10 
Earthquakes at other plate boundaries hardly ever trigger tsunami that reach the North 
Atlantic Ocean. However, the tsunami of December 2004 was also recorded on the 
European coasts. Model simulations showed that it reached the continental shelf off Ireland 
after about 30 hours (Geist et al. 2006). 
The reported periods of 10-20 minutes are characteristic of earthquake-induced tsunami, but 
also periods of more than one hour are not unusual (McNeil 2001). 
Earthquakes have also occurred in the North Sea itself. Huge waves were reported in the 
Channel at the time of the 1580 earthquake. The authors of the British report (Kerridge 2005) 
concluded, however, that most likely the 1580 earthquake did not cause a tsunami, and that 
the huge waves observed had other sources. 
Fig. 3.1.2: Plate boundaries (blue: convergent, red: divergent, black: transform fault) and 
active volcanoes (Press etal. 1995, Fig. 5.27) 
3.2 Volcanic eruptions 
Volcanic eruptions may cause tsunami by different mechanisms (cf. Table 7.1, Bryant 2001, 
Latter 1981). 
A tsunami can be triggered by seismic tremors accompanying an eruption. 
Volcanic eruptions may lead to pyroclastic flows. Most of these flows consist of two 
parts: a mixture of hot rock and gas that moves along the ground and a cloud of hot 
ash that rises above it. Both parts can result in a tsunami when they reach an ocean 
surface. In the past, hot ash having a higher density than sea water has caused large 
tsunami remote from the eruption site due to large masses of ash sinking to the 
seabed and flowing along the bottom. 
Underwater volcanic eruptions in the upper 500 m of the water column result in 
tsunami due to pressure fluctuations. Steam explosions followed by huge tsunami 
occur when water comes in contact with hot magma. 
The collapse of a vulcano summit may form a caldera. Water rushing into such 
calderas has been the cause of historical tsunami. 
During slope failures on high, steep volcanoes, the avalanching slope material 
reaches very high flow speeds before it enters the water. The resulting tsunami may 
be extremely high. 
Lateral eruptions may cause major debris flows into the water. 
Also mixtures of ash and water from crater lakes or ash and glacier ice from the crest 
of the volcano (lahars) have triggered high local tsunami. 
Another potential cause is lava flows into the ocean.