REGIONAL TSUNAMI SOURCES The warning time for tsunami from regional sources is about 1–3 hours, and presents a real challenge to monitoring and warning agencies. To locate an event, evaluate its tsunami potential and issue a warning in so short a time is problematic, requiring pre-planning and scenario development. Self-evacuation of residents will be required at short notice. As outlined in the following sections, regional source tsunami may represent a significant hazard and risk, and these may be catastrophic on rare occasions. Regional sources include earthquakes and volcanoes (eruption and flank collapse) from tectonically active regions to the north of New Zealand, and south of New Zealand from about 50-60°S. Regional sources of tsunami to the east and west are highly unlikely (but note that the Solomon Islands subduction zone is a distant source that primarily affects the west coast of New Zealand, even though it occurs in the ‘SW Pacific region’). Hence, the coasts most at risk from regional source tsunami are the northern half of the North Island and the southern half of the South Island. The following sections outline what is known about the historical impact of regional source tsunami, about the sources of potentially damaging tsunami, and what has been learnt and can be learnt from numerical modelling, and from geological studies of pre-historical tsunami. They form the basis for what is known about the frequency and magnitude of events that New Zealand might expect to experience. Earthquakes In New Zealand’s historical record, the largest earthquakes along the arc between New Hebrides (Vanuatu), Kermadec Islands and Tonga have been less than magnitude 8.5. Only two of these are known to have caused tsunami with run-ups in New Zealand approaching 1m. Although the record of run-ups in New Zealand may be incomplete, we would expect a large event in historical times to have been noted. To the south of New Zealand, only a few large earthquakes have occurred since the 1960s, when the installation of a worldwide seismic network allowed large earthquakes to be identified and located. The only three large earthquakes in the last 40 years had magnitudes between 7.8 and 8.4, and all were in areas of the plate boundary where earthquakes with predominantly horizontal (strike-slip) movement along the fault occur. These earthquakes do not usually generate large tsunami and none had run-up of > 1 m in New Zealand (along the south and west coasts of the South Island).
Posted on: Sat, 15 Mar 2014 00:22:37 +0000
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