Wild Land

Another moderately-strong earthquake struck near Christchurch yesterday afternoon, nearly five years after a sixth magnitude earthquake killed 185 people.

Monday, 15th February 2016

A magnitude 5.7 earthquake struck beneath Pegasus Bay, 15 km east of Christchurch at 1:14 on Sunday afternoon. The undersea quake was 15 km deep according to GeoNet.

The U.S. Geological Survey reports the earthquake as magnitude 5.8, centred 17 km east-north-east of Christchurch, 21 km south-east of Kaiapoi, at a depth of 8 km.

[click for larger image] The earthquake left a strong trace on GeoNet’s national seismograph network. GeoNet’s website reports that the earthquake was widely felt, with more than 3000 reports of the earthquake being felt from Waikato to Southland.

Christchurch’s infrastructure coped remarkably well, with no reports of serious injury or major damage caused by this earthquake. Shaking intensity in the city was lower because of the quake’s location but goods were thrown from shelves in homes, offices, supermarkets and stores, and some of the city’s shopping malls were closed yesterday afternoon to assess damage and clean up shops.

Liquefaction was reported in New Brighton which was closer to the earthquake epicentre, but to a lesser extent than previous moderate or strong earthquakes. Parts of some suburbs were without power for a time, but it was restored by evening except in a few cases where feeder wires had been torn away from individual houses by movement during the shake. Water supplies were disrupted when automatic shut-off valves triggered at some city reservoirs and had to be manually reset. Some water mains were broken but repairs are nearly complete.

Spectacular rockfalls occurred when cliffs collapsed into the sea at Sumner. This has brought demolition work on nearby houses damaged in the 2011 earthquake to a halt until stability of rock faces is assessed.

Mobile telephone networks suffered minor damage (damaged towers) which initially added to the network congestion as users sent text messages and made calls.

Many Christchurch residents talk of frayed nerves, having returned to normal life after the lengthy aftershock sequence that followed the magnitude 7.1 quake of September 4th 2010 and the disastrous magnitude 6.3 quake of February 22nd 2011. The return to an active series of aftershocks makes many wonder what the future holds.

In a discussion with RNZ National this morning, Professor Mark Stirling of Otago University said that the Canterbury region was experiencing an extended period of aftershock activity, similar to that which followed the magnitude 7.1 Inangahua earthquake of 1968. It is thought that, with aftershocks of the Inangahua earthquake still being recorded, the surrounding area may experience a century-long aftershock sequence. In the case of Canterbury, GeoNet seismologists indicate that the aftershock activity could continue for decades before the region returns to lower earthquake levels experienced prior to the 2010 Darfield earthquake.

Yesterday’s earthquake, centred under Pegasus Bay, continues the eastward trend of significant, isolated earthquakes which began with the magnitude 7.1 earthquake near Darfield in 2010. The magnitude 6.3 Christchurch earthquake of 2011 was centred on a fault below the Port Hills near the city. However, it is not a steady march eastward away from the city. The last earthquake to cause shaking of severe intensity in Canterbury was the magnitude 5.2 quake of 25th May 2012. This quake was centred 20 km east of Christchurch, about 5 km east of yesterday’s event.

The earthquake fault which hosted yesterday’s earthquake has yet to be firmly established. However, Prof. Stirling suggested that earthquakes like yesterday’s are releasing strain that has built up in old, less active faults in the area. These faults have slowly accumulated strain resulting from the collision between the Pacific and Australian tectonic plates in the vicinity of New Zealand over a very long period of time. Until comparatively recently, the area around Christchurch had been seismically quiet for several decades. The large Darfield quake releasing some of the built-up strain inland from Christchurch changed the “balance” in the fractured crust in the region, bringing some faults nearer to rupture causing the 2011 and 2016 earthquakes.

Prior to yesterday’s earthquake, GeoNet’s modelling of likely aftershock activity (prepared in November) showed a 49% chance of a magnitude five earthquake in the Canterbury area over the next year. This has now risen to 63%.

Writing on the GeoNet website, Sarah McBride summarised the revised aftershock probability for the next twelve months as follows:
Ӣ It is likely (63%) that an earthquake of magnitude 5.0-5.9 will occur.
Ӣ It is very unlikely (8%) that an earthquake of magnitude 6.0-6.9 will occur.
Ӣ It is extremely unlikely (less than 1%) that an earthquake of magnitude greater than 7.0 will occur in the next year.

This projection applies to the region shown here, and not just to the area east of Christchurch where recent earthquakes have occurred.

[Compiled from data supplied by GeoNet and the U.S. Geological Survey]

This entry was posted on Monday, February 15th, 2016 at 7:08 pm and is filed under Earthquakes - Observations. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.