settling, and the region is likely to keep experiencing tremors, and the anxiety and damage that come with them, for years to come.