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.