By Roger Ball and Glenn Stevens*
Following the tragedy in Japan (11 March 2011) many people are asking, what is the risk of tsunami in our region? Understanding the tsunami risk to our area is a topic of ongoing research. Based on what we know, however, the tsunami hazard to the Nelson Tasman region can be grouped into distant, regional and local sources. Each of these has different implications.
Distant tsunami sources are large undersea earthquakes that generate tsunami which then cross the ocean to strike New Zealand. For example, the 2010 Chilean earthquake and subsequent tsunami. Because of the travel times for the tsunami to cross the ocean there is realistic warning of many hours. Most of these distant sources lie to the east of New Zealand and the Nelson Tasman region is not directly exposed to the Pacific Ocean. For a tsunami crossing the Pacific Ocean to affect the Nelson/Tasman region it needs to propagate through Cook Strait where it would lose some of its energy.
Regional tsunami sources are where a tsunami may strike within 1 to 3 hours of the trigger event. For our region such tsunami will typically arise from undersea earthquakes relatively close to the New Zealand coastline.For example, undersea earthquakes along the east coast of the NorthIsland. Limited warning of a regional sourced tsunami may be possible but not in all cases.
Local tsunami sources Local tsunami sources in the Nelson Tasman region are primarily local earthquakes where the sea floor is ruptured. The undersea extension of the Waimea-Flaxmore Fault system and faults in Cook Strait and around Marlborough and Wellington are potential sources of local tsunami. Local source tsunami may also include landslides into the sea, undersea slumping or even volcanic activity. Tsunami from local sources could strike the closest parts of the coastline within minutes of the trigger event. Other than feeling a significant earthquake there will be no warning of such a tsunami.
In terms of likelihood, distant sourced tsunami can be expected to occur much more frequently than regional sourced tsunami, and regional sourced tsunami are more likely than a local sourced tsunami. In terms of size (i.e. surge or wave height) the largest tsunami to affect the Nelson Tasman region will be generated from local and regional sources. Large locally sourced tsunami in the Nelson Tasman region are very infrequent events (i.e. return periods in the order of 2,500 years on average according to one report done by GNS).
Not all earthquakes will result in tsunami. For example, the large 1929 Murchison earthquake and 1968 Inangahua earthquake did not produce a tsunami. Nor did the recent Christchurch earthquakes. It is when earthquakes cause vertical displacement of the sea floor that large tsunamis are generated.
Overall, the Nelson Tasman region has a lower tsunami risk compared to other parts of New Zealand’s coastline. In European times there have been a number of tsunamis in the Nelson region, though fortunately none have been devastating. The research relating to tsunami in pre-European times is limited but there is evidence of sizeable tsunami having occurred in places such as Abel Tasman Park.
Because there is tsunami risk in our region, and because we often travel to other regions with higher tsunami risk, it is important that you and your family know what to do in the event of a possible tsunami.
IF you are at or near the coast and you experience any of the following:
THEN get to high ground and/or go inland.
During a tsunami alert, listen to the radio, inform your neighbours. If you have power/internet during the emergency, visit these sites for updates: www.nelsontasmancivildefence.co.nz or www. civildefence.govt.nz. If the official advice is “don’t go to the beach”, follow that advice. Sometimes the official advice will change as more information comes in about the tsunami - eg, the restriction on going to the beach could be eased or an evacuation could be ordered. So stay tuned to the radio. Visit www.civildefence.govt.nz for information about being prepared including information about tsunami.
*Roger Ball is Manager of the Nelson Tasman Emergency Management Office. Glenn Stevens is a Resource Scientist with Tasman District Council.