A tsunami hit New Zealand on June 23, 2001. Gauges in the Chatham Islands detected a sudden 55 cm rise in sea level, and several hours later mainland New Zealand recorders noted a 30 cm rise.
Warnings had been transmitted from the Pacific Tsunami Warning Centre in Hawaii, so New Zealand seismologists knew the wave was coming and were prepared.
The tsunami was triggered by a distant earthquake near Arequipa, on the coast of Peru. It was plate business, as usual, at a depth of 40 km within the subduction zone where the oceanic crust of the small but fast-moving Nazca Plate (east of the Pacific Plate) is drawn down beneath the thick continental crust of the South American Plate.
This event was the biggest earthquake the world has seen in the past 25 years. At magnitude 8.3, it can rightly be described as a “great” earthquake. It ruptured a length of seafloor some 300 km long parallel to the coast and more than 100 km offshore. The tsunami it generated, travelling at 700 km/h, took just 15 minutes to reach the Peruvian coast. It arrived in the Chathams 14 hours later.
Amazingly, the earthquake and tsunami claimed fewer than 150 lives. Had they struck on a summer’s day when that section of 5 coast was packed with beachgoers, the casualty total would have been much greater.
We do not hear of tsunami in New Zealand very often, but they do constitute a major hazard. Historic tallies of tsunami deaths for New Zealand are comparable to total deaths from earthquakes within the past 200 years: between 200 and 300 lives. At least six significant tsunami have hit New Zealand in this time period. There is a record of several hundred Maori being killed by a huge wave in Fiordland in 1826, and of a Chatham Islander being drowned in a tsunami that struck Kaingaroa (on the north coast of Chatham Island) in 1868, probably generated from the same undersea fault as the recent event.
Tsunami are triggered by sudden massive displacements within ocean water masses such as are caused by earthquakes, volcanoes and landslides, especially submarine landslides. The deadliest tsunami are those triggered locally, for which there is little or no warning and no time to escape.
The tsunami generated by the Arequipa earthquake, for example, was 12-15 m high. As it romped inland, traversing up to a kilometre of flat land on the coastal plains nearest the epicentre, it destroyed virtually everything in its path.
Earthquakes and tsunami are rude monsters that raise their ugly heads at will, it seems. What can be done about them? The old Maori story of the boy and the taniwha says it all. The boy asks his grandfather: “How can I fight the taniwha?” The old man replies: ” First, get to know the taniwha.”
New Zealand scientists and engineers are doing just that: getting to know tsunami. They are visiting places which have been struck by tsunami, such as Sissano Lagoon, Papua New Guinea, which suffered a massive tsunami in July 1998, and now the Arequipa coast in Peru.
The risk to humans from hazards such as earthquakes and tsunami grows daily as humanity spreads its infrastructure over the Earth’s surface. Recall the horrors of the 1999 Izmit earthquake in Turkey: more than 20,000 lives lost. Or the 1995 Kobe earthquake, with more than 5000 dead. The impact of the Kobe earthquake has been likened to the New York Twin Towers disaster. Certainly, a similar number of lives were lost.
Arequipa was a subduction-zone earthquake. Although we have such earthquakes in New Zealand, there have not been any major ones in historic memory. This is a significant difference between New Zealand and Peru: they have had them, we haven’t … yet!
There is another difference of a geological nature between the two countries. The Earth’s crust where New Zealand is situated is thinner than under Peru. Ours is 25 km thick, compared with their 40 km-plus. So, because we are more “thin-skinned,” the land surface is that much closer to the subduction zone—where the Pacific Plate grinds beneath the Australian Plate. Should there be a major earthquake within that subduction zone, the intensity will he much greater than that of a Peruvian event, and hence more damage will be done.
New Zealand’s current systems for monitoring such hazards are based on old technology, with old instruments that are sparse and expensive to maintain. But with substantial funding from the Earthquake Commission announced in March 2001, the Institute of Geological and Nuclear Sciences (GNS) is modernising surveillance equipment for monitoring New Zealand’s non-biological natural hazards, namely earthquakes, volcanoes, landslides, geothermal activity and tsunami.
These new instruments will be managed and maintained as an integrated system called GeoNet and based at GNS in Gracefield, Lower Hutt. GeoNet will greatly improve hazard monitoring, forecasting, disaster-management response and disaster recovery.
As an adjunct to GeoNet, an educational programme designed as a resource for schools has been launched.
“Quake Trackers” was produced in 1998 by GNS and Victoria University to improve science education and earthquake-hazard awareness in secondary schools. To date, 14 high schools have Quake Trackers, and there is a flourishing website-based national school seismograph network which enables students to develop skills in computer science, earth science, physics and seismology.