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Geosciences column: Shelter island – building a barrier to protect the coast

27 Jun

The latest Geosciences Column features recent research into tsunami hazards and explains how island building out to sea can help protect buildings on the shore…

Barrier reefs are well known for holding off the wrath of the ocean and sheltering the serene lagoons that stretch between them and the mainland. Barrier islands possess the same protective power, taking the impact of waves that have built up across the ocean and dissipating their energy before they break on the continent. Now, a team of Spanish and Columbian scientists have shown how this barrier island effect can be harnessed to protect communities from the worst of ocean waves – the tsunami.

Tsunamis are generated when vertical faults beneath the seabed slip, causing a large earthquake (over magnitude 5 on the Richter scale) and displacing a huge volume of water. They pose a greater hazard than earthquakes alone, and in seismically active coastal areas they are a significant concern. One such area is the seismic belt that shadows the coastline between Ecuador and Columbia, where the Nazca Plate subducts beneath the South American.  There have been six major quakes along the belt in the last century, the most recent of which was a magnitude 7.7 quake that resulted in a devastating tsunami and the destruction of an entire island within the Mira River Delta in 1979.

Spot the difference. Top: the island of El Guano prior to the 1979 tsunami; bottom: the same area following the tsunami. (Credit: Otero et al. 2014)

Spot the difference. Top: the island of El Guano prior to the 1979 tsunami; bottom: the same area following the tsunami. (Credit: Otero et al. 2014)

In the Columbian department of Nariño alone, the 1979 tsunami resulted in the loss of over 450 lives and 3080 homes. But the devastation would have been greater if it weren’t for El Guano, a sandy barrier island that was once present just off the country’s Pacific coast. By modelling tsunami as it happened, and how it would unfold if it occurred again today, Luis Otero and his colleagues from the University of Norte, Columbia, and the Environmental Hydraulics Institute IH Cantabria, Spain, showed just how good a barrier the island was – cutting the energy transferred to the island city of Tumaco by up to 60%.

It’s not the first time natural defences have been shown to protect the coast. Indeed, studies of the 2006 Boxing Day tsunami in Indonesia have shown that reefs, mangroves, beaches and dunes all provide the coast some protection by absorbing the tsunami’s initial impact and slowing the speed of the advancing wave.

How flooding would differ if El Guano island was present: (a) shows the current situation and (b) shows what would happen if the island was recreated. The white regions represent the areas that are not flooded and the black line shows the shoreline. (Credit: Otero et al. 2014)

What a difference an island makes: (a) shows the current situation and (b) shows what would happen if the island was present. The white regions represent the areas that are not flooded and the black line shows the shoreline. (adapted from Otero et al. 2014)

Tsunami hazard in this region is both high and likely, and the team show that rebuilding the island would be a worthwhile engineering effort if the government hopes to afford the area the same protection it had in ’79 in the future. Elongating the island would increase its protective potential even further, as would reshaping the it to form three similarly shaped barriers to cut the energy transferred to the Columbian coastline beyond.

Otero’s tsunami model showed such engineering would offer tremendous protection to Tumaco and the other inhabitants of the Mira River Delta in the event of a tsunami – particularly one that occurred at high tide. But because Tumaco is such a sizable coastal city, some unprotected areas would remain.

Currently, the government’s focus is on establishing a swift and effective early warning an evacuation strategy, but a barrier island could provide a big boost to the safety of the local population and the security of local infrastructure.

By Sara Mynott, EGU Communications Officer

Reference:

Otero, L. J., Restrepo, J. C., and Gonzalez, M.: Tsunami hazard assessment in the southern Colombian Pacific basin and a proposal to regenerate a previous barrier island as protection, Nat. Hazards Earth Syst. Sci., 14, 1155-1168, 2014.

Imaggeo on Mondays: Shaken, not stirred – sediment shows signs of past earthquakes

23 Jun

Nore Praet, a PhD student from Ghent University in Belgium, brings us this week’s Imaggeo on Mondays. She sets the scene for an investigation into past earthquakes and explains how peering through a lake’s icy surface and its murky waters, and into the sediment below can help scientists find out more about the impact of earthquakes in the future…

Early this year, I set off with a group of scientists (including Koen De Rycker, Maarten Van Daele and Philipp Kempf) from the Renard Centre of Marine Geology to conduct fieldwork in South-Central Alaska. The reason for our stay was the search for past megathrust earthquakes (earthquakes produced by the subduction of an oceanic tectonic plate under a continental plate), which had an unusually high magnitude and destructive power.

