Question on Irish earthquake

A question was posted to an old post on the largest earthquake near Ireland, about where it was felt.

So, here is the map to answer that question, from the British Geological Survey:

Read about the European intensity scale numbers here.

British seismotectonics

Good question posed in the comment after my last entry by Hypo-theses blogger Ian Simpson. Did St. Patrick chase the earthquakes out of Ireland? That would be a nice story indeed, but maybe we can come up with a scientific reason.

I won't claim to be an expert, but a Google search (shhh, don't tell my students) came up with a very nice PowerPoint from Dr. Brian Baptie of the Earthquake Seismology team, Earth Hazards and Systems Programme, British Geological Survey (excellent web site). The paper is on Seismo-tectonics and state of stress of the British Isles, presented at the 8^th International Symposium on Upper Rhine Graben Evolution and Neotectonics. You can find and download the presentation here.

Baptie does indeed note a strong variation in spatial distribution of seismicity throughout the United Kingdom. Earthquakes occur in a north-south band along the length of Britain. But there is an absence of seismicity in northeast Britain, the northwest Atlantic margin and Ireland (as we pointed out yesterday). The rather striking pattern is shown in the figure below.

The figure above (click on it for an enlarged version) shows historical seismicity of the UK (yellow) from 1832 to 1970 for earthquakes of magnitude above 3.0 and instrumental seismicity (red) from 1970 to present for earthquakes with magnitudes > 2.0 (from Earthquakes in the UK).

Baptie concludes that a prevailing stress field due to northwest compression is dominant, suggesting a combination of plate boundary forces plus post-glacial rebound may be controlling re-activation of old faults.

Earthquake-phobic? Go live in Ireland

According to the Irish Seismic Network:

"The largest recorded earthquake to have occurred in Ireland was a magnitude 2.3ML in Co. Leitrim in November 1994. There were no felt reports for this event.

"The largest recorded earthquake we have had offshore Ireland was a magnitude 5.4ML near the Welsh coast on 19 July 1984. This was felt mainly in Wales and adjacent parts of Britain, and over a significant area of the east coast of Ireland. There was some structural damage reported from North Wales adjacent to the epicentre of the event. More significant damage would have resulted had the event occurred at less than the estimated 20km depth."

Sounds pretty safe, earthquake-wise!

Happy St. Patrick's Day!