Mayo earthquake
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The Mayo earthquake of June 2012
Dr Brian McConnell

An earthquake of magnitude 4.0 occurred 60km off the Mayo coast at a depth of 3km shortly before 9 o’clock on Wednesday, 6 June 2012. Tremors were felt in counties Mayo, Sligo and Galway.

The networks of seismometers operated by the British Geological Survey and the Dublin Institute for Advanced Studies calculated the epicentre to be 54.17 -10.86 and 54.151 -10.904 respectively (see ref. 1, 2). This location lies slightly eastward of the mapped trace (ref. 3) of the main normal fault bounding the Slyne Basin. The Corrib gas field lies approximately 20 km to the north west of the calculated epicentre, within the middle of the basin. The well boreholes have no connection to the basin-bounding faults.

The Slyne-Erris-Rockall basin system has been created through progressive down-to-the-west subsidence since Triassic times (at least 200 million years). Basin formation is related to crustal extension that led to opening of the Atlantic Ocean, which continues today. While seismicity of this magnitude has not been recorded along the fault system in this area since instrumental records began in 1970, the region is still subject to far-field stresses from extension on the mid-Atlantic ridge (and compression in the Mediterranean as well). Thus, while the location and magnitude of this event were unexpected, there is a clear geological reason for it. Future seismic activity along the fault system may be anticipated, but not of significantly larger magnitude. The magnitude was far below the threshold for generation of a seismogenic tsunami.

While the location and magnitude of this event are unusual, smaller earthquakes are quite common around Ireland, although they generally cluster in northern Donegal and in the Irish Sea between Wexford and North Wales.

Mayo earthquake
The Mayo earthquake as recorded on the Irish National Seismic Network