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south-west of the Celtic Sea such that a large earthquake might be felt weakly in
south-west Ireland but not elsewhere. And the likelihood of finding felt reports from
south-west Ireland at this period is very low.
Furthermore, although the edge of the Celtic sea is a very low seismicity area, one
anomalous earthquake has been observed there on 17 February 1980, with a reported
magnitude of 4.5 ML (according to the agency LDG - Laboratoire de Detection et
de Geophysique, France, as reported in the bulletin of the International Seismo-
logical Centre). This seems to show that some structure in this area is capable of
reactivation. The event is too poorly recorded for a focal mechanism to be possi-
ble. Alternatively, this could possibly be an explosion from underwater disposal of
munitions, but the records seen do not suggest that this is likely.
Consequently, one cannot rule out a passive margin tsunamigenic earthquake
as impossible on seismological grounds. Whether the 1607 event
was
a tsunami
or not is another matter. The alternative is that the inundation was a storm surge.
Horsburgh (2006) has shown that contemporary weather records mention prolonged
high winds before 20 January 1607, and that other parts of England also suffered
from flooding. Tidal conditions for 20 January 1607 can also be calculated, and
they were particularly high (Horsburgh, 2006). Thus the conditions were in place
for a storm surge event, and consequently there is no need to invoke any tsunami to
explain the historical observations.
It is concluded, therefore, that there was no large passive margin earthquake on
20 January 1607.
5.4 1686
The remotest inhabited place in the British Isles used to be the island of St Kilda (it
was evacuated in the mid 20th century), 170 km west of the Scottish mainland. In
historical times the island was home to a small community of illiterate subsistence
farmers who lived almost without any contact with the outside world. Given that the
seismicity of mainland Britain is not that well known for the 17th century, it would
seem amazing that one could know anything about an earthquake in this period felt
on St Kilda, yet by strange chance this is the case.
In 1697 an estates administrator called Martin Martin, an aspiring naturalist with
a university education, travelled to St Kilda to make notes on natural history. In the
course of his excursion, he learnt from the islanders that they had felt an earthquake
in 1686 (Martin, 1716; Musson, 1998). Since Martin worked on the Outer Hebrides,
the nearest islands east of St Kilda, and seems to have been unaware of this 1686
earthquake previously, it may be taken that St Kilda was the only place where it was
felt. Thanks to the chance visit of Martin to St Kilda, and the fact that he published
what he learnt there, we know today, against all odds, that this earthquake took place.
This earthquake presents an extreme case of the coastal problem. The only report
is from a speck of an island surrounded by sea, and one cannot tell whether the
epicentre was north, south, east or west, never mind how far from shore it was. One
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