Christchurch
EARTHQUAKE RESPONSE
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Tsunami factsheet
Information about tsunamis
Why didn’t the September 2010 and February
2011 earthquakes cause tsunamis?
The September 2010 and February 2011 earthquakes
did not cause tsunamis because they happened
on land and did not displace the sea floor. The
aftershocks from these earthquakes are also too
small to cause a tsunami, even if the earthquake
happens offshore. An earthquake out at sea
usually needs to be greater than magnitude 6.5
to cause a tsunami.
Local and distant tsunamis
A tsunami caused by an earthquake at sea but
relatively close to Christchurch is called a local
tsunami. A tsunami caused by an earthquake in the
Pacific Islands or further across the Pacific Ocean is
called a distant tsunami.
Local tsunamis
What is the risk of a local tsunami?
The likelihood of a tsunami generated close to
the Christchurch coast is very low. Based on the
information we have, there are no faults large
enough to cause a significant tsunami in Pegasus
Bay.
NIWA scientists have just completed a seismic survey
of the sea floor in southern Pegasus Bay to better
understand the geology and to find out if there are
any faults we are not currently aware of. The results
of this survey will be available in the coming weeks.
Would an offshore earthquake as large as the
September one cause a tsunami?
The faults that caused the September 2010 and
February 2011 earthquakes mostly involved sideways
motion, with only a small amount of up-and-down
motion. Should another earthquake of September’s
magnitude occur offshore, we expect any resulting
tsunami would be relatively small – less than 1.5
metres at the Christchurch shore. Only beaches,
estuaries and very low lying coastal land along
Pegasus Bay and northern Banks Peninsula would
be affected, however currents would be created that
could affect boats.
What would happen in Christchurch if a tsunami
occurred in Kaikoura?
Should a tsunami occur in the Kaikoura Canyon just
south of Kaikoura, it might affect the South Kaikoura
coastline, but would be too small by the time it got to
Pegasus Bay to affect Christchurch.
What should we do if we are near the sea and
there is a strong earthquake?
It is very unlikely the earthquake will cause a tsunami
but like anywhere in New Zealand, if you are at the
beach or in a low lying coastal area and feel strong
ground shaking for more than 20 seconds, or notice a
sudden rise or drop in sea level, immediately move to
higher ground or inland from the evacuation zone for
distant tsunamis (see map overleaf).
Why did the recent Japanese tsunami cause so
much devastation?
The Japanese tsunami was caused by a massive
earthquake from a known large earthquake fault
not far off the Japanese coast. We do not have a
known fault like this off the Canterbury coast, so
a large tsunami like the one in Japan would not
happen here.
Distant tsunamis
What is the risk of a distant tsunami?
The likelihood of a distant tsunami causing damage
to Christchurch is low. Such a tsunami would most
likely originate from across the Pacific Ocean (e.g.
South America) and would take around 12-15 hours
to cross the Pacific Ocean. This would allow time to
issue warnings and evacuate coastal areas if land is
likely to be flooded.
What will happen if a distant tsunami threatens
Christchurch?
Most distant tsunamis that reach New Zealand are
too small to flood coastal land, although they can
create strong currents in water that can affect ports,
boats and people in the sea and at river mouths.
However, sometimes a distant tsunami can be big
enough to flood coastal land. If the Ministry of Civil
Defence & Emergency Management issues a tsunami
warning that includes a threat to land, the Police,
Fire Service and Civil Defence will start evacuating
Christchurch
EARTHQUAKE RESPONSE
the area indicated on the map below. They will doorknock to make sure people evacuate and will also
use vehicle sirens and public address systems to
alert people to leave. Evacuation messages will also
be provided on the radio, television and the internet.
Why were we not evacuated during the February
2010 Chilean tsunami?
Coastal areas were not evacuated during the
February 2010 Chilean tsunami because the tsunami
was not large enough to flood land in Christchurch.
However, it did create strong currents in the sea and
in the estuary, and that is why people were warned
to stay off beaches and out of the sea.
When will warning sirens be installed?
Sirens are being installed along coastal areas
of Christchurch to warn of distant tsunamis.
They were due to be operational by July 2011,
however this timeframe may be delayed by
a few months because of the February 2011
earthquake. A series of community meetings
will be held once the sirens are operational to
explain what to do if the sirens are activated.
It is very unlikely the sirens would be used to
warn of a local tsunami because the tsunami
would reach the coast before the sirens could
be activated.
Things to know and do if you need to
evacuate
• Be prepared by having a household
evacuation plan and storing important
personal items and documents where you
can quickly gather them.
• Share any evacuation messages you get
with neighbours.
• Check to see if your neighbours need
assistance evacuating.
• Secure your home and leave a telephone
book outside your door to let emergency
services know you have left.
• Move to higher ground or inland from the
areas shown on the map. You may need
to stay with friends or family outside the
evacuated area, or go to a designated
emergency welfare centre which will be
set up by Christchurch City Council.
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• Do not return home until the tsunami warning
has been cancelled and Civil Defence gives the
all clear. The first surge in a tsunami may not be
the largest and surges may continue for many
hours. The extent of any flooding will also
depend on the tides at the time the surges arrive.
More information
You can find information on the Coastal Evacuation
Plan for Christchurch at http://bit.yl/eeCTyX. The
pamphlet will soon be updated once the coastal
sirens are installed. You can also find more
information on Canterbury’s tsunami hazard in
the booklet Q Files: Tsunamis on the Environment
Canterbury website at www.ecan.govt.nz/qfiles.