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Student Activity - Something creepy is happening
Activity idea
By the end of this activity, students should understand:
- how slow slips are caused
- how researching slow slips is important for studying earthquakes
- how to read and plot data on slow slips
Slow slip events
The Pacific plate is continuously diving down below the eastern North Island, which is on the Australian plate. The Pacific plate often gets stuck under the North Island as it dives down and this pushes the Australian plate to the west. Usually this stress gets released as an earthquake that we can feel or at least that shows up on a seismograph. As the Pacific plate gets deeper, it gets warmer and softer and it isn’t stuck any more. The two plates start to glide past each other, and the Australian plate may spring back deep underground much more gently, causing slow earthquakes 15–30 kilometres deep beneath the North Island. This is what causes slow slips on top of the ground.
Instructions
Read the article: What are slow slips?
- Describe what a slow slip event is.
- How far has the ground been found to move in a typical slow slip event?
- How do we know that slow slip events have occurred even when we can’t feel them?
- Why is GPS used to track slow slip events?
- In your own words, try and describe how tectonic plate movement causes slow slip events.
- Why do slow slip events occur above the area where the plates are slipping past each other?
- When one plate subducts under another wet sediments are also taken under ground. This water warms up as the plate subducts deeper. Do you think that this water may play any part in causing slow slip events?
- These slow slip events may be warning us of more violent earthquakes. Can you suggest how they might indicate a more violent earthquake?
- It has also been suggested that slow slip events may be relieving stress and preventing more dangerous earthquakes. Can you suggest how they might be relieving stress?
A slow slip on the Kapiti Coast
A few years ago a GPS site on the Kapiti Coast recorded a slow slip event, while another site not far away in Wellington did not.
This activity compares data from these two GPS sites (Wellington airport and Paekakariki on the Kapiti Coast) over a six-year period.
You will be graphing one piece of data from each year. (Usually, actual data are collected and plotted continuously, and computer generated graphs are produced to show short-term changes in detail.)
This table shows land surface movement towards the east over time, relative to the Australian plate. (Tectonic movement was taking place before July 2000, but movement is recorded as zero on this date because this was when data collection started.)
|
Data |
Movement (mm) at Wellington GPS site |
Movement (mm) at Paekakariki GPS site |
|---|---|---|
|
July 2000 |
0 |
0 |
|
July 2001 |
30 |
25 |
|
July 2002 |
60 |
50 |
|
July 2003 |
90 |
75 |
|
July 2004 |
120 |
90 |
|
July 2005 |
150 |
115 |
|
July 2006 |
180 |
140 |
Plot the Wellington data on a line graph, with time on the x-axis, movement (mm) on the y-axis.
- How far does the land at Wellington move each year? This is part of normal tectonic movement.
- Does this rate change during the data collection period?
Now plot the Paekakariki data on the same graph.
- What is the rate of movement at Paekakariki (mm per year) between July 2000 and July 2002?
- When does it change?
- Does the movement get faster or slower?
- When does it change back to the rate shown at the beginning of the data?
Paekakariki is not far from Wellington, but it experienced a slow slip event while Wellington only recorded normal tectonic movement. No other GPS recording stations in the North Island picked up unusual movement in that period. What does this tell us about the size of the area that the slow slip covered?
