The Irish National Seismic Network (INSN) has said that two earthquakes that were recorded on Sunday 25th August in the Irish Sea in a region off the northwest coast of England were most likely the result of glacial rebound, the process whereby stresses built up the weight of glaciers from the last Ice Age are slowly released.

A magnitude 2.2 earthquake struck at 5:37AM UTC approximately 23km from Fleetwood, 25km from Blackpool, 60km from Liverpool, 74km from Ramsey, 172km from Belfast and 185km from Dublin. A much stronger magnitude 3.2 earthquake struck the same location at 9:58AM UTC at a depth of 8km, and was felt on the British mainland in Fleetwood, Blackpool and Thornton-Cleveleys. The larger earthquake was also recorded by INSN seismometers as far away as Donegal and Wexford in Ireland. The INSN, which monitors seismic activity in the region, says the magnitude 3.2 earthquake was the strongest quake recorded in Britain or Ireland since 29th May 2013 when a magnitude 3.8 earthquake struck off the coast of the Llŷn Peninsula in North Wales.

INSN Director Tom Blake indicated that only residents closest to the epicentre of today’s earthquakes are likely to have felt any shaking. He added that further tremors were possible during the coming days. Mr. Blake from the School of Cosmic Physics in the Dublin Institute for Advanced Studies (DIAS) explained: “This part of the Irish Sea has not seen any significant seismic activity in recent years but ultimately their cause is likely to be no different than other earthquakes in Britain and Ireland. The last earthquake recorded in this part of the Irish Sea occurred in 1843 and is estimated to have been a magnitude 4 quake.” He continued: “Although Britain and Ireland are far from any plate boundaries, much of the Region is still experiencing quakes due to the removal of the weight of ice sheets that once covered the land. Occasionally this post-glacial isostatic rebound – the phenomenon of the land surface gradually returning to its pre-glacial contours – results in earthquakes of this magnitude, particularly in the northern half of the islands.” Mr. Blake said: “it is impossible to tell if stronger earthquakes will occur in the coming days and weeks, but aftershocks can be expected even if most if not all will be too weak to be felt.”

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Other earthquakes have been recorded in the Irish Sea and along western parts of Britain in the past 6 weeks. They include quakes at Llanrwst, Conwy in North Wales on July 15^th (mag 1.4), Irish Sea off Anglesey on 29^th July (mag 1.2), Islay, Argyll/Bute on 31^st July (mag 1.4), Knutsford, Cheshire on 11^th August (mag 2.1) and the Isle of Arran on 12^th August (mag 1.3).  The largest known British earthquake occurred near the Dogger Bank in 1931, with a magnitude of 6.1. The largest earthquake to impact Ireland occurred on the Llŷn peninsula on 19 July 1984. The 5.4 magnitude earthquake was the largest ever recorded earthquake on mainland Britain and was felt throughout Ireland’s east coast, Wales and England. Aftershocks from the quake measured up to 4.3 on the Richter scale and some structural damage resulted along the east coast of Ireland.  A magnitude 2.7 earthquake was recorded off the northwest coast of Ireland on November 21st 2012. On June 6th 2012, a magnitude 4.0 earthquake was recorded off the coast of County Mayo, close to the Corrib gas field.  The quake happened at 9am, 60km west of Aughleam near Belmullet.  The most significant land earthquake to be recorded in Ireland in recent years occurred when a 2.7 quake hit Lisdoonvarna in County Clare in May 2010.   It also was the first earthquake to be recorded in the west of Ireland in modern times.

The DIAS began modern seismic recordings in 1978. The INSN now features 6 permanent stations. 2013 marks 20 years since the beginning of digital seismic recording of the INSN.