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.

Ed heads North

Ben, Daria and Isobel investigate the stratigraphy

Following Emma's departure back to the UK, I headed north from Valdivia for part 2 of my field season in Chile. Three buses, each slower and more decrepit than the last took me to Temuco, then Carahue and finally Tirua. Here, I met up with Marco Cisternas again, along with Lisa Ely, Alan Nelson, Rob Wesson and Isobel Hong from the US. Ben Horton and Daria Nikitina swelled our numbers to 8 when they arrived yesterday.

We've spent the last four days working at Quidico, a small river valley 20 minutes drive north of Tirua. We've been digging deep pits, cleaning river bank exposures and taking thin columns of sediment using a coring device to try to understand the sequence of sediments that has been laid down at the site. The presence of sand layers may suggest inundation by tsunamis.

As the end of the first field season approaches

After almost three weeks in Chile, the end of the first part of our field campaign is approaching. Emma is leaving tomorrow and Ed is travelling around the Arauco area (further north) and then Chiloe (further south) for another couple of weeks of field work with colleagues from the US and Chile.

All in all it's been a successful first field season - we've been to 6 new sites, collected 225 modern marsh samples, and nearly 4 metres of sediment to analyse when we get back. That's going to be a lot of time spent looking down a microscope counting diatoms!!

Tiny bits of rock and very small living things...

Taking great care to not leave any trace of
where we've sampled on the marsh!

Our next stop has been Rio Lingue, a site about 60 km north of Valdivia. Locals have been telling us of the devastation and land subsidence caused by the 1960 earthquake, and also told us how the 2010 tsunami reached this far south.

We found an excellent marsh to work on, which allowed us to take lots of samples to help us properly understand the present day environment. In our work we need a sound understanding of what is happening today to be able to interpret what happened in the past. We take small samples along a transect across the marsh from the mudflat exposed at low tide, across the low marsh, high marsh, and up to where freshwater vegetation grows. We look at how diatom communities (small microscopic single-celled algae) change across these different zones as the amount of tidal inundation decreases the further you get from the sea. We mainly find marine diatom species in mudflat samples and freshwater diatoms in the high marsh samples. Then when we look at our sediment cores from the past, if we find lots of marine diatoms we interpret the environment at this time was most like the present day mudflat. From this we are able to reconstruct how the land level has changed through time.

Taking a modern sample from the low marsh

Freshwater diatom of the Pinnularia genus (about half the width of a
human hair - thanks to Melanie Leng for giving this comparison!)

A few less common words (part 2)

Following on from part 1, here's the rest of the explanation of the common word summary of our work. We were delayed posting this by a wild goose chase to find a highly inaccessible site...

When the shaking happens the land can go up and down. Some places change from being high up before the shaking to being lower down after. Other places change from low down to higher up.

We're describing the vertical component of coseismic deformation resulting from a subduction zone earthquake. This is typically less than a few metres (either uplift or subsidence) for sites on the Chilean mainland. R. Grapenthin, Univ. Alaska, niftily visualises this phenomenon for the 2011 Japan earthquake.

We look for things that can tell us how and when the land has changed in the past and when waves have covered the land. To do this we look at tiny bits of rock and very small living things.

We're particularly interested in working out when past earthquakes and tsunamis have happened and how big they've been. As part 1 mentioned, historical records may be too short to provide a realistic assessment of seismic hazards. Instead, we turn to sediments (tiny bits of rock) from coastal environments. Certain low energy settings, like the tidal marshes north of Valdivia, preserve evidence for both tsunami inundation and coseismic deformation. The very small living things are diatoms, single-celled algae. More on how we use diatoms to look at land-level change in the next blog post.

We have found the biggest shaking and largest waves happened four times in the past 1000 years in the area we are working.

Ed's PhD, based on sites a few hundred kilometres south of this project, found sedimentary and diatom evidence for the 1960 and 1575 earthquakes, as well as two older earthquakes predating written records in Chile.

A few less common words (part 1)

We thought we'd expand on our last blog post today and use some less common words to explain what we're up to. There were many other great common word distillations of projects, research areas and whole careers in science generated in response to the Up-Goer Five text editor. You can read them in the tumblr curated by Chris and Anne from Highly Allochthonous.

So, here's what we were trying to convey in the thousand word post (or ten hundred as those pesky Americans keep saying). The common words are in italics.

When the ground shakes a lot it can kill many people.

Earthquakes! You got that bit, right?

Ground shaking can also form very big waves of water which cover lots of the land, and kill more people.

Tsunamis. Created in this instance by strain release during an earthquake resulting in deformation of the sea floor. Tsunamis can also be formed by landslides, volcanic eruptions, meteorites and iceberg roll events.

It is important to understand when and where this happens so that we can make people safer in years to come.

The past is the key to future preparedness and hazard mitigation. We want to know how frequently earthquakes and tsunamis occur and their maximum possible sizes. Historical records are valuable, but in many areas do not go back far enough. Even in Japan, home to the most comprehensive records of tsunamis, historical records may underestimate the size of previous seismic events. See this from the Japan Times for more.

Part 2 to follow...

Our research using only common words

Here's a summary of our past and present research in Chile using only the 1000 most common words used in the English language:

When the ground shakes a lot it can kill many people. Ground shaking can also form very big waves of water which cover lots of the land, and kill more people. It is important to understand when and where this happens so that we can make people safer in years to come.

