Some of you will remember the slow-slip events that we observed on the Hikurangi subduction plate boundary following the M7.8 Kaikoura earthquake using the GeoNet GPS network. The Hikurangi subduction zone is where the Pacific Plate and Australian Plate meet beneath the North Island and northern South Island.

So, here’s what we know now: one slow-slip event has stopped (offshore the east coast of the North Island), and the other one has slowed down (beneath the Kapiti region). But, the most interesting part of our update is that we have now detected slip occurring on the subduction zone beneath the upper South Island. All of these slow slips started immediately after the earthquake and are a direct consequence of the magnitude 7.8 Kaikoura earthquake.

The newly discovered upper South Island slip

A swath of new data has revealed that the upper South Island started slowly moving immediately after the Kaikoura Earthquake. This movement was most active during the first month after the earthquake and has slowed down considerably since the start of the year. Like the slow-slip events in the North Island, most of the land movement we are currently detecting in Marlborough is caused by the subduction plate boundary slipping beneath the upper South Island, where the plates meet about 25km below the Earth’s surface. Following the Kaikoura earthquake, the plate boundary has slipped up to half a metre — slowly releasing energy over the past few months equivalent to a magnitude 7.3 earthquake. This type of slow slip (often called “afterslip”) typically happens following large earthquakes, either within the zone of earthquake rupture, or in the region immediately surrounding the zone of rupture. New work on what happened during the Kaikoura earthquake(led by GNS Science) was also published last week, which also shows that this same area of the subduction zone may also have moved during the initial M7.8 Kaikoura Earthquake.

We were only recently able to get a full picture of this post-earthquake slip happening on the subduction zone beneath the upper South Island, partly because there are fewer GeoNet continuous GPS sites there. GNS scientists installed new GPS sites after the earthquake and then had to physically go out and retrieve all of the data over time, instead of relying on GeoNet instruments that relay data straight back to us every hour of every day. This slow-slip event involved up to 15 cm of movement offshore on the Hikurangi subduction zone between Gisborne and southern Hawke’s Bay. It took only two weeks to occur, and was mostly finished by the end of November. Over that period it released enough energy to be equivalent to a magnitude 7.1 earthquake. Slow-slip events on the east coast often trigger small to moderate earthquakes nearby. There were more than 200 earthquakes associated with this slow slip event off the East Coast of the North Island, including a magnitude 5.5 on 22 November near Porangahau.

Kapiti slow-slip event

The slow-slip event occurring beneath the Kapiti coast and Marlborough Sounds is still trucking along, but at a slightly lower rate compared to immediately after the Kaikoura Earthquake. The tectonic plates have moved past each other by several centimetres since November, and thus far this event has released energy equivalent to a magnitude 7.0 earthquake. Based on the behaviour of past Kapiti slow-slip events, we expect it to continue for at least a few more months.

What this all means

Slow-slip events are a relatively newly-recognized phenomena; we’ve only been able to detect them in New Zealand for the last 15 years. Even though the slow-slip events relieve built-up stress between the tectonic plates without earthquakes, they can transfer stress onto other faults around them. We’ve detected dozens of slow-slip events in the past that haven’t caused large earthquakes, although it is possible that the Cook Strait earthquake sequence in 2013 was triggered by the Kapiti slow slip event, which was occurring at the time. Because so much slow slip has been happening since the Kaikoura earthquake, our scientists are currently trying to develop new ways to figure out how these slow slips might influence the likelihood of future earthquakes. Our current models suggest that another earthquake of a similar or greater magnitude than the Kaikoura M7.8 has a 5% likelihood within the next year. While 5% is still a small chance, this chance is approximately six times greater than it was prior to the Kaikoura earthquake. We are currently working on updating our calculations based on the new information we have on the slow slip events following the M7.8 earthquake. As we’ve mentioned previously, we’re aware that these messages could be unsettling, and that’s a very normal reaction. What we do want you to take away from this (and this applies to all New Zealanders, at all times—not only now) is to follow Civil Defence’s advice and make sure that you’re prepared for earthquakes and tsunamis. We know that being prepared makes a real difference in helping you get through an event and recovering afterward. Many of you have already got you and your family prepared, so well done you guys!

Contact Scientist: Laura Wallace