GeoNet and Tsunami - Part Two

Introduction

In my last tsunami blog I outlined GeoNet’s role operating the real-time tsunami gauge (sea level) network, and the use of these gauges for tsunami modelling, characterisation and warning.

GNS Science does not operate an official warning centre, but are the science advisors (using the GeoNet capability) to the Ministry of Civil Defence & Emergency Management (MCDEM), the New Zealand agency responsible for tsunami warning. International and New Zealand data are used to characterise the potential of tsunami generated by distant or regional earthquakes to threaten the New Zealand coast.

Distant and Regional Source Tsunami

Distant source tsunami take many hours to reach New Zealand allowing adequate time for warning and evacuation if required. Regional tsunami sources have travel times of between one and three hours and usually originate from the South-west Pacific region. In this case although there is less time official warnings are still possible.  For both distant and regional source tsunami New Zealand relies on the Pacific Tsunami Warning Centre (PTWC), located in Hawaii to alert us to possible tsunami threats. PTWC serves as the operational headquarters for the Pacific Tsunami Warning and Mitigation System (PTWS). The PTWS is governed by Pacific member countries of the Intergovernmental Oceanographic Commission (IOC) which is a body under the United Nations Educational, Scientific and Cultural Organization (UNESCO). In a later blog I will outline how New Zealand contributes to PTWS.

The PTWC monitors an expansive seismic and sea level network (provided by member countries of PTWS) in the Pacific and issues tsunami bulletins which are used to trigger the New Zealand response. Once a notification is received from PTWC (via a variety of communications channels) the likelihood of serious impact in New Zealand can be assessed.  A brief consultation between the GeoNet and MCDEM Duty Officers takes place and this can lead to the issuing of either a “no threat”, “potential threat” or “warning” message. While a “warning” will be issued by MCDEM as a default action if an earthquake exceeds certain thresholds, in most cases no action is required because the event is too distant or small to be a danger to New Zealand. As a first response the GeoNet Duty Officer uses the best available information on the earthquake size and location and a catalogue of tsunami forecast models to quickly estimate the likely tsunami impact in pre-defined coastal zones around New Zealand (see Figure 1). This information is provided to MCDEM as a first estimate of the likely actions required by responding agencies.

If time permits, the GeoNet Duty Officer calls a meeting of the Tsunami Experts Panel to provide a more detailed estimate of the likely impacts on New Zealand. The panel is comprised of New Zealand experts from GNS Science, the National Institute of Water and Atmospheric Research (NIWA), New Zealand universities and private organisations. Extra observations and modelling techniques are employed by the Duty Officer and members of the Tsunami Experts Panel who give continuing updates to MCDEM on the probable impacts of the tsunami. As part of this process a Science Liaison Officer is provided to the National Crisis Management Centre (NCMC, located in the Beehive basement) if the centre has been activated. This provides a seamless connection for science advice to the emergency responders.  This process of review and update continues until the threat posed to New Zealand passes.

Figure 1: The tsunami threat level map produced at the time of the March 2011 Japan Tsunami. Note that the colours used for the threat levels have changed to avoid confusion with evacuation zones. For more details refer to the Tsunami Warning and Advisory Plan (page 13) on the MCDEM website.

Local Source Tsunami

What about local source tsunami warning? Here we mean tsunami with a travel time of less than one hour to the nearest coast. The greatest local source tsunami threat to New Zealand is from the subduction zone along the East Coast of the North Island, where the Pacific and Australian plates meet. This could potentially cause a huge tsunami similar to the one that struck Japan in 2011, but unlike Japan we have very little indication that such a tsunami has ever occurred.

 New Zealand does not have a dedicated local tsunami warning capability.  While MCDEM will issue warnings in the same manner as described above in the case of a nearby large earthquake, these warnings are unlikely to be timely enough for effective response so it is important people know the natural warning signs and act on those. Examples from Indonesia, Samoa, Chile and Japan suggest that people are much more likely to survive a tsunami if they heed the natural warning signs and self-evacuate. Waiting for an official warning often means losing those vital few minutes with fatal results.

So, people in coastal areas should watch out for:

• strong earthquake shaking (hard to stand up);
• weak earthquake shaking lasting for a minute or more;
• strange sea behaviour such as the sea level suddenly rising and falling, or the sea making loud and unusual noises or roaring like a jet engine.

If any or all of these are observed – don’t wait for an official warning – let the natural signs be the warning. Take immediate action to evacuate the predetermined evacuation zones, or if they don’t exist go to high ground or go inland (both is best).

It is important to note that the hardware to provide a dedicated local tsunami early warning system, even when fully developed only provides a small part of what is required for a robust, sustainable, end-to-end local tsunami early warning capability. The warning messages need to reach the community at risk and the community must have pre-planned response procedures if effective local tsunami warning is to succeed. And this must be sustained for decades. Additionally, it is important that any warning system not undermine self-evacuation (mentioned above as so important) triggered by natural warning signs. Education is a cornerstone of a sustained tsunami risk awareness and response programme.

GeoNet and Local Tsunami Early Warning

By its nature GeoNet does have some of the tools required to provide local tsunami early warnings, including a broadband seismograph network, a tsunami gauge (sea level) network, expert staff and access to international data feeds. However, several components required for a robust local warning capability are lacking. For example, New Zealand currently has no offshore deep sea tsunami detection capability, and relies on other countries’ sensors. And further developments of the earthquake systems are required:

• Improved offshore earthquake location capability. Because of the long thin nature of New Zealand earthquake location and depth estimation accuracy drops off quickly for offshore events;
• Improved earthquake size (magnitude) estimation (using both seismic and GPS techniques);
• Fast earthquake source characterisation – is it the kind of earthquake which may cause a tsunami?;
• Tsunami (slow source) earthquake identification capability – is an earthquake of the kind that appear to be smaller but can cause large tsunami?

These capabilities are being researched or are under development but not yet available. Even with all these capabilities, I believe an effective local tsunami early warning system would require at least some offshore deep ocean sensors off the East Coast of the North Island. This would provide good capability for that region (the most destructive of the possible local sources), with capabilities in other regions mainly limited to warnings based only on earthquake size, depth and location. A further requirement of an effective local tsunami early warning system is a fully staffed 24/7 operations centre. GeoNet Duty Officers are currently “on-call” and can respond from home or work, but are not full time in the role. Automation can be employed as much as possible, but with current and envisaged levels of technology all countries attempting local tsunami early warning have 24/7 staffed operations centres. 

The bottom line is that GeoNet could play a small but significant part in the national effort to establish a fully operational and effective local tsunami warning capability. But an extra zero would need to be added to the GeoNet budget if this were to become a reality, and a coordinated effort by many New Zealand organisations would be required.