Here we are on the shores of lake Christabel
situated in the mountains of the South Island, New Zealand.
We are here because the alpine fault
is located 7km to the west and dips away beneath us.
This fault produces great earthquakes -
earthquakes with a magnitude higher than 8.
and these lake basins
are nature's answer to the seismometer, because when the fault ruptures
in a great earthquake, the sediments that accumulate in the bottom of the lake
record deposits indicative of those
really big events that disturb the landscape.
When we choose a lake site
to investigate the first thing we do it is a geophysical survey,
and this provides an insight
into the spatial distribution
of the sediment at the bottom of the lake.
So we can identify the best sites to sample the lake sediment by coring it.
We deploy a Mackareth coring system.
The way the system works is we lower it off the side of the boat,
it travels down through the water column to rest upright on the bottom of the lake.
The coring works by pushing compressed air
which depresses the 6m length of core tube into the sediment.
To get the corer back, we have to use a salvage bag,
and usually the tonne of lift force that the salvage bag provides
brings the whole system with our intact core -
our sedimentary record to the surface.
Typically we take between 3 to 6 sediment cores
from the lake, we ship them out of the lake and transport them to the lab
where we analyze sediments.
Here we've got a section of one of our sediment cores
that we have retrieved from a lake.
Before an earthquake we have normal conditions
and we just have material raining out
of the water column, forming the layering that you can see.
We have strong shaking produced by an earthquake
and this results in deformation or the buckling and bending
of those finely layered sediments that have just recently accumulated.
The shaking also de-stabilizes material on the margins of the lake,
producing underwater landslides
that form this thick deposit that you can see here.
course and dark at the base and becoming finer as we go through the unit,
and that represents the occurrence of an earthquake.
The shaking not only disturbs the lake,
it also disturbs the wider landscape,
causing landslides on hill-slopes and the production of lots of sediment.
The rivers work to move that sediment from the catchment into the lake basin.
We can see that process depositing
these lighter coloured sediments through here.
Over decades all that sediment is
washed by the rivers into the lakes and we have a return to normal conditions.
this tiny dark fleck next to the tip of the knife
is a leaf in the sediments,
and this is all we need to produce a radiocarbon date.
Radiocarbon dating allows us to determine precisely these earthquakes occurred.
The precise dating allows us to corelate events between sites
spaced along 300+ kilometres of the Alpine Fault,
determining how much of the fault broke in individual earthquakes.