When the shaking happens the land can go up and down. Some places change from being high up before the shaking to being lower down after. Other places change from low down to higher up. We look for things that can tell us how and when the land has changed in the past and when waves have covered the land. To do this we look at tiny bits of rock and very small living things. We have found the biggest shaking and largest waves happened four times in the past 1000 years in the area we are working.

Thanks to Chris Rowan and Anne Jefferson of the Highly Allocthonous blog for passing on the idea. Head over to the Up-Goer Five Words Box to try it for yourself.

Marshes, mud and crazy postmen

Since the last post, we've had a successful few days in Queule, Tolten and Rio Lingue (about 80km north of Valdivia). These locations lie on a wide coastal plain; subsidence of a couple of metres during the 1960 earthquake changed low lying but valuable farmland into waterlogged marshland. The tsunami swept up rivers and across the coastal plain, reaching several kilometres inland. As the area is so flat, with few safe higher areas, escaping the waves would have been difficult.

We collected lots of samples of sediment from intertidal marshes close to the mouths of the rivers that cross the plain. We're particularly interested in tidal marshes as they provide evidence for how the land or sea has changed in level over time. On a positive note, we found some good sites and collected lots of samples. Unfortunately we seemed to spend a lot of our time watching the tide go up and down, sometimes until it got too dark to actually see what we were doing!

Ed at Queule with sediments that we think
were deposited before and after the 1960
earthquake and tsunami

We've also been back to Valdivia and sampled a marsh on Isla del Rey, an island in the Valdivia Estuary. We're pretty pleased with the samples from here, not just because we found a good site, but also because we managed to get to the site in the first place! First we needed to find a boat to take us there, then were trapped and had to escape from a very talkative postman on the island (who wanted to tell us everything from the entire history of Chile to the material used to surface the road in response to us asking who owned the land), and finally we needed to get a boat back. The latter proved to be the most tricky, but it seems in Valdivia it's possible to hail a boat like you hail a taxi!

What are we looking for?

After a few more days looking for potential sites, we thought it was probably time we gave an update on what makes a good site and what exactly we're looking for...

Ed digging a pit / pogo-sticking!

So we're working on tidal marshes which preserve evidence of previous great earthquakes. During the largest subduction earthquakes, land may suddenly subside or uplift by up to a few metres. This coseismic deformation results from extension of the upper plate during the earthquake (as in this diagram from Natural Resources Canada). The area that we are working in subsided during the 1960 earthquake. Low lying land subsided into the sea, inundating farmland and killing trees (see picture 3 of our last blog post). 

10cm of tsunami sand (light grey)
separating two different types of sediment

We have been digging pits in marshes and looking at incised banks of rivers in the hope of finding abrupt changes in sediment type which might reflect the sudden changes in the level of the land with respect to the sea. A change from a soil with lots of organic matter to a sandier, less organic sediment could reflect a change from a freshwater to a tidal flat with no vegetation. 

Fieldwork so far in pictures

Rudolfo at Valdivia's Seismological Museum

The local neighbourhood in Valdivia!

Trees killed by the 1960 Valdivia earthquake

Tsunami deposits south of Valdivia

Too many horseflies!

Highs and lows of Chaihuin

Today we travelled further south from Valdivia to the site of Chaihuin, on the Pacific coast. There's a good marsh here and even better, far fewer horseflies than at previous sites! After a few problems ascertaining land ownership due to our miserable grasp of Spanish, we eventually got onto the marsh to sample. We collected lots of samples from the present day marsh, as well as finding tsunami deposits and some promising older sediment sequences.

Just a shame about the 3 hours it took us to get back, after getting stuck behind a lorry full of shells travelling at 10 km/hr and it taking an hour and a half to get a ferry across the estuary!

Valdivia and the 1960 earthquake

We've travelled 800km south from Valparaiso to Valdivia, the city which gives its name to the largest earthquake since the beginning of instrumental records over a century ago. The Valdivia earthquake of 22nd May 1960 measured 9.5 on the moment magnitude scale. The resulting tsunami crested over 25m high in Chile and caused fatalities across the Pacific. The release of built up strain also caused the south American plate to deform, with the area around Valdivia sinking by over two metres. Sea water inundated forests, meadows and parts of the city. You can still see dead trees killed by the salt standing in the estuary.

Yesterday we visited the Seismological Museum in Valdivia, which does a fantastic job of explaining the causes and effects of the 1960 earthquake and the development of modern seismology.

We're heading out around now to see if we can find sedimentary evidence of the Valdivia earthquake and tsunami.

Valparaíso

Emma, Marco and Ed

A productive first day in Chile. We spent most of it with Prof Marco Cisternas of the Catholic University of Valparaíso. He's provided a lot of useful insights into the sites that we will be visiting over the next few weeks.

Pleasingly the School of Marine Sciences smelt of the sea; however, lunch looked distinctly like chicken! Marco's office looks straight out over the bay, though you'll have to squint to see it in the photo.

We're now heading south on the overnight bus to Valdivia for the start of the fieldwork. Thanks to Ulises at Camila 109 (where we stayed last night) for helping us navigate Chilean bus ticket booking.

Welcome to Chile

We (Ed and Emma) are in Chile looking for sedimentary evidence for past earthquakes and tsunamis. This is the first field season of our new Chilean Palaeoseismology project. We'll be tweeting in the field from @EarthquakeEd and @QuakeHunting and will try to keep this blog up to date with longer posts and photos.