Wednesday 21 February 2018

Land below to go on trembling and shaking

n The answer to earthquakes in our region lies in the last ice age.
n The answer to earthquakes in our region lies in the last ice age.

While we think of the crust of the Earth beneath our feet as being solid it often trembles, shakes and quakes as it relieves itself of the ever-changing stresses that are stored elastically within its rocky skin.

Fortunately for us, big earthquakes are rare events. The last big Irish event was the earthquake that occurred under the Llyn Peninsula in north-west Wales on the morning of 19 July 1984. That quake measured 5.4 on the Richter scale and tremors from it were felt by people all along the east coast of Ireland.

The energy released during an earthquake event is quantified by a number on the magnitude scale devised by Charles Richter in California in 1935. Little tremors go largely unnoticed. Anything above 2.5 on Richter's scale is regarded as significant and there have been five such earthquakes in the Irish Sea area so far this year.

So why is the ground beneath us quaking? The answer lies in the last ice age that peaked about 22,000 years ago. In parts of south-west Ireland the ice was only about 100m thick but where the ice dome reached its maximum thickness off the north-east coast of Scotland the ice was well over 1km thick.

Sea level dropped dramatically to provide the water to make all that ice. Water is heavy so when sea level dropped part of the tremendous pressure on the sea floor also dropped. However, the weight of ice on the land introduced new pressures. It all happened over tens of thousands of years so the underlying rocks had time to slowly adjust to the new stresses and strains.

All of the ice melted relatively quickly some 11,600 years ago. Pressures on the land were relieved but pressures on the sea bed increased as sea level rose again. It happened so relatively quickly that the underlying rocks are still recovering. As pent-up stresses are relieved the ground shakes and we experience earthquakes.

As Ireland bobs up after being pressed down by the weight of ice the rebound is uneven: the island is tilting along an axis running from Donegal to Wexford with the coast of south Kerry sinking at the rate of some 5cm per century and north Antrim coast rising by some 10cm per century.

Experts in these matters reckon that our green and pleasant land will go on trembling, shaking and quaking for another 10,000 years before it reaches some kind of equilibrium and is still.

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