Geoscience Reference
In-Depth Information
4 Likely Effects of a Large Offshore Earthquake
We can begin by considering what might be expected to be observed and reported if
a large earthquake occurred on the NW Europe passive margin, similar in magnitude
to the 1929 Grand Banks earthquake. In Fig. 2, estimated isoseismals are plotted for
such an event, based loosely on the isoseismals of the 1929 Grand Banks event
(Smith, 1966) evened out to remove ellipticality, and with a notional epicentre near
the Geikie Escarpment on the edge of the Hebrides Shelf. Obviously, actual isoseis-
mals could vary in different ways, and these are intended only to be indicative. If
such an earthquake were to have occurred in the late modern period when commu-
nications were good, one might expect the isoseismals would be such as could be
reconstructed quite well from macroseismic data, as has been the case with other
large British earthquakes. However, the impact on the historical record in earlier
centuries could be expected to be different.
The area over which damage would be observed, within the isoseismals 6 and
7 EMS (European Macroseismic Scale), are in the remote north and north-west of
Scotland. In this area, before the 18th century, the population consisted of subsis-
tence farmers who spoke Gaelic rather than English, and were illiterate. The Gaelic
language in Scotland never had a literary tradition in the way that other Celtic lan-
guages such as Welsh and Irish did have. Communications with the rest of Britain
were very poor; there were no roads until the 18th century when the first roads into
the Scottish highlands were built for military purposes. Thus, no information could
be expected to come out of this area about the damage that occurred.
This can be illustrated by the case of the earthquake of 8 November 1608. A
plot of places from which reports exist (Fig. 3) shows a dramatic divide along the
“Highland Line” - the geological divide between the mountain country of the north
of Scotland and the lowlands of the Central Valley. South of this line there are many
reports; to the north there are none. This is nothing to do with the distribution of
earthquake effects, but entirely to do with the distribution of literacy.
Returning to Fig. 2, over the rest of the British Isles the shaking would vary
from strong (5 EMS) but not damaging in the Scottish cultural centres (Stirling,
Edinburgh, St Andrews, Glasgow) to weak (3 EMS) over most of the wealthiest
(the southern) parts of England.
Therefore, for the period before the 17th century (and one might except the later
16th century as well) the sort of report that one might expect to survive would be
along the lines of information that an earthquake was felt over a very wide area, but
without any description of effects such as damage (since there would be no notable
effects in the areas from which one might expect reports to come). One might also
find it reported that the earthquake was stronger in Scotland; however, generally
historical reporting is less good for Scotland than it is for England, partly due to the
loss of documents in the violence of the Scottish Reformation.
In fact, there are a number of earthquakes that more or less meet this description,
especially for the medieval period. Clearly, though, in the case of an earthquake
that is mentioned in chronicles as “felt throughout England” with no further detail,
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