The most recent Alaskan megathrust earthquake was the Great Alaskan Earthquake in 1964, which represents the second largest earthquake ever instrumentally recorded (9.2 on the moment magnitude scale). In order to have an estimate when such a large earthquake may strike again, we need to study the recurrence pattern of past earthquakes. Since megathrust earthquakes typically have recurrence periods of several centuries, historical archives will not suffice. This is where natural archives, like lake sediments, come in. These records have the advantage of going much further back in time, and they are what brought us to Alaska’s Eklutna Lake.

Zooming in on some individual ice crystal aggregates (few centimeters across) and geometric frost patterns on the frozen surface of Eklutna Lake in Southern Alaska. (Credit: Nore Praet via imaggeo.egu.eu)

Zooming in on individual ice crystal aggregates (few centimeters across) and geometric frost patterns on the frozen surface of Eklutna Lake, Alaska. (Credit: Nore Praet via imaggeo.egu.eu)

Lake sediments can contain very distinct earthquake traces because seismic shaking produces underwater landslides that leave well-defined sediment deposits in the lake basin.

Long sediment cores, extending some 15 metres through these deposits, make it possible to construct a palaeoseismic record. From this we can make an estimate of the recurrence rate of megathrust earthquakes. This will be crucial for understanding seismic hazard in South-Central Alaska and, in particular, in the densely populated city of Anchorage.

This photo, taken in early February, captured the moment where we officially started the fieldwork, after checking the thickness of the ice and its stability.

The constant struggle with the almighty Alaskan cryosphere was the real common theme during the fieldwork. The freezing cold, together with ice formation on the coring equipment, seriously hampered the efficiency of the coring operation. It took some time to accept these conditions and adjust to the demanding laws of this harsh wilderness, but once you are willing to invest the energy into working here, every day nature surprises you with her astonishing beauty.

By Nore Praet, Ghent University

Imaggeo is the EGU’s open access geosciences image repository. Photos uploaded to Imaggeo can be used by scientists, the press and the public provided the original author is credited. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. You can submit your photos here.

It’s not my fault

1 May

A line on a map is important. In the Beverly Hills region of Los Angeles a series of mapped fault lines are now the cause of a major controversy. Communities have been alarmed, money has been lost and legal proceedings are ongoing.

It started in 1992. James Dolan and Kerry Sieh, two earthquake geologists at the California Institute of Technology, published a map in a field trip guidebook about the Los Angeles Basin. The map in question included suggestions of two potentially active earthquake faults in the north of the basin – the West Beverly Hills Lineament and the Santa Monica Fault – though neither of them had been confirmed by detailed study.

In the two decades since the situation has slowly spiralled out of control. Dolan and Sieh’s original map has been included in a whole series of academic papers and in 2005 the faults were added –as dotted lines – to an official map from the United States Geological Survey. Then, in 2010, the state published a map in which it labelled both faults as ‘active’.

Map released by the Los Angeles metro company at the press conference in 2011. Red lines show suggested active faults and BHHS is the Beverly Hills high school.

Map released by the Los Angeles metro company at the press conference in 2011. Red lines show suggested active faults and BHHS is the Beverly Hills high school.

Eldon Gath, from Earth Consultants International, has been reflecting on the escalating situation at the EGU General Assembly 2014. “What was a hypothesis,” explains Gath, “became a fact without any new information. Every subsequent map has reproduced these faults without even questioning whether they are right.”

These careless assumptions have now come back to haunt the people of Beverly Hills. Metro, the Los Angeles underground company, are proposing a new ‘Subway to the Sea’, an underground line from Los Angeles to the coast that would pass directly through Beverly Hills and Santa Monica. In 2011 they held a press conference to announce the results of some geotechnical surveys they had commissioned and duly claimed that a whole series of active earthquake faults ran through the proposed train route and, crucially, right under the local Beverly Hills high school. The metro company also proposed a new route for their underground line to avoid some of the faults.

Beverly Hills high school. (Credit: Eldon Gath)

Beverly Hills high school. (Credit: Eldon Gath)

“It came as a big shock to everyone,” says Gath, “this was dropped on the local community at a press conference and there was no advance notice that the map was going to be released. The high school found out about this fault map when the Los Angeles Times called them. That’s just not good public relations.”

