Not our fault - but be prepared
The Seddon earthquakes have no direct relevance to Nelson and its criss-cross network of fault lines, geologist Mike Johnston said today.
But they do show that New Zealand sits astride an active plate boundary in which stress is accumulating.
"Earthquakes will occur anywhere in the country and should be prepared for," Dr Johnston said.
Currently attending a conference in Manchester, Dr Johnston is a specialist in the geology of the Nelson region and has closely studied the Nelson urban area. He is a consultant to both the Nelson and Tasman councils.
He said the two sources of earthquake ground shaking in Nelson-Richmond were local and distant quakes. Local quakes would come from the Waimea-Flaxmore fault system, which included faults that extended from the Alpine Fault near Lake Rotoiti along the foot of the eastern hills through Richmond and Nelson and into Tasman Bay.
Shorter cross-faults, such as the Grampian and Bishopdale faults within the city boundaries - the latter classed as active, as it has offset the ground surface - join the larger ones.
"Movement on relatively short lengths of the major faults or movement on the lesser faults are expected to generate magnitude 6 to 6.5 shallow earthquakes similar to those currently occurring near Seddon," he said. Damage would be significant if the epicentres were near populated areas.
"However, the frequency of when and where such fault ruptures might occur is largely unknown but it is in the order of thousands of years on any one particular fault in the Waimea-Flaxmore fault system."
The two local authorities had funded a recent study of the fault system involving trenching to gather data on previous earthquakes. Results were still being assessed, Dr Johnston said.
More distant earthquakes that would give strong ground shaking such as experienced in 1848, 1855 and 1929 could be expected on average about every 80 years, but would be less intense than if the quake came from the Waimea-Flaxmore system, he said.
Previously, Dr Johnston has said it was easily shown that Nelson had more than its share of fault lines - the Tahunanui and Port Hills, the Grampians and the eastern hills, each range progressively higher, were all created by fault activity.
He has also said liquefaction is a major threat, particularly for the Port Nelson reclamation but also for coastal areas around Tasman and Golden Bays.
Talking in March 2011 about the risks to this region he said the Waimea Fault near Brightwater had moved three times in 18,000 years, most recently 6000 years ago. "It means you're getting closer to the next one, and that's just one of the faults in that system, so the risk is high . . . one of the local faults could move today."