Earthquakes

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The Earth Moves in Ireland
Mary Carter - GSI
Tom Blake - DIAS

On Friday 7th December 2012, there was an earthquake in Japan, Mag 7.3 off the east coast of Honsu. It occurred in the oceanic lithosphere of the Pacific Plates close to where it subducts beneath Japan. The shock waves travelled around the globe and the movement of the earth in Ireland was recorded on a number of seismometers. The Seismology in Schools project in Ireland is coordinated by Tom Blake, Dublin Institute for Advanced (DIAS) studies http://www.dias.ie/sis/, and Tom is also the Director of the Irish National Seismic Network (INSN), and a recording of the earthquake from this network can be seen in Figure 1. The Geological Survey of Ireland (GSI) sponsored four seismometers in 2008 as part of the Seismology in schools Program, and has a seismometer and PC on view. On January 9th – 11th DIAS staff will demonstrate the equipment in the BT Young Scientist Exhibition on the Geological Survey of Ireland stand see article on this...

 Seismogram of earthquake
Figure 1.  Seismogram of earthquake from Inch Island, Co. Donegal, Ireland, recorded by DIAS

On a global basis the Seismology in schools project is coordinated by IRIS, and below is an image of their Event Information page, showing which schools uploaded data on this earthquake (Figure 2).
Honshu earthquake
Figure 2.  View of IRIS website showing location of earthquakes and the schools that recorded it (as of 12th Dec, 2012)

Of the 15 schools which have uploaded data on this earthquake, 5 are from Ireland. The plate boundary region surrounding the December 7, 2012 earthquake hosts moderate to large earthquakes fairly regularly – 12 events of M7 or larger have occurred within 250 km of this earthquake over the past 40 years. These historic events include the M9.0 Tohoku earthquake of March 11, 2011, which ruptured a large portion of the subduction zone plate interface to the west of the December 7th event, and which spawned a major tsunami that caused significant devastation along the Honshu coast. A series of aftershocks of that 2011 megathrust event also occurred to the east of the plate boundary within the Pacific plate, including a M 7.6 normal faulting earthquake 60 km to the northeast of the December 7 2012 event. Large tsunamogenic earthquakes are not common in the North Atlantic, due mainly to the extensional nature of the Mid-Atlantic Ridge and the absence of subduction zones for the most part. Exceptions are along the Azores-Gibraltar Fracture Zone, which has both lateral and vertical sense of displacement and was the source of the Lisbon earthquake and Tsunami of 1755 (which reached Ireland) and the subduction zones of the Caribbean which caused the Haiti earthquake and Tsunami of 2011 for example.

As far as the history of earthquakes in Ireland goes, most seismicity in Ireland is very low, Magnitude 1.5 approx, but all recorded and historical earthquakes in and around Ireland are at the link below. In the absence of a Magnitude against an event on this the link, not enough stations recorded the event to make an accurate magnitude estimation to be made. Ireland is stable compared to Japan, and other places which are near plate boundaries. But overall, the earth is in constant motion.

http://www.dias.ie/irish_seismicity/