How Mallet hit on a seismic shift

SO, at long last, the rogue Gulf of Mexico oil-well has been capped.

1816 was ‘the year of no summer’, 1978 was ‘the year of three popes’ and the ‘9/11’ atrocity defined 2001. Will 2010 be ‘the year of the spillage’? Closer to home, the saga of the Mayo gas pipeline continued but, on the positive side, June 3, saw the 200th anniversary of an Irish scientist whose work proved crucial to oil and gas exploration.

Robert Mallet, the son of a factory owner, was born near Capel Street in Dublin in 1810. After studying engineering he joined the family’s foundry business. The ironwork of the Fastnet Rock lighthouse and the railings around Trinity College have R & J Mallet stamped on them. The firm also produced a giant mortar, which fired a one tonne shell a distance of two and a half kilometres. It was intended for use in the Crimea, but the war there ended before this weapon of mass destruction could be deployed.

But Mallet’s most outstanding contribution was in earth science. In October 1849 he and his son buried kegs of gunpowder in the sand at Killiney Beach, south of Dublin. Charges were detonated and the effect measured half a mile away. It was a crude experiment, but it proved that shock waves travel through sand. For comparison, the energy of waves passing through rock was recorded by an instrument on Dalkey island. Setting off explosions and examining the waves reflected from layers under the ground would, in time, enable deposits of oil and gas to be located. The technique is now used in oil exploration.

The famous experiment was recreated for the BBC television programme Coast, which celebrated Mallet’s contribution to seismology. Seismos is Greek for ‘earthquake’ but the field also covers volcanoes and tsunamis. Early seismometers consisted of a heavy pendulum with a pen attached. When the ground shook the pen scratched out a trace on a soot-covered glass plate. In modern instruments earth movements trigger remotely-controlled electronic sensors, producing graphs on slowly rotating tapes. With seismometers positioned on bedrock at strategic points, the source, or ‘epicentre’, of an earthquake can be determined. Temperature variations, pressure changes, man-made vibrations and noise complicate matters; processing seismic data is a job for the computer.

We don’t worry too much about earthquakes in Ireland, although, if the Annals of Ulster are to be believed, 1,000 people were killed by one in 824.

Following the quake which struck Lisbon on the 1st November 1755, killing tens of thousands of people, three-metre high waves were recorded along the Cork coast.

The strongest local tremor of recent years occurred in 1984. With an epicentre near Anglesey, it registered 5.4 on the Richter scale, about one ten-thousandth the strength of the Lisbon quake.

So the ground beneath our feet is not as solid as it seems.

According to Thomas Blake of the Dublin Institute for Advanced Studies, an ‘earthquake swarm’ was recorded in Donegal last January. A tremor of 1.7 on the Richter Scale occurred on January 7th, followed by a 1.6 shock on the 26th and a 1.7 on 27th.

Such rumblings are not that unusual for Donegal, but three events in a row is slightly odd. Ireland is nowhere near a major fault, so what is going on? Perhaps the land is still stretching its limbs after the long imprisonment of the Ice Age.

The country then was covered in ice several kilometres thick. When this melted, an enormous weight was lifted. The compressed ground, released like a coiled spring, expanded.

Levels rose as the tension eased. The expansion, though much diminished, is still going on and energy locked up in the ground for millennia is being redistributed.

The Donegal quakes were detected on a seismometer at St Egney’s Primary School, Desertegney, one of 50 taking part in the Seismology in Schools Programme, organised by the Institute. Working with schools in Britain and the United States, Irish pupils are helping to locate tremors taking place all over the world.


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