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can guess it was less likely to be to the east, as this would put it closer to the Outer
Hebrides and the mainland.
Clearly this cannot have been a large earthquake of 6 Mw or greater, as then it
would certainly have been more widely observed. However, it shows that, despite
contemporary aseismicity, the Atlantic waters west and north-west of Scotland have
produced earthquakes in the past large enough to be felt. It also shows how much of
a lottery is the preservation of historical records of such events.
5.5 23 May 1847
In the course of the 19th century, a number of anomalous tidal fluctuations in the
south-west of England were reported, some of which resemble weak tsunamis, and
which were considered to be earthquake-related at the time. These were particularly
chronicled by Richard Edmonds (Edmonds, 1846, 1856, 1860, 1869) and all have
recently been listed and reviewed by Dawson et al. (2000). Some of these may be
due to storm surges. It is curious that so many are reported from Cornwall and yet
not from south-west Ireland or from Brittany.
One such event will be considered in detail here, which is particularly signif-
icant as it was accompanied by an actual earthquake report. This is the event of
23 May 1847. On this day strange tidal fluctuations were observed along the coast
of Cornwall and Devon, as far east as Plymouth and also in the Scilly Isles. The
maximum amplitude was from 1.0 to 1.6 m (Edmonds, 1869). In the Scilly Isles
a strange noise was heard, as if underground (Falmouth Packet 5 June 1847 p8).
The previous evening a slight earthquake was felt by many people in the Penzance
district (Falmouth Packet 29 May 1847 p8).
This is another case of an offshore event where one has a choice between adopt-
ing a small near-shore solution or a larger event further away. How close to the
Scilly Isles was the epicentre? The very fact of a marine disturbance makes a small
near-shore earthquake less likely. The generally flat bathymetry of the coastal waters
around England make slumping-induced events unlikely, so either the marine distur-
bance was not seismic, or the earthquake was probably distant, and therefore fairly
large. One can hypothesise a large earthquake on the passive margin south-west of
the Celtic Sea with an elliptical felt area that reached to the Scilly Isles without
touching Ireland or France (Fig. 5).
There are still two problems. The first is the Penzance earthquake the night be-
fore, which appears must have been a local event the timing of which just before a
larger offshore earthquake was complete coincidence. This is the solution proposed
by Musson (1989). The other is the duration of the marine disturbance. According
to Edmonds (1869) the fluctuation began as early as 05 h near Penzance, reached a
peak at 17 h, and at Plymouth did not peak until between 20 h and 21 h. This duration
is too long to be credible for a tsunami. Even if, say, the time of the onset was
misreported, the fact that the disturbance continued all day with varying magnitude
and peaked late, rather than peaking with one of the first waves, is inconsistent
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