The Case for Large Earthquakes Felt in the UK in Historical Times - Historical Seismology: Interdisciplinary Studies of Past and Recent Earthquakes - page 180

1 Introduction

The issue of maximum credible magnitude, important in seismic hazard analysis,
has always been a difficult subject to handle in intraplate areas, where the seismic
cycle is longer than the historical record. In this paper the issue of the possibility of
large earthquakes in the UK is examined, with particular emphasis on the possibility
that such a large earthquake may actually have occurred in historical times without
being recognised from the earthquake record.
In considering this topic in the general sense, the occurrence of large ( 7Mw)

earthquakes in quiet seismic areas (such as the 1989 Tennant's Creek, Australia
earthquake or the 1993 Latur, India earthquake, to name but two) naturally raises
speculation as to whether such events can occur in other low seismicity areas. The
estimation of maximum magnitude cannot proceed on the basis of fault length, since
the existence of ancient major faults (like the Great Glen Fault in Scotland) does not
mean that these faults could reactivate along their whole length in a major earth-
quake in present day tectonic circumstances, quite different from those of the time
they formed. One approach is through palaeoseimic investigation (e.g. (Camelbeeck
and Meghraoui, 1996, 1998), (Camelbeeck et al., 2001)) though this can be difficult
in glaciated areas. Attempts at palaeoseismic studies in Scotland (Davenport and
Ringrose, 1985; Fenton and Ringrose, 1992) have been considered unconvincing by
Stewart et al. (2001).
Examining the historical record for the British Isles, Ambraseys and Jackson
(1985) write:

∼

The seismicity of the UK is clearly different from that of eastern USA or W Africa in that
either (i) no earthquakes of M 0 occur, or (ii) 700 years is not long enough to reveal

such events in the UK, whereas 100 years is more than adequate in the eastern USA and W
Africa.

≥ 6 .

... Is there anywhere on the continents seismically quieter than the UK?

In some respects the comparison is unfair, since the eastern USA is over four times
as large as the UK, and western Africa is larger still. One could easily find parts of
the eastern USA or western Africa of equivalent size to the UK that have comparable
or lower levels of seismic activity.
Nevertheless, the lack of even moderately large earthquakes with onshore epi-
centres in Britain is rather striking. Taking the period 1570-2005 one can distin-
guish eighteen events of magnitude 5.0-5.4 ML in Britain with epicentres onshore
or just offshore to the west (i.e. excluding events in the North Sea and English
Channel). The number of events larger than 5.4 ML in the same area is zero. The
Gutenberg-Richter b value that one would derive from earthquakes 0ML in

this area (admittedly eighteen is a very small sample) is 1.8. However, in the same
period, there are four earthquakes

≥ 5 .

4 ML in the English Channel and North Sea
(south of 60 ◦ N). If one assumes that there is no difference between Great Britain
and the offshore extension to the south and east as regards producing earthquakes
larger than 5.4 ML, then the probability of all four larger events occurring in the
offshore area by chance is about 0.015.

> 5 .