Your best warnings for a tsunami are:
- a long rolling-motion earthquake that’s longer than a minute,
- a strong earthquake that makes it hard to stand up, or
- unusual ocean behaviour: loud or strange noises coming from the sea (e.g. like a jet plane or train) or sudden sea level changes.
You need to evacuate the red and the orange evacuation zones if you experience or observe these warning signs.
It is unlikely that a tsunami has been created, but it is better to be safe than sorry. Tsunami waves come in series. The first wave may not be the biggest, so you need to be prepared to stay out of the evacuation zones for many hours. If a tsunami is coming from close to our shore, there will not be any time to issue an official warning, sound sirens or to send an emergency mobile alert. The earthquake is your best warning.
An official tsunami warning can be issued for a tsunami coming from outside the Canterbury region or from across the Pacific Ocean. You will hear Civil Defence official tsunami warnings on the radio, on television, on social media, or via an Emergency Mobile Alert, telling you which zones to evacuate.
What is a long or strong earthquake?
A long earthquake is an earthquake that shakes for longer than one minute.
The bigger the earthquake magnitude, the longer the shaking. So if the earthquake lasts for longer than a minute, you know that it is big, somewhere, probably over magnitude 7.5 which is big enough to possibly create a damaging tsunami if it was under the ocean.
A strong earthquake is when the shaking is so strong that it is hard to stand up – the sort of earthquake where furniture starts moving around and things fall over.
Why the long or strong message?
If you are near a big earthquake, the shaking will be long and strong. A big earthquake further away from you may be felt as a long but mild or moderate, rolling earthquake.
A big earthquake far away may still have created a tsunami, it will just take longer to reach you than if it was just offshore. So this is why the message is long or strong, rather than long and strong.
Sirens and when to return home
The sirens will be activated for a tsunami with more than three hours of warning time, and if time allows, the sirens may be sounded for a tsunami with one to three hours of warning time.
Sirens installed along the Christchurch coastline are intended to warn you about a tsunami generated in the Pacific Islands or across the Pacific Ocean, and if there is time, for a tsunami generated further afield from Canterbury i.e. in the North Island's east coast.
Sirens are not intended to warn you about a tsunami created close to our shore. A long or strong earthquake will be your only warning of a tsunami created close to our shore.
Safe to return, 'all clear' messages will be broadcasted by Civil Defence using radio, TV, radio, Council's Newsline(external link), and social media.
You can also do your bit by sharing official warnings with friends and family in the evacuation zones.
Emergency Mobile Alerts
Emergency Mobile Alerts are messages broadcasted by authorised agencies to keep people safe. The alerts can be targeted to areas affected by serious hazards, including tsunami. If your phone is on, capable and inside the targeted location, you should get the alerts.
It is important that you still evacuate after a long or strong earthquake and not wait. Emergency Mobile Alerts are just one way of finding out about serious threats, so ensure you have an emergency plan and know where to find more information during an emergency.
Emergency Mobile Alerts will only be used for serious hazards that involve threats to life, health or property, or in some cases for test purposes. If your phone is on, capable and inside the targeted location, you should get the alerts. You don’t have to download an app or subscribe to a service, just ensure your phone is capable and updated.