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Earthquake House, Comrie

Historically, the Comrie area of Scotland has experienced fairly frequent earthquakes by UK standards.

James Melville recorded in his diary an earthquake felt across Perthshire in July 1597. The first recorded Comrie shock was in 1788. The Rev.s Taylor & Gilfillan documented the strange movements, noises and tremors around the end of the 18thC.

With the 'Great Earthquake of 1839' The Comrie Pioneers, postmaster Peter Macfarlane and shoemaker James Drummond, began to keep records.

In 1841, a Committee for the Investigation of Scottish and Irish Earthquakes was formed - the term seismometer was first used, they discovered how to evaluate the Epicentre (a point on the Earth's surface where the shock is greatest) and Prof J.D.Forbes designed the first seismometer using an inverted pendulum writing onto a concave disk above.

Over the years to 1844 a number of instruments were in use around Comrie. Things quietened so interest was lost.

With renewed activity the Committee began work again in 1869. New and more sensitive instruments were sought. In 1874 the Earthquake House was built on solid rock to hold the Mallet seismometer.

This consisted of two boxwood planks N-S & E-W. Onto these, boxwood cylinders of increasing width and therefore stability are placed. With a shock cylinders fall over up to a certain width and this allows a measurement of the strength of the earthquake. Sand around the planks 'catches' the cylinders and prevents them from rolling and knocking down others. Simple but very effective.

Technology became more sophisticated with time. From 1911 the building had become redundant and fell into disrepair.

In 1988 it was decided that Earthquake House be restored and modern equipment was supplied by the British Geological Survey. Large windows have been fitted so the visitor can observe both the old and new seismometers installed.

Fortunately the damage done by Comrie quakes has been minor. But why does this area experience them?

Up until the last century, scientists were unsure about the source of earthquakes. Were they geological or meteological?

Pioneer James Drummond believed they were the explosions of natural gas underground. As evidence accumulated, it became clear that they were the result of movement of great fractures in the Earth's crust called Faults. There is resistance so the two sides move in infrequent jolts - releasing built up energy as earthquakes.

In fact it was not until the formulation of the theories of Plate Tectonics in the 1950s & 60s that a full explanation was offered. Great crustal plates move across the Earth's surface floating on the semi-molten layer below. Where these 'scrape and grind' together the great earthquake zones - of say Japan and California - occur.

But what about Comrie? The great Highland Boundary Fault which separates the Highlands from the Lowlands lies just outside Comrie. This fault was highly active 400Myears ago when Strathearn like much of Britain was a hot dry desert. At that time the Fault must have been driven by global plate movements. Are the quakes just some minor settling of this once great fault? Possibly.

The epicentres lie just north of Comrie while the movement is actually about 15km down. The HBF lies on the surface just to the south of the town. The HBF is actually very complex and may slope northward with depth. Alternatively another unknown fault may exist at depth and be causing the shocks. This theory is supported by many geologists on the basis that the HBF is inactive elsewhere.

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