For Teachers: A practical field trip lesson plan to Harcourt Park, Upper Hutt

This is a world class geological site showing the interaction between river erosion and an active fault.

Getting there: The best place to start your visit is at California Park, which gives a good visual introduction to the landforms. Turn off the River Road (SH2) at Upper Hutt into Totara Park Road. Cross the first roundabout and continue over and right at the second into California Drive. After about 700 metres the California Park is visible on the left.

You could also start your visit at Harcourt Park. From State Highway 2 at Brown Owl, Upper Hutt, turn into Akatarawa Rd - Harcourt Park will appear on the left after about 400m.

Introduction

What you can see there: As you drive towards California Park you will be driving down California Drive – you can see that houses on the left side of the road are higher than those on the right – you are driving along the fault scarp! California Drive shows how thoughtful urban planning can reduce earthquake risk. At California Park the topographic expression of the Wellington Fault is well defined. You can see an obvious straight scarp, this is called the fault scarp, and the ground surface has been uplifted on the northwest side.

Between California Park and Harcourt Park, the Hutt River flows across the Wellington Fault at a right angle. Over many thousands of years the Hutt River has built up (aggraded) and cut down (eroded) sediment, these natural river cycles create step-like river terraces. At Harcourt Park the river terraces have been have been offset sideways along the Wellington fault as a result of past earthquake ruptures of the fault. Here you can measure how progressively older landforms have been displaced by greater amounts across the fault.

The Geological Background:

There are at least 4 major active fault lines running through the Wellington Region (e.g. the Wairarapa, Wellington, and Ohariu Faults), and one big one underneath Wellington (the ‘subduction zone interface’ or plate boundary). The famous earthquake of 1855 was caused by the rupture of the Wairarapa Fault which moved about 15 m sideways – the greatest sideways movement ever recorded in a historic earthquake anywhere in the world.

The Wellington Fault itself last ruptured about 200 to 450 years ago. It has an average repeat interval of about 500 – 1000 years and moves roughly 4 – 6 m sideways each time. There is also some vertical movement that has, in places, uplifted land to the west of the fault and subsided land to the east of the fault. The combination of these displacements, and modification by river and marine erosion, have created the prominent fault-line escarpment running from Wellington City, along the edge of Wellington Harbour and up the western side of the Hutt Valley.

River Terraces River terraces are created by interactions between sediment supply and tectonics. The supply of sediment to a river varies over time due to many factors; it will increase during periods of cold climate (resulting in fewer trees in our mountainous areas) or periods of very stormy weather. During these periods of high sediment supply rivers can build up (aggrade) terraces. When sediment supply to the rivers decreases the river cuts down (degrades or erodes) through the terrace it has previously created. These cycles of aggradation and degradation occur repeatedly over thousands of years. If the land stays stable then each successive cycle would erode and build up a river terrace at the same level. But if the land is being uplifted by tectonic forces then the terraces are progressively uplifted above river level and the terrace is abandoned by the river. This creates stepped sequences of river terraces with the higher terraces being oldest and the terraces closer to the river being the youngest. The embankment separating two different terrace levels is known as a ‘riser’. The Hutt River has some well developed sequences of terraces and risers.

The youngest terrace is the lowest - the oldest is highest. Where the fault cuts across the river, the terraces have been displaced sideways:

Offset terraces at Harcourt Park:

The Wellington Fault is close to the path through Harcourt Park between the river and the play area. On either side the risers (slopes) of river terraces can be seen extending away at approximately right angles to the path. Notice that the risers do not continue across the fault because they have been offset by fault movements (earthquakes).

The photo shows the area near the car park at Harcourt Park. The flat area at the top is a river terrace and the sloping area on the left is a riser.

The diagram below shows more detail, including the river section. The terraces were created in the order 1 first, then 2 then 3. The arrows alongside the fault show that there is horizontal as well as vertical displacement. Careful observers can see that the higher terrace risers are offset by greater amounts across the Wellington Fault than the lower terrace risers. This is because the higher terraces are older – they have been through more earthquakes, and more displacements, than the younger terraces.

For Teachers

New Zealand Curriculum: Planet Earth and Beyond Level 3 - 4: Investigate the water cycle Level 6: Investigate processes that shape surface features of the Earth Level 7: Causes of natural hazards and interactions with human activity Nature of Science levels 3 - 4: Use science knowledge to consider issues of concern

See also: Earthquakes; Feeling the Earth Move Building Science Concepts Book 40

Time You should allow about 2 hours for the visit excluding travel time, depending on how much background or follow up discussions you do at school and which activities you choose to include.

