Carbon dating earthquakes
When an Alpine Fault earthquake occurs, the fault uplifts by about a metre, creating a space into which the creek brings loose rock debris (gravel, sand and silt) washed down the river from landslides caused by the groundshaking.Another Alpine Fault earthquake uplifts the scarp, and a new earthquake debris layer is deposited, adding another record to the fault rupture history.The great thing about peat layers is that they provide plenty of carbon material for radiocarbon dating.The time of past earthquakes is at the horizon where the peat changes upward into landslide derived sand and silt, so by taking samples at this layer, the earthquakes can be dated.The first picture shows how the low fault scarp blocks the stream, ponding the water and creating a swamp.This gradually fills up with carbon rich plant material (peat) so that eventually the surface of the swamp becomes level with the top of the scarp and the river flows straight across it.
These diagrams show how the amazing record of earthquakes was created at Hokuri Creek.This oblique Google Earth view of the West Coast shows the relative uplift on the eastern side of the fault that has created the Southern Alps.The Hokuri Creek location indicated on the image is the site of a very important record of Alpine Fault earthquakes going back for the last 8000 years.Records of the two or three most recent 'quakes, missing from this sequence, have been found in a more recent study at the nearby John O'Groats swamp.
You can see how scientists used a ladder to access good sampling points.
Amazingly, they were able to trace 24 earthquakes going back over the last 8000 years.