Sunday, October 14, 2012

GeoNet and Tsunami - Part One

One of GeoNet’s roles is as science advisers to the Ministry of Civil Defence & Emergency Management (MCDEM) on tsunami response. Currently this is mainly confined to regional and distant source tsunami caused by earthquakes. So how do we carry out our role?

There are three major aspects of the role – data and information, expert advice and warning systems and international engagement. I will outline each of these in turn – in this blog I will just concentrate on the tsunami (sea level) gauge network.

As a part of GeoNet we operated a tsunami gauge network of 17 sites around New Zealand and on offshore islands. These sites have twin pressure sensors in the ocean to record sea height change. The network is operated in partnership with Land Information New Zealand (LINZ) with the GeoNet Earthquake Commission (EQC) contribution supporting the data communications and processing. All the data is made available to the international data centres, particularly the Pacific Tsunami Warning Centre, (PTWC) in Hawaii, as well as being available from the GeoNet website.

A question we are often asked is: do these sites provide warning? And the answer (I am a scientist after all!) is yes and no. Yes, the gauges on offshore islands will provide an hour or two warning of a tsunami “surges” heading for mainland New Zealand, and ones on the mainland coast will provide some warning for other parts of New Zealand. But a gauge very close to you will be no help to you for warning. Tsunami warning is very international so we rely on information from other countries gauges, and other countries rely on our gauges – particularly our cousins across the ditch (for non-Australians or New Zealanders, that is the Tasman Sea) who may be threatened by a large earthquake at the bottom of New Zealand’s South Island.

Another really important use for these tsunami gauges is the calibration of tsunami forecast models. Since the Indian Ocean Tsunami on Boxing Day 2004 there has been huge progress with models that forecast the likely impacts from earthquake caused tsunami once accurate information earthquake is available. This is particularly true for ocean basin wide tsunami, where the tsunami waves may travel for many hours before being a threat on a distant shore. If the likely impacts can be forecast in advance then effective evacuation is possible without the economic losses of over evacuation or the issues caused if people are asked too often to evacuate but no tsunami occurs (the “cry wolf” affect).

Figure 1: Recordings of the Japanese Tsunami around the New Zealand Coast on the LINZ GeoNet Tsunami Gauge sites. Note the largest surges at Gisborne and the Chatham Islands are four to six hours after the first arrivals.
Recent Pacific Ocean basin wide tsunami have provide a rich data source for refining tsunami models. For example, the Japanese tsunami of 11 March 2011 was well recorded around the New Zealand coast (see Figure 1) with surge heights of around a metre in places. These measured values were very close to the forecast levels and increase our confidence that we can effectively warn New Zealand communities without closing the whole New Zealand coast. Figure 1 also demonstrates a really important observation – tsunami surges from distant tsunami just keep coming. In Gisborne and the Chatham Islands the largest surges occurred many hours after the first arrivals. If you cannot see the shark any more it may be safe to go back in the water (see Jaws if you don't understand the reference), but with tsunami take extra care for many hours after you see the first rise and fall of the sea.

In my next tsunami blog I will go into more detail about how we use the data and information and expert advice to advise on likely tsunami impacts.

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