You will need:

• Paper and pencils for sketching - each student • 30 metre or longer tape measure - one per 5 students approx • A small portable whiteboard (eg about 50cm across) and marker is useful for the teacher to sketch explanatory diagrams

Answers to questions from Activities for Students – Discussion Points: (below) 1. Compare the two measurements: The offsets are roughly 25 - 30m and 18 - 22m – the exact amounts are not too important, what is important is that students should be able to show that the higher terrace riser is offset by a greater amount along the fault than the lower terrace riser. 2. Brainstorm reasons to explain any difference in the two measurements you made: These might include: inaccurate measurements, incorrect estimation of the positions of the risers, or that the differences are actually real. 3. Relate the difference in riser offset to the earthquake history of the area: You could discuss the relative ages of the landforms – the higher terraces are older than the lower terraces. The difference in terrace riser offsets is because the The lower terrace is youngest and has had less time to be moved by earthquakes, and is therefore less offset. Extension: 4. How many ruptures of the fault would have been necessary to create each of the offsets that you measured? Eg: Roughly 20 m of offset would require 4 ruptures of 5m each. 5. Estimate the approximate age of the oldest offset terrace measured (a)For a measurement of about 25m (5 ruptures of the fault), if the last rupture was about 250 years ago, then an additional 4 ruptures at one every 750 years would make the terrace at least 3250 years old. (b) For a 30 m displacement (6 ruptures in total) this would give a minimum age of 4000 years. 6. Over many thousands of years, and over many earthquake cycles, what is the average rate (per year) of lateral movement on the Wellington Fault? Displacement of 5 m (or 5000 mm) every 750 years equals a rate of movement of 5000 / 750 = 6.6 mm/yr 7. If the Pacific and Australian plates are moving relative to each other in the Wellington region at a rate of about 35 mm/yr, what percent of this plate movement is accommodated by movement on the Wellington Fault? 6.6 mm/yr divided by 35 mm/yr times 100 equals about 18 % 8. Comment on how the rest of the plate motion is accommodated Deformation of the crust and earthquakes on the other faults in the region – Wairarapa Fault, Ohariu Fault, and the subduction interface

Activities For Students

1. Walk through California Park The Wellington Fault which runs right through the North Island is aligned along the centre of California Drive, with the houses set back on either side. Should the fault rupture, the houses will be severely shaken but at least they won't be torn apart. (Note: In other parts of the Wellington Region, houses have been built across the fault line itself.)

The path through the park runs along the northwest side of the fault. The slope just to the southeast of the path is the scarp of the Wellington Fault created by both lateral (sideways) and vertical movement of the fault during relatively recent earthquake ruptures (the fault last ruptured in a major earthquake about 200-450 years ago and, on average, it ruptures once every 500-1000 years).

Follow the footpath northeastwards towards the river. There are some green concrete survey pillars in California Park that have been put in to measure if the fault is creeping (ie moving gradually and continuously), or not. Fault lines in other parts of the world have been observed to move by creep, this means the fault gradually and continuously moves and offsets the ground surface without large earthquakes. Repeated measurements of the positions of the survey pillars in California Park show they have not moved relative to each other, therefore the fault is not creeping. The Wellington Fault is currently locked – both sides of the fault are stuck together – it is accumulating strain that will eventually be released in an earthquake.

At the Hutt River, walk slightly upstream to see where the fault is exposed in the cliff on the far side. Grey bedrock is visible in the lower part of the cliff to the left (west) of the fault, whereas on the right there is no bedrock visible (exposed), only faulted river gravel. The ground surface level is higher on the left hand side of the fault.

Cross the footbridge and follow the same line along the fault into Harcourt Park. Near the river the fault scarp is a very prominent feature. Continue along towards the playground to the sign "Wellington Fault Crush Zone" which is a good base for the next activities.

2. Sketch Map of Harcourt Park Draw a simple plan view of the path and terrace risers extending away to the sides. Focus on the terraces, terrace risers and fault scarp rather than man made objects or trees etc.

3. Measuring the offset terraces Work in pairs or small groups to measure the offset distances of the risers along the fault. You will need to estimate the position of the top edge of each riser as close to the fault as possible and then measure the separation of these positions along the fault. Start about 50m along the path from the car park, near the sign showing the “Wellington Fault crush zone".

• Stand at top of terrace risers on opposite sides of fault (footpath). Note: The top edge of each riser has been rounded off by erosion. Use your own judgement to choose the best place to measure from • Measure the separation distance along fault (path) • Repeat for the lower terrace further down the path

Discussion points

1. Compare the two measurements

2. Brainstorm reasons to explain any difference between the two measurements you made

3. Relate the difference to the history of river erosion and earthquakes that ruptured the fault in the area

Extension:

(For these calculations, assume that on average the Wellington Fault ruptures every 750 years with about 5 metres of horizontal displacement).

4. How many ruptures of the fault would have been necessary to create each of the offsets that you measured?

5. Estimate the approximate age of the oldest offset terrace measured.

6. Over many thousands of years, and over many earthquake cycles, what is the average rate (per year) of lateral movement on the Wellington Fault?

7. If the Pacific and Australian plates are moving relative to each other in the Wellington region at a rate of about 35 mm/yr, what percent of this plate movement is accommodated by movement on the Wellington Fault?

8. Comment on how the rest of the plate motion might be being accommodated by the Earth’s crust in the Wellington Region.

End of Field Trip