The Case for Large Earthquakes Felt in the UK in Historical Times - Historical Seismology: Interdisciplinary Studies of Past and Recent Earthquakes - page 188

5 Case Studies

It is all too easy, in considering a subject such as this, to extend an idea too far
and start to interpret all obscure earthquake reports as possible large distant events,
when there are much more straightforward explanations that don't require large
earthquakes.
A general consideration is the difficulty of interpreting any historical offshore
earthquake from sparse macroseismic data on a single coast. In the absence of other
information, one can usually posit either a small earthquake just offshore, or a larger
one further out to sea, and there may be no constraints on how one interprets this
other than the general rule that small earthquakes are inherently more common phe-
nomena than large ones. In assigning epicentres to historical offshore earthquakes
in the UK earthquake catalogue (Musson, 1994), the standard practice was to adopt
the closest credible epicentre to shore and thus the smallest probable magnitude.
An interesting case in point is the earthquake of 4 January 1879. This was weakly
felt on the island of Unst, which is the most northerly of the Shetland Isles. This was
the only place in the UK where it was felt, but it was also felt in Norway (Kolderup,
1913) at Flesje, on the other side of the North Sea. We can thus be certain that this
was a moderately large earthquake between Shetland and Norway, most likely in
the Viking Graben, and with an estimated magnitude of around 4.8 ML. Now, there
are also a number of other reports of earthquakes felt only on Unst at around the
same period, and these are not matched with Norwegian records. Are these other
largish events in the North Sea which were less strong to the east? Or similar events
to the north or west of the Shetlands? Or small local earthquakes? It is not possible
to tell, although given that seismicity in the Viking Graben and South Møre Basin
is much greater than any other seismicity in the area, one is inclined to suspect
that these solitary reports from the Shetlands (often from lighthouses) represent
earthquakes

> 4 . 5 ML at some distance to the east and north-east of the point of

observation.

5.1 11 August 1089

This earthquake will stand for some other medieval cases. Like so many medieval
events in the UK, the available information is brief and unsatisfactory. The account
in the Anglo-Saxon Chronicle (Ingram, 1823), one of the major sources of informa-
tion for the 11th century, reads

There was also over all England much earth-shaking on the third day before the ides of
August ...

The fullest account is by the chronicler William of Malmesbury (Stevenson, 1853-
1856), who writes:

... a great earthquake terrified all England with a horrid spectacle; for all the buildings were
lifted up, and then settled again as before.