The Case for Large Earthquakes Felt in the UK in Historical Times - Historical Seismology: Interdisciplinary Studies of Past and Recent Earthquakes - page 195

Positive identification might be achieved if evidence from Ireland could be
obtained, but the limitations of the documentary record from Ireland before the
18th century makes this unlikely. A sequence of chance events preserved a piece
of oral history that an earthquake was felt in Antrim (Northern Ireland) around
1600 by Sir Hugh Clotworthy. A woman was told that this happened before she
was born; in her old age she passed the information to someone else. Because
an earthquake was felt in Dublin in 1690, that someone (Sir Thomas Molyneux)
thought to include the information in a letter to his brother; the letter survived and
was published in a university magazine in the mid 19th century (Marsh, 1841).
There is no other information on this earthquake (unless it is a distant report of
the 23 July 1597 earthquake felt over much of the north of Scotland). It is only
by this chain of happenstance that there is any information at all, and if informa-
tion from Ireland c. 1600 survives only through luck, one cannot read much into
the absence of reports from Ireland in 1508. The story of the 1686 earthquake,
given previously, is another example of how the historical seismologist is dependent
on luck.
What are the consequences of all this? Firstly, the judgement of Ambraseys and
Jackson (1985) previously quoted is in need of revision, at least to the extent that
large magnitude earthquakes may have occurred in the British Isles in the last 1000
years - we are just not able to recognise them for what they are with the evidence
on hand. To generalise from this that an earthquake with magnitude 6Mw or

even 7 Mw is possible in mainland Britain (which would have some implications
for seismic hazard) is another matter.
If we assume that large magnitude earthquakes can affect the British Isles, but
only from the passive margin, the effect on hazard assessment for onshore struc-
tures would be very limited. In intraplate areas in general, probabilistic seismic
hazard assessment (PSHA) is not very sensitive to decisions made about maximum
magnitude, except perhaps at very long return periods. This is because even if one
allows the possibility of large earthquakes, they must be such rare events that they
contribute little to the hazard compared with more moderate-sized earthquakes that
are much more common, and may still produce high ground motion values through
scatter. If one introduces large magnitude earthquakes that are also at a considerable
distance offshore, the effect is infinitesimal.
The implications are much more significant with respect to offshore hydrocarbon
exploration, as a large earthquake could either directly damage offshore installa-
tions, or cause damaging submarine slope failure. However, this danger is already
recognised (Baltzer et al., 1998).
The other potential issue is tsunami hazard. One cannot tell how rare large passive
margin earthquakes in north-west Europe are, beyond saying that the earthquakes
themselves are rare, and that should one occur, the chances of it being also tsunami-
genic are low. Therefore the conditional probability of a tsunami occurring could be
very low indeed. Even so, given the danger to human life, and the fact that tsunami
warning systems can be fairly inexpensive if combined with other monitoring sys-
tems, it may still be advantageous to pursue such a system for the north-east Atlantic
(Kerridge, 2005).

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