“U.K. earthquakes and John Milne's contributions to the science” Dr Ian Stimpson, FGS On Wednesday, the 20th March, Dr Ian Stimpson FGS, of the School of Physical & Geographical Sciences, Keele University. talked to an audience of 36 members and guests.
Ian last visited us two years ago, just after the disastrous earthquakes in Japan and New Zealand, when he gave us a splendid lecture on them in lieu of a talk on UK quakes. This talk is an updated version of the one we missed.
The first part of Ian’s talk focussed on John Milne, a geologist who could reasonably be described as “The Father of Seismology” because his legacy is so huge. In spite of this, he is comparatively unrecognised. This year is the centenary of his death.
John Milne was raised in Rochdale, read Natural Sciences at Kings College, London (Precursor of the Royal School of Mines) and then Mineralogy at the University of Freiberg in Germany, before surveying for coal in Newfoundland. He then took up a post as Professor of Geology and Mining at Imperial College, Tokyo. Typically, being of an adventurous nature, during 1875-76, he went there the difficult way, overland, making geological notes along the way! The earthquake which occurred on his first night in Japan triggered his lifelong interest in them. By 1889 he was a Fellow of the Royal Society, a considerable achievement for a geologist.
During this period in the 1880s there was a transition from a belief in the supernatural origins of earthquakes to a scientific one and in 1893 at the Koto convention, it was mooted for the first time that faults caused earthquakes rather than being the results of them.
In the late 1800s there was no accurate recorder of seismic vibration before Milne and Gray developed the Milne-Gray seismograph which measured the three components (north-south, east-west and up-down). His invention was not confined to geology, he developed a vibration detector for steam engines which facilitated the improvement of the track.
In 1895 when his home and laboratory burnt down he ended his 20 year stay in Japan, came back and set up home with his Japanese wife at Shide on the Isle of Wight. By 1908 the Milne-Shaw seismograph was the standard instrument for a worldwide network reporting back to him at his home, establishing the foundation of modern earthquake research, a remarkable achievement as he was in receipt of no government money.
His published log of recorded earthquakes clearly maps out what we came to understood as plate boundaries some 50 years later.
He died of Bright’s disease in 1913 at the young age of 62.
The scale of magnitude of earthquakes is based on the log of the size of the biggest displacement in the seismic trace, so from one step in magnitude to another is the equivalent of an increase of 32 times the amount of energy released. Shocks go from M 9.5 as the largest, recorded in Chile, to minus 5, when you break your pencil.
Most quakes are related to plate boundaries, but intraplate quakes occur, as in the UK. The risk is small here, but we are subject to magnitude 5 quakes and most of us remember one or two local “bumps”, including the Bishops Castle one in 1990, of magnitude 5.1. Strangely, we have more M 5 quakes than M 4 ones.
The UK is subject to compression leading to movement on ancient faults. Some small shallow quakes are related to coal extraction, when the stronger sandstones which separate the coal seams break; but larger quakes tend to be triggered at depth leading to only minor damage.
The distribution of earthquakes in the UK is problematic, being concentrated along the west coast, plus the Welsh borders, with the odd small patch round Dover. Very strangely, Ireland (North and South) has almost no quakes. We get on average one M 5-6 earthquake every 8 years. Fracking leads to shocks of about M -1 to M -2, of which we get about 23 every year.
Central within the UK there is a stable, roughly triangular, Midland Craton, within a complex patchwork of ancient terranes, all separated by faults along which movements can occur when intraplate pressure builds up. The overall distribution is however difficult to explain and some geologists invoke a “Hot Mantle Anomaly” to explain the larger deeper quakes which originate some 20 km down.
Colin Humphrey wound up the meeting by thanking Ian for a remarkably interesting talk and making the point that we should see our way to commemorating the life of John Milne, considering the huge legacy we have inherited from him.
At the indoor meeting, on the 17th April, Duncan Hawley will be giving a talk on “Early Geological Maps”.