Welly quakes likely to continue, and some could be larger - geophysics Prof

"Earthquakes are likely to continue near Wellington for some time – no-one can say how long," says Victoria Univesity geophysics professor Euan Smith in the wake of 5.8 and 6.5 quakes that struck the capital today. 

"Some of them could be as large, or larger, than the ones we have just had."

Therefore, anyone with property or possessions which are vulnerable to being knocked over or down and damaged should take steps to secure them, Prof Smith says.

GNS' GeoNet unit weighed in with "Statistical analysis of the sequence to date provides estimates of the probability of future earthquakes in central New Zealand. GNS Science estimates that in the coming week there could be up to nine magnitude 5.0 or greater events, with an approximately 30% probability (a 1 in 3 chance) of a magnitude 6.0 or greater. The most likely period for this to occur is the next 24 hours, when the probability is approximately 20% (a 1 in 5 chance)."

Earthquakes less than magnitude 7.0 do not usually generate a tsunami, GeoNet says.

"However, it is possible for undersea landslides triggered by earthquake shaking to produce a tsunami. The impact of these types of tsunami is usually confined to the coastline close to the earthquake epicentre, and would reach the coast within 10 to 20 minutes following the earthquake."

Prof Smith adds, “Few events in the natural world are unprecedented.  While Wellingtonians reflect on the latest reminders that we all live in a dynamic place, they may be interested to know that something very similar happened in January 1950.

“Then, between January 6 and about January 28 more than 30 earthquakes above magnitude 3 occurred in Cook Strait, about 50 km north and slightly further west than the current quakes.  This swarm featured four earthquakes above magnitude 5, between January 7 and 21.  We have so far had two of magnitude greater than 5, on Friday and this morning (Sunday, 21 July2013), accompanied by numerous smaller ones.

“These two larger earthquakes are scientifically interesting because of their faulting type.  Fault types can be inferred from the way in which earthquakes radiate waves all around the world.  The USGS Earthquake Information Centre uses these waves to estimate the location, depth and faulting type.

“Their models show that our two bigger ones resulted from the squeezing of Wellington in the boundary between the Pacific and Australian tectonic plates.Their depths are consistent with them occurring on the boundary surface between the Pacific plate, which is sinking northwestwards under the North Island and northern South Island, and the overlying Australian plate.”