July 21, 2013

Earthquakes shake NZ capital

Posted by Austin Elliott

New Zealand is at it again–or as usual–seismically. This time a series of strong earthquakes in the Cook Strait culminated Sunday morning with a M6.5 between the South and North Islands, shaking the national capital of Wellington and exacting a fair bit of damage around the region.

Seismogram from Wellington, NZ, Sunday July 21, 2013. Click for live feed.

Seismogram from Wellington, NZ, Sunday July 21, 2013. Click for live feed.

The shocks started with two oddly steep convergent events, a 5.3 Friday morning and a 5.8 in the wee hours Sunday morning. Each of these had their own aftershock sequences, but it’s notable that the larger one came second… and of course it’s notable that 10 hours later a M6.5 was unleashed from the strike-slip faults underlying the Strait.

An animation from Weather Media shows the sequence unfold over the course of the day Sunday:


The relatively populous capital Wellington is sprinkled with a modest distribution of all the hazards expected when an earthquake hits a city: power failures, underground utility ruptures, sidewalks sprinkled with shattered glass, and street curbs buried in masonry rubble.

Despite strong shaking and considerable damage to the contents of stores, there appears to be only modest structural damage. Photos show ground failure in the port, which is unsurprising as shipping ports commonly occupy weak, made land. New Zealand has learned important lessons from the Canterbury earthquake sequence, and so residents, employers, and building owners alike are duly cautious in reopening and reoccupying CBD high-rises only after thorough inspection.

Overall Wellington is operating nicely as a prepared city struck by a modestly dangerous earthquake. The USGS and New Zealand’s GeoNet are frank about the substantially increased seismic risk in the next few days to weeks, and if the last few days of activity are any guide, Wellingtonians will understand the potential for larger or further damaging earthquakes. Here’s the critical bit from GNS Science:

Statistical analysis of the sequence to date provides estimates… that in the coming week there could be up to nine M5.0 or greater events, with an approximately 30% probability (1 in 3 chance) of a M6.0 or greater.

Earthquakes less than magnitude 7.0 do not usually generate a tsunami; 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.

Seismic hazard information for the city of Wellington is available on the country’s ShakeOut webpage from the national earthquake drill last year. The areas at highest risk occupy the soft sediment and filled land around the industrial shoreline of the city.

Here are a few decent videos of the earthquake happening. I’ll update this as I find more. Feel free to add in the comments any that you come across!

The noisy earthquake was captured downtown by one enthused resident. That was a big earthquake, eh?

It was also captured by a tripod-mounted camera in a Wellington office. One of these women does the right thing, the other runs around like a chicken with her head cut off.

A pretty tame CCTV surveillance video captures the jiggling in a hardware store

Update [7/21/13 14:24 PDT]: This stuff.co.nz article contains a video with footage of the quake from parking lots and as it interrupts an indoor “football” game.