How prepared are we for a major marine earthquake?
This week, New Zealand's leading coastal scientists, engineers and planners are attending the New Zealand Coastal Society 20th Annual Conference in Auckland. At the conference, NIWA's Dr Philip Barnes will explore the question of how well do we know New Zealand's submarine earthquake hazards.
Drawing on NIWA's experience from Canterbury earthquakes, and how this work feeds into a bigger picture of earthquake risk in New Zealand, Dr Barnes says, "We have to be prepared for these sorts of marine earthquakes in many New Zealand locations."
GNS Science has recently led a major revision of the New Zealand National Seismic Hazard Model (NSHM), and have worked closely with NIWA to capture submarine faulting. The model includes a range of earthquake hazard estimates.
A number of newly-identified active fault sources have recently been incorporated into the NSHM model, increasing the total by around 200, to about 530. The recent increase is largely attributed to the inclusion of the newly-identified offshore fault sources, including previously unknown faults in the bedrock under Pegasus Bay, Canterbury.
A particularly large offshore fault was discovered when NIWA's research vessel Kaharoa conducted a seismic survey of 800 square kilometres in southern Pegasus Bay, shortly after the February 2011 Christchurch earthquake.
In the last decade, there have been many major earthquakes causing severe ground-shaking and tsunamis around the world. The Boxing Day tsunami of 2004 killed more than 200,000 people and resulted in widespread destruction. One of the most powerful earthquakes on record struck the north-east coast of Japan in 2011, triggering massive tsunami waves with up to 40 metres of run-up height.
"There are many earthquake faults on the seafloor around our coasts, and the next major earthquake could be centred offshore. So it highlights the need for coastal marine investigations of active earthquake faulting, for both ground-shaking hazards and tsunamis," says Dr Barnes.
"NIWA's geosciences team responded rapidly to the Canterbury earthquakes, working closely with the Natural Hazards Research Platform and other authorities to inform the recovery."
Some locally significant changes in probabilistic earthquake hazard were observed in some areas when the methodology of the NSHM was revised, and the large number of new active fault sources were included in the model.
"If a major earthquake occurs on a seafloor fault, we might already have information about that fault.
"We also now have some experience in how to respond and learn more about the cause of a major earthquake if such an event occurs. The more background information we have on active faults, the better," says Dr Barnes.
"We have worked offshore from Fiordland, right through Canterbury, Marlborough, Cook Strait, Kapiti, Hikurangi margin (on the east coast of the North Island), New Plymouth and Bay of Plenty. We've identified faults and characterised them, and in some areas we are now working on the history of past earthquakes."
The New Zealand NSHM project is a collaboration between GNS Science and NIWA, with input from Victoria University of Wellington and the University of Canterbury. The new model informs engineering design and the insurance sector.
"Incorporating the NIWA offshore active fault data into the NSHM is a shining example of our very fruitful GNS-NIWA collaboration" says Mark Stirling, leader of the NSHM project.
The New Zealand Coastal Society Annual Conference – 'Making Waves, 20 years and beyond' is being held 14 – 16 November 2012, at the Royal NZ Yacht Squadron in Auckland.
NHMS hazard map, comparison of 2002 and 2010 hazard maps
By means of colour coded maps, or site specific hazard graphs, the NSHM produces a range of earthquake hazard estimates for New Zealand. The maps, by themselves, do not represent comprehensive engineering advice; they are simply a graphic expression of information contained in the National Seismic Hazard Model.