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Reports of earthquakes
Among many fascinating folders in the Somerset Studies Library is one
on local earthquakes.
On July 12, 1747 people who were sitting talking or supping in Taunton
felt their seats move under them, and those who were in bed were awakened.
In 1863 on October 6 a loud report was first heard, which could easily
have been imagined to be a terrific explosion at a distance, and many
people believed that Taunton Prison had been blown up by Fenians until
their own eyes convinced them otherwise, This was followed by a shaking
of houses, bedroom furniture, windows etc. and many people, imagining
that their houses were being ransacked by burglars, hastily slipped on
garments and searched for the intruders. The shocks were also felt at
Curry Rivel and in the Parrett Valley between Stoke-sub-Hamdon and Langport.
In Bridgwater the ships lying in the river fairly rocked to and fro, and
some portion of a large body of ice in the river near the bridge was broken
up. This was a large earthquake with an epicentre in Herefordshire, felt
over most of England and Wales. Incidentally, when was ice last seen on
the river in early October?
On January 4, 1680 of this year, about seven o'clock in the morning,
there was an earthquake at Bridgwater, Chedzoy and for some miles around.
It shook the houses pretty much, and was attended with a noise resembling
a sudden gust of wind, or as others imagined, the shock and noise was
not unlike that of some great thing thrown upon the ground.In the same
year (15 May 1201) there was a great earthquake in the town, which is
called Montacute, and for seven miles round, lasting for nearly two
hours, so that the seats in the Monks choir were seen to shake, and
the monks themselves trembled and were stupefied (Annnales Wintoni).
Mathew of Paris recorded the effects of the Wells' earthquake on
December 21, 1248:
...an earthquake occurred in England, by which (as told to the writer
of this work by the Bishop of Bath in whose diocese it occurred) the
walls of the buildings were burst asunder, the stones were torn from
their places, and gaps appeared in the ruined walls. The vaulted roof
which has been placed on the top of the church of Wells by the great
efforts of the builder, a mass of great size and weight, was hurled
so as to strike great terror into all who heard it.....
The Somerset and Dorset Notes and Queries for 1895 gives details about
a number of shocks in the Priddy, Wells, Shepton Mallet and Glastonbury
area on December 30, 1893. At Wookey it seemed as if the earth was assuming
an undulating motion, such as is observed on the waves of the sea. The
first of the shocks, for there were several, was preceded by what seemed
to be 'a terrific explosion' (Anon, 1895). In fact many accounts
of earthquakes mention what sounded like large explosions.
Davison (1896) gives an account of the Exmoor Earthquake of the following
year-January 23, 1894. Figure 3.1 shows where the effects were observed.
Davison suggested that the earthquake was associated with movement on
east-west faults.
Clearly there have been a significant number of earthquakes during historical
time in Somerset yet the record cannot compare with the number and intensity
of earthquakes in places such as Turkey or Alaska. However, we still need
to know what are the chances of an earthquake compromising the safety
of the reactors at Hinkley Point Nuclear Power Station. The Environmental
Statement produced for the public enquiry into plans for a third reactor
concluded that risks were minimal (CEGB, 1987). British Geological Survey
(BGS) seismologists at Edinburgh have been investigating historical records
of earthquakes in order to assess the degree and intensity of earthquake
activity in Britain. There is an authoritative discussion of the 1248,
1747, 1894 and other earthquakes in the region in Musson, (1994).
Regional stresses and the pattern of faults
The distribution map of earthquake foci in Figure 3.2 has been specially
prepared by the BGS Global Seismology Research Group. It is important
to note that the open circles are pre-1970 events and the black circles
represent post 1970 earthquakes whose location is based on BGS instrumental
monitoring. We may note that the latter have averaged one per year including
two near Taunton. Clearly there are stresses in the geological foundations
of S W England.
Earthquakes are shock waves resulting from the release of stresses by
sudden movement along faults. Faults in Somerset are the product of a
long history of compression and tension. The most dramatic, the N-S compression
which took place during the Variscan Orogeny, included thrusting, folding
and considerable NNW-SSE strike-slip (sideways) movement along faults
including the Cothelstone Fault. There followed N-S crustal extension
which involved E-W faulting and the deposition of Permo-Triassic, Jurassic
and Cretaceous sediments in subsiding basins. NW-SE compression during
the succeeding Tertiary Period resulted in folding, thrusting and reactivation
of Variscan faults.
The more recent episodes of compression have produced arrays of N-S/NNW-SSE
vertical cracks (joints) in the rocks which can be seen in almost every
Exmoor quarry, the Liassic cliffs and foreshore at Kilve and the quarry
workings at Ham Hill near Yeovil among many other sites.
Is it possible to associate the recorded earthquakes with known faults
and the present-day stress-field? Dollar (1963) has speculated on this
subject but Dr Roger Musson, of the British Geological Survey Global Seismology
Research Group in Edinburgh, makes the following comments (pers. comm.):
The issue of present stresses and active faults is highly contentious.
One can say that the direction of principal compressive stress is roughly
NW-SE from the opening of the Atlantic, and assuming that faults are more
likely to be reactivated in a strike-slip sense, this would mean that
faults trending N-S or E-W are more likely to be reactivated. But it only
takes a small fault to make a British earthquake, and there are plenty
of them. Given the three-dimensional uncertainty in any hypothetical location,
it is very hard, if not impossible to pin blame for any particular quake
on an individual fault, and my colleagues are always very doubtful of
any attempts to do so. I would go so far as to question whether the concept
of 'active fault' (inherited from California) is even useful
in a British setting.
Evidence for earthquake activity in the rocks
It is reasonable to assume that earthquakes must have occurred in geological
time as there is plenty of evidence that faulting took place which must
surely have disturbed sediments on the seafloor. Yet very few possible
examples are reported; this may be because geologists do not recognise
the effects of earthquakes when they see them! However on the west side
of St Audries Bay, some 3km east of Watchet (ST 103 431), one bed stands
out from the evenly-bedded strata; it shows that the sediment has been
plastically deformed into mini-buckles and highly contorted slump-folds
as shown in Figure 3.3. The tops of the folds are penetrated by shrinkage
cracks and cut by an erosion surface. The deformed sediments are composed
of calcareous mudstones and lenticles and laminations of siltstones and
very fine sandstones (Mayall, 1983). (Visitors may find the bed obscured
by rock falls.)
Mayall suggests undulating ground motion caused liquifaction in the saturated
sediment; the sudden loss of porewater would cause the sediment to lose
its strength. The erosion surface and shrinkage cracks suggest that this
episode was followed by elevation and exposure of the seabed. A bed with
similar features dated to this time has been found from east Devon to
Cheshire and this would indicate a widespread event. Elsewhere we mention
blocks of Blue Lias at Taunton Castle and Staple Fitzpaine which contain
numerous small brachiopods (Calcirynchia calcaria) scattered through the
rock and usually in an unabraded state. Presumably these creatures were
once attached to the seafloor. Did they suffer sudden upheaval and burial
as a result of earth tremors or were storms the cause?
Recent detailed mapping by the British Geological Survey, including geophysical
surveys, have shown that some sedimentary formations in the Wincanton-Shaftesbury
area vary in thickness either side of a fault thus suggesting deposition
more or less contemporaneous with movement on the fault (see Bristow et
al., 1995 and Barton et al., 1998 for examples).
The study of earthquakes leads into many areas of geological and human
concern and has recently extended to the planets. A wide range of related
topics are covered in a popular publication from the British Geological
Survey entitled Earthquakes: our trembling planet (van Rose and Musson,
1998).
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