9
3 Sources of relevance to the North Sea
Before considering the establishment of a tsunami warning service for the German North Sea
coast, possible causes of tsunami that may affect the North Sea have to be studied and
evaluated.
The following is a collection of possible causes of tsunami events, i.e. volcanic eruptions,
submarine slides, and meteoritic impacts, under the aspect of their potential occurrence in
the North Atlantic Ocean and Arctic Ocean.
In the past, the most devastating destruction has been caused by tsunamigenic events close
to the coast, e.g. the Lisbon tsunami in 1755. The UK reports (Kerridge 2005, Smallman
2006) also discusses simulations of tsunami events caused by North Sea earthquakes.
However, in the present report, it is assumed that tsunami affecting the North Sea are
generated outside the North Sea. An exception might be tsunami caused by meteoritic
impacts.
3.1 Earthquakes
Submarine earthquakes are the best-known causes of tsunami events. However, most
submarine earthquakes do not result in tsunami.
An earthquake in the subduction zone between tectonic plates and near transform faults (Fig.
3.1.1) will only cause a tsunami if it generates a vertical displacement of the seabed. Such
displacements are rare near transform faults, where the plates slide past each other. The
only subduction zone in the North Atlantic Ocean is the Caribbean Arc system, and the
longest transform fault in this region extends from the Azores into the Mediterranean Sea
(see Fig. 3.1.2).
Fig. 3.1.1: Types of plate boundaries (Press et al. 1995, Fig. 1.14)
The earthquake magnitude it takes to produce a significant elevation of the water level
depends on the depth of the earthquake epicentre. Strong earthquakes and tsunami felt as
far as the North Sea are rare events. However, the Lisbon earthquake of 1755, which had a
Richter magnitude of 8.5-9, caused a devastating tsunami. The epicentre of the earthquake
was located on the shelf, close to the boundary between the Azores and Gibraltar plates.
About 40-60 minutes after the earthquake, the water retreated from the coast and returned a
few minutes later as a 15 m wave. More waves followed. The tsunami also propagated along
the western European coasts. About 4 hours later, a 3-4 m high wave with a period of IQ-20
minutes travelled through the English Channel towards the North Sea. Besides the
tsunami caused by vertical water displacement, the seismic waves of this earthquake
triggered seiches in Scottish, Swedish, and Finnish lakes (Bryant 2001). Model hazard
assessments (Smallman 2006) give extreme sea levels around the Cornish coast from 3.4 to
5.2 m.
In the Caribbean region, the Caribbean plate is moving toward the North American plate.
Tsunami are frequent in this region (Grundlay et al. 2005), but their effect on northern Europe
is minor.
