Wairarapa residents’ awareness of earthquakes went up on the ‘Richter scale’ last month, so the presentation of “It’s Our Fault” in Masterton on July 25th attracted a large crowd. The powerpoint address – with the subtitle ‘Wellington and Wairarapa Rock’ was given by Dr Robert Langridge from the GNS Science research team based in Wellington.
One GNS task is to map faultlines, so the latest spates of earthquakes centred near Seddon, which occurred on a previously unmapped one, were a surprise. The ‘new’ fault, like the also unmapped Greendale one responsible for the first big Christchurch event, has now been added. Many Wairarapa faultlines have been mapped, but not all of them.
Dr Langridge explained the severity of an earthquake depends on two major factors. Distance from the epicentre is the obvious one; and the second, perhaps surprisingly, is the building code for the area concerned. GNS staff have produced graphs showing effects from past earthquakes, and building standards are thus now able to be set at a level well above damage expected.
The audience then learned about the five main effects of an earthquake. These are: ground shaking, liquefaction, slope failure, surface fault rupture and finally, tsunami. The likelihood and severity of these effects depend primarily on the land structure. Thanks to GNS research again, we now have a map of the soil types in the Wairarapa region. Soil types are graded from A [largely marble rock] through to D [weak, unstable soil] and even E which is located in sandy areas such as river banks and lakes. Our local towns are built on D type soils!
The other factors which are important are the type of fault line, the location and depth of the epicentre, and the time the earthquake strikes.
Finally, of course, the NEXT BIG ONE? Thanks again to the extensive studies of past earthquakes, scientists can now extrapolate the time between events and the chance of a “big one”. Luckily for us and Wellington, the 8.2 event which occurred here 300 years ago has ‘unloaded’ the main Wellington fault, so the estimated dates for future events have been extended.
However, there is no room for complacency. A simple equation explains that the level of risk is a product of ‘hazard’ and ‘vulnerability’. It is the latter that we can minimise. Thanks to the experts at GNS, it is now known that not all land is ‘created equal’; so buildings must be strengthened, designed, located and built accordingly. And Dr Langridge’s concluding advice – “BE PREPARED”.
