The Case for Large Earthquakes Felt in the UK in Historical Times - Historical Seismology: Interdisciplinary Studies of Past and Recent Earthquakes - page 182

tsunami warning systems should be implemented in the North Atlantic in the wake
of the 26 December 2004 Sumatran earthquake (Kerridge, 2005).
3 Geological Evidence for Large Passive Margin Earthquakes
Near Britain
That significant earthquakes have happened on the north-west European margin at
some time in the past has already been mooted in the literature with regard to the
triggering of submarine slope failures (Paul and Jobson, 1987; Baltzer et al., 1998;
Holmes et al., 1998; Jackson et al., 1999). Submarine landslides are common fea-
tures along passive margins (Embley and Jacobi, 1977; Embley, 1982; Mienert et al.,
2003; Canals et al., 2004; Huhnerbach et al., 2004) and at least fifteen large slides
or mass flows can identified along the NW European margin from 50 ◦ Nto70 ◦ N,
including the massive Storegga slide (Bugge et al., 1987; Mienert and Weaver, 2003)
as shown in Fig. 1. That large submarine slides may be due to earthquake triggering
is well known (Hampton et al., 1996); the case of 1929 Grand Banks has already
been mentioned, and some other cases are reviewed by Embley (1982). In the case of
Europe, the relationship between seismicity and slides is ambiguous; it is suggested
by Mienert et al. (2003) that earthquake triggering may not be the most important
reason for slides. One other mechanism that has been suggested, for instance, is
that slides may be triggered by escape of gas hydrates (Bugge et al., 1987; Vogt
and Jung, 2002). Recent studies, though, incline towards viewing earthquakes as
the most likely external triggering factor (Canals et al., 2004; Sultan et al., 2004).
Holmes et al. (1998) consider that earthquakes may have triggered slide movements
on the Barra Fan, and Baltzer et al. (1998) conclude that at least one earthquake
(probably just before or during the early Devensian, i.e. around 70,000 years BP)
was responsible for triggering the debris flows of the Sula Sgeir Fan. Strachan and
Evans (1991) consider that earthquake triggering was the most likely explanation
for the sediment failure below the Geikie escarpment. (These locations are close
together; only the Geikie Escarpment is named on Fig. 1.)
The discussions of seismicity in these references unsurprisingly make most ref-
erence to known modern seismicity in the area, which is limited and of small
magnitude. The existence of a few events like the 13 April 1980 Hebrides Terrace
Seamount (4.0 mb) shows that the area is not entirely aseismic (Jacob et al., 1983).
Paul and Jobson (1987) estimate that an earthquake of magnitude 6 could have
caused failure on the Hebrides slope. Baltzer et al. (1998) then go on to say that
a magnitude 6 might have taken place during times of major isostatic adjustment
(i.e. the early Devensian). This is no doubt a sound argument; but by analogy with
other passive margins, and given a long period of observation, it is not actually
necessary to invoke greater isostatic adjustment in order to allow for the possibil-
ity of a large earthquake. The same is true of some the examples discussed in the
COSTA Project (Canals et al., 2004); seismicity is considered to be strongly indi-
cated as a causative factor in several examples, and where examination of modern