The Case for Large Earthquakes Felt in the UK in Historical Times - Historical Seismology: Interdisciplinary Studies of Past and Recent Earthquakes - page 184

and historical data shows very low activity, it is suggested that seismicity may have
been elevated in immediate post-glacial times. In fact, rare passive margin earth-
quakes are a possible factor even in periods where seismicity is not elevated as a
result of deglaciation.
The most recent of these slides is probably the Afen Slide in the Faroe-Shetland
Channel, which is not older than about 2880 years BP (Wilson et al., 2003, 2004),
putting it within range of the historical period, though of course accurate dating is
not possible. The morphology of the slide suggests that it was due to triggering by
an earthquake, possibly associated with the Victory Transfer Zone (Canals et al.,
2004; Sultan et al., 2004; Wilson et al., 2004), and an attempt has been made to
model the effects of earthquake loading on slope stability in the area (Jackson et al.,
2004).
Thus the existence of the various slides shown in Fig. 1 is evidence in favour
of large earthquakes having occurred on the passive margin at least some time in
the past.
Two other pieces of evidence can also be cited. The first is the existence of two
deformation ridges off the continental slope south-west of the Celtic Sea. These
were first shown by Masson and Parson (1983) and are also marked on Fig. 1. The
southern feature is described by Masson and Parson (1983) as a complex faulted
monocline, with crustal shortening accommodated by two zones of reverse faulting.
Superficially, they resemble coseismic ridges which can develop in cases where
faulting does not break through the surface in a fault scarp, as in the case of the
Reelfoot Scarp, which was created by the 1812 New Madrid earthquake in Ten-
nessee (Russ, 1979). They could therefore reflect large earthquakes. They are not
easy to date. Masson and Parson (1983) consider that they must be later than the
Early to Middle Eocene boundary. It has been suggested that they may be later than
this (Tate, 2005, pers. comm.) but this is unproven.
The second is the existence of an apparent sea-floor-cutting fault on the northern
flank of the Wyville-Thompson Ridge (Tate et al., 1999). This is shown on a seismic
section (Figure 3 in Tate et al., 1999) as a steep, south dipping fault cutting the sea
bed. Uplift of the Wyville-Thompson Ridge started in the late Eocene and continued
intermittently throughout the Oligocene, and possibly until Recent times (Tate et al.,
1999). This north-bounding fault is possibly the only known instance of seabed fault
displacement in UK waters, and it does rather suggest palaeoseismic evidence of a
large earthquake. Precise dating is not possible; it may be recent, but this is debat-
able. Other faults have been identified in the vicinity of the Afen Slide, with offsets
of several metres, displacing Quaternary sediments and occasionally reaching the
sea floor (Canals et al., 2004).
To sum up: it is probable, from physical evidence, that large earthquakes have
occurred on the NW European passive margin, at least since Devensian times, and
possibly more recently. The remainder of the paper will address the possibility that
such an event may actually have occurred in historical times, and that such a large
earthquake may be sitting in published earthquake catalogues without anyone hav-
ing been able to recognise it.