Sunday, 23 June 2013

Our new paper on Chilean earthquakes

Our first paper on Chilean earthquakes is now available online in the journal Quaternary Science Reviews. The paper is open access, so the full text is free for anyone to read.

The work we present sets out how we can use diatoms, microscopic algae, to quantify the changes in land level which occur during plate boundary earthquakes. We're interested in reconstructing these land level changes as they may provide a way of working out how big past earthquakes have been. Others have used similar approaches to identify and increase understanding of the threat posed by large earthquakes in Alaska, Japan and along the west coast of the United States. We apply the techniques to Chile, demonstrating that estimates of land level change using diatoms are accurate and useful.
A slide stuffed full of diatoms, viewed down
a microscope

So how does this diatom thing work then? Well, different species of diatoms prefer different environments. They all live in wet places, but some prefer salt water, while others can only live in fresh water. This means that particular species characterise different elevations within the intertidal zone. We look at samples from tidal marshes to find out which species live where. Then, when we look at a sediment core (a stack of layers of mud and sand that have been deposited over time) we can identify the changing environments through the changing diatom species.

But how is that linked to earthquakes? Earthquakes are caused by the build up and release of strain where the earth's plates meet. Where one plate is pushed beneath another, this build up and release of strain results in a cycle of changes in land level. Typically, between shocks the land slowly moves in one direction, while the inevitable earthquake suddenly reverses this. Along the coastline, this means the land suddenly shoots up or down, leaving marshes high and dry or submerged beneath the waves. Its these sudden changes, reflected by the diatoms, that we look for.

The two main conclusions of our paper are:
1. Tsunamis (which the largest earthquakes often cause) don't always leave clear sand layers full of marine diatoms (as some have previously suggested)
2. Diatoms can, however, be used to look at land level changes. Wahey!

There's another paper to follow, which uses the techniques described here to reconstruct changes in land level during some older Chilean earthquakes. Stay tuned.

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