Dozens of slow-slip events (also known as "silent" earthquakes) have been detected in New Zealand since 2002. They occur up to 60 km below the earth’s surface where the Pacific Plate meets the Australian Plate, along the Hikurangi Subduction Zone (marked by the orange zone on the image below). Slow-slip events can move faults the equivalent of magnitude 6+ earthquakes over a period of weeks to months. Movements caused by these slow-slip events are so slow that they are undetectable by both humans and GeoNet's seismographs. GeoNet, in partnership with LINZ run a network of GPS stations around the country that are able to detect land movement as little as a few millimeters resulting from slow-slip events.

Read this article to learn more.

The relationship between slow-slip events and earthquakes

There are several examples from both New Zealand and overseas of earthquake swarms accompanying slow-slip events. Because slow-slip events occur over a large area, the amount of stress they transfer to other faults is diffuse. This is unlike a large traditional earthquake that has a large stress transfer concentrated in a relatively small region. For this reason, a magnitude 7.1 slow-slip event is probably not going to have anywhere near the associated triggered earthquake activity that we saw after the magnitude 7.1 Darfield earthquake, but it will increase stress in surrounding areas, and could push an already stressed fault closer to rupture. In essence, it can be the straw that breaks the camel’s back. This is possibly what happened with the January 2014 Eketahuna quake, where the Kapiti slow-slip event loaded stress on the fault that broke. The fault in the Eketahuna earthquake would most likely have ruptured in the near future, but the added stress may have caused it to rupture earlier.

It is important to note a few caveats when looking at the transferred stress events from slow-slip events (or regular earthquakes):

• Slow-slip events do not universally increase stress on surrounding faults, they also relieve stress in some areas, and therefore may postpone an earthquake in an area of decreased stress.
• As we can’t directly measure the plate movement (because it is well below the surface and most often offshore) we have to use models to determine what is going on and how the stress is being distributed. As such, the model will hopefully give a good indication, but will not be perfect, as there are many details that we cannot resolve using solely land-based instruments.

Why we care when slow slip causes no shaking

Before discovering slow-slip events, earthquakes were thought to be the only way the Earth’s crust could relieve the pent-up stresses caused by the moving tectonic plates. With the discovery of slow-slip events, this thinking has been drastically altered, as slow-slip events accommodate a large proportion of the effects of the converging plates without knocking a single ornament off a shelf. Slow-slip events in themselves don't pose any risk to people, but they are a major part of how the tectonic plates move in a subduction zone. The other major part is earthquakes. So if we better understand the slow-slip events, we should better understand the earthquake potential of subduction zones.

The subduction zones where slow-slip events occur (in our case where the Pacific and Australian plates meet) are responsible for generating the world’s largest earthquakes – ‘great earthquakes’ or ‘mega-thrust earthquakes’ – which have a magnitude greater than 8. These types of earthquakes can also produce tsunami with deadly consequences as we’ve seen in recent times in Japan and Sumatra. Scientists think that a future mega-thrust earthquake with a magnitude of 8 or larger is possible on New Zealand’s northern subduction zone – the Hikurangi subduction zone. An earthquake this large would produce damaging shaking for much of the North Island, and could produce a significant tsunami affecting much of the country (and also some coastal regions around the Pacific). However, scientists still don’t know how much, and how often, the Hikurangi subduction zone ruptures in megathrust quakes. The best evidence at the moment suggests they are relatively rare, happening every several hundred years or more. This is an area of active scientific research in New Zealand, and the more we know about the slow-slip events, the more we understand the subduction zone as a whole, and ultimately the better prepared we, and other nations, can be.