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Before 1570 it is very difficult to estimate magnitudes. Thus some medieval
earthquakes such as 20 February 1247 and 11 September 1275 were most likely
over 5 ML, but it would stretching slender evidence to attempt to categorise whether
they were larger than 5.4 ML (Musson, 1994).
On the available information, therefore, it does appear that there is a significant
lack of earthquakes of 5.5 ML and over in mainland UK in the last 400 years (this is
about 5.2 Mw). The purpose of this paper is to explore the possibility that there may
actually be much larger earthquakes in the historical record of the UK, but which are
hard or impossible to recognise because they have offshore epicentres. The reader is
warned in advance that much of this paper is highly speculative. Given the difficulty
of the subject matter this is unfortunately inevitable.
2 Passive Margin Seismicity
The first comprehensive study of intraplate seismicity worldwide, with a view to
distinguishing areas most at risk from unexpectedly large events, was made by Sykes
(1978). After a lengthy review of global intraplate seismicity, he concluded that a
particular locus of large-magnitude intraplate events was on the passive margins of
continents. He particularly considered that the intersection of passive margins and
oceanic transform fracture zones marked locations of concern. He argued that such
fracture zones initiated at the earliest stages of oceanic rifting at the sites of pre-
existing weaknesses in the continental crust, and that these weaknesses continued to
act as possible earthquake sources even when oceanic rifting was well advanced.
A subsequent major study of worldwide intraplate seismicity was commissioned
by the Electric Power Research Institute in the US, which confirmed Sykes's find-
ings that passive margins are significant, but without the emphasis on the intersec-
tion with oceanic fracture zones (Johnston et al., 1994). In a summary in Johnston
(1989), it is noted that
Of [the] eight largest SCI [Stable Continental Interior] earthquakes, seven occur at passive
margins or in extended crust resulting from margin formation.
A classic example is the 18 November 1929 Grand Banks earthquake (7.2 Mw).
This occurred off the coast of Newfoundland in an area with no previous or subse-
quent seismicity barring aftershocks of the 1929 earthquake itself. The earthquake
triggered a large submarine slide on the continental slope, which in turn produced a
damaging tsunami which killed 28 (Smith, 1966; Stewart, 1979; Fine et al., 2005).
It was also widely felt over the onshore area (Smith, 1966). Sykes (1978) notes that
the epicentre is near the intersection of the passive margin with the Newfoundland
Fracture Zone.
This raises two questions with respect to north-west Europe: could an earthquake
similar to the 1929 Grand Banks earthquake occur on the north-west European con-
tinental margin? And is there any evidence that such a thing might have occurred
already? The question is particularly pertinent given recent concerns about whether
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