How do we know about Earth movements?

Aotearoa New Zealand experiences a lot of geological activity – the land is right at the junction of the Australian and Pacific tectonic plates – making it a shaky place to live! The Pacific plate is being forced down (subducted) under the Australian plate.

GNS Science reports the country receives over 15,000 earthquakes each year but only 100–150 of the quakes are large enough to be felt. We know about the smaller movements because they are recorded by seismographs. These machines are very sensitive to earth movements, but some movements are so small and occur over time that they don’t register. These movements are called slow slip events.

Scientists were surprised to discover that large areas of land in Te Ika-a-Māui North Island are silently and slowly moving towards the east. The movement is very, very slow. In human terms, imagine standing at one spot and taking more than 2 weeks to move a couple of millimetres. Now you can see why slow slips weren’t even noticed in Aotearoa until 2002.

Exploring the slow slip mystery

Slow slips were first observed in Japan and Canada. Scientists in Aotearoa began their own observations by setting up networks of continuously recording global positioning system (CGPS) stations. There are over 50 monitoring sites across the North, South and Chatham Islands. Satellites send signals to the receivers at these sites, enabling the stations to record very precise positions on the Earth’s surface. This continuous sending and receiving allows scientists to measure even the smallest of movements – and the bigger ones too.

GNS Science reported more than a dozen slow slip events in the first 10 years of monitoring. They have occurred in Gisborne, Hastings, Wanganui, Ashhurst, Dannevirke and Paekākāriki, and offshore in the Gisborne and Hawke’s Bay regions.

Why we need to monitor slow slips

If the ground is moving so slowly, why go to the time and effort to track movements of just a few millimetres? It’s because scientists want to know if there is a relationship between slow slips and earthquakes. Sometimes it appears that slow slips can relieve the stress that builds up in fault lines. Other times it appears that slow slips increase the stress in surrounding areas and push the fault closer to rupture (breaking). Collecting data about slow slips provides a better understanding of this relationship. It might also help to inform models that warn us about earthquakes.

Build a satellite to track Earth movements

Have a go at building a fit-for-purpose satellite to monitor slow slips! Choose the ‘Monitor Earth movements’ scenario. Once you’ve received the GNSS data, use this activity to analyse the slow slip movements around Māhia Peninsula.