Usually we know that earthquakes have occurred by shaking of the ground and objects, obvious ground movement or by seismometer readings but scientists have discovered to their surprise that large areas of land in the North Island are silently and slowly moving towards the east by up to 30 millimetres, but this movement doesn’t even show up on seismographs. These movements are called slow slip events.
What causes slow slip events?
New Zealand lies above a subduction zone where the rocks of the Pacific plate are pushing under the Australian plate. Many things happen at the interface between the two plates. Deep down, perhaps 70 kilometres or more, the rocks are heated and softened, and the plates slide steadily past each other without much sticking, but nearer the surface, there is a lot of friction between the moving plates. They tend to stick, building up tension – this is called the ‘locked interface’.
Every now and then, the rocks slip suddenly, releasing the stress and sending waves of energy through the surrounding rock. We feel this as an earthquake. Between these two areas may be a transition zone, where the rocks are only partly stuck and where friction is such that stresses between plates only build up slightly before being released. This results in slow rather than fast releases of energy, though still enough to deform the surface. A movement of 30 mm at the surface possibly resulted from a slip of 300 mm at the interface.
Spotting slow slips
Slow slip events have probably been going on for a long time, but they have not been recognised. Most of New Zealand’s surface is gradually moving as a result of normal tectonic movement. Until recently, annual measurements could only give a broad snapshot that showed a fairly constant rate of change. This all changed with the use of networks of continuously recording global positioning system (CGPS) stations, which is able to accurately detect tiny ground movements – the equivalent to you moving about 20 cm very slowly over two weeks!
Daily data collection helped scientists see that the gradual surface movement in some regions occasionally changed direction and speed for periods ranging from days to months. These slow slip events were first observed in New Zealand in 2002, shortly after they had been found in Japan and Canada.
Studying slow slips
Slow slip events have occurred in Gisborne, Hastings, Wanganui, Ashhurst, Dannevirke and Paekākāriki. Some take a few days, and others take many months. One of the best documented slow slip earthquakes in New Zealand occurred from January to June 2005 beneath the Manawatū region. This caused the land to move 10 to 30 mm to the east. This slow earthquake may have triggered some of the small earthquakes in the lower North Island early in 2005.
A slow slip event on the Kapiti coast in 2003–2004 was followed by a swarm of small earthquakes near Upper Hutt. So is there a link between slow slips and ordinary 'fast' earthquakes? One possibility is that slow slips change the stress in the stuck interface at shallower depths above it, triggering earthquakes. If this is the case, a better understanding of slow slips may help give an extra level of earthquake warning.
Nature of Science
Accurate data collection is an essential part of science. The development of continuous GPS has transformed the study of earthquakes and related events in New Zealand. This is not only because of the increase in accuracy of CGPS over previous methods but also because of the ability to collect data continuously, which enables short-lived changes in ground deformation – such as slow slip events – to be detected.
Slow slip events seem to occur just below the edge of the highly locked areas where ordinary earthquakes are generated. By measuring slow slips at the surface, scientists will be able to map the edge of the locked interface. This would provide some indication of where major earthquakes are most likely to occur and their possible size. This information could be of use to the Earthquake Commission and insurance companies, who would then be more aware of high and low risk earthquake areas.
GNS scientists like Laura Wallace are working with colleagues in America to develop an automated method of detecting the start of slow slip events. At the moment, this is done by someone visually checking data daily or weekly, looking for hard-to-find patterns. An automated way of finding the start of an event would mean that seismographs could be rushed to the right place to make detailed measurements of an event as it is occurring.