Immediately after the press conference the school decided to launch its own investigations. It commissioned new boreholes and a trench that stretched from the front door of the school right across the proposed location of the fault. If the fault was there, then the trench would reveal layers of sediment that had been offset in previous earthquakes. But nothing was found; 50,000 year old sediments were entirely undisturbed. At a nearby location one fault was found, but the sediment ages revealed that it hadn’t moved for at least the last 200,000 years. After all the disruption, it appears that the faults don’t even exist.

Trench excavated in front of the school. No evidence of past earthquakes was found in the trench. (Credit: Eldon Gath)

Trench excavated in front of the school. No evidence of past earthquakes was found in the trench. (Credit: Eldon Gath)

To date the school has spent an estimated three million dollars on these studies and at least three other property owners in the vicinity have commissioned similar work. “It’s unfortunate that they had to spend so much money,” says Gath, “but the lawyers are now feasting at the trough.” Multiple lawsuits have been filed and a couple of cases have already been successful.

But why were these mistakes allowed to happen? “I think there was a paradigm-driven interpretation,” explains Gath. In the original work done by the metro company, “every kind of mismatch was interpreted as a fault. It was a fault-driven interpretation. But they didn’t pay any attention at all to the sediment ages.” It is also alleged that competing financial and political interests have played a role.

The current situation can hardly be blamed on Dolan and Sieh’s original map, but their initial tentativeness has been lost somewhere in the Chinese-whispers-style translation.

In the future, we must make sure that someone takes responsibility for the faults.

By Tim Middleton, University of Oxford

Reference:

Gath et al., 2014: The West Beverly Hills Lineament and Beverly Hills High School: Ethical Issues in Geo-Hazard Communication. Geophysical Research Abstracts, Vol. 16, EGU2014-1131-1

GeoTalk: Steven Smith on fossil faults and fantastic faulting

24 Apr

This week in GeoTalk, we’re talking to Steven Smith, a Lecturer from the University of Otago. Steven takes us on an Earth-shaking journey, explaining how ancient faults reveal what’s happening under the Earth’s surface and delving into the future of fault zone research.

First, could you introduce yourself and tell us a little about what you are currently working on?

Last September I started as a Lecturer at the University of Otago in New Zealand. My work focuses on understanding the structure and evolution of tectonic fault zones in the continental crust. I graduated from Durham University (England) back in 2005 and remained there to complete my PhD, studying a large fault zone exposed on Italy’s Island of Elba. My Italian connection continued as I then moved to Rome for 4 years to undertake a post-doc with Giulio Di Toro and Stefan Nielsen at the INGV (National Institute of Geophysics and Volcanology). In Rome I was working on a large group project funded by the European Union. The project involved integrating field and experimental work to better understand some of the extreme deformation processes that occur in fault zones during earthquakes. When my post-doc finished, I relocated to New Zealand, attracted by the research opportunities available here and the wonderful places waiting to be explored!

Steve on the summit of Lobbia Alta (3,195 m), a peak in the Adamello area of the Italian Alps. This area is part of a large Tertiary magmatic batholith that is cut by several fault zones Steve studied as part of his post-doc work. (Credit: Steven Smith)

Steve on the summit of Lobbia Alta (3,195 m), a peak in the Adamello area of the Italian Alps. This area is part of a large Tertiary magmatic batholith that is cut by several fault zones Steve studied as part of his post-doc work. (Credit: Steven Smith)

Last year, you received a Division Outstanding Young Scientists Award for your work on the structure of shear zones. Could you tell us a bit more about your research in this area?

I work mainly on so-called “exhumed” fault zones – those that were once active and have been brought to the surface of the Earth by millions of years of uplift and erosion. By studying the structure of exhumed fault zones we can understand a lot about the physical and chemical processes that are active when faults slip. I have worked on fault zones in Europe that were exhumed from both the middle and upper crust. I’m particularly interested in the crushed-up rocks in the cores of fault zones (better known as fault rocks) and what these can tell us about fault zone rheology and deformation processes.

In the last few years, we have identified a number of previously-unrecognised textures in the cores of large normal and thrust faults in Italy. These include faults with highly reflective “mirror-like” surfaces and small rounded grains resembling pellets. Our work focused on trying to understand the significance of these textures, and in doing so we had to develop new experimental methods to deform fault zone materials under more realistic conditions. By comparing our field observations to the experimental data, we realised that the textures we identified probably represent ancient “fossil” earthquakes preserved in the rock record.

When thinking of a fault zone, you don’t often think small. How can you scale down what’s going on to create a laboratory-sized experiment?

That’s a very good question, and something that experimentalists have to be aware of all the time. It’s generally not possible to reproduce in the laboratory all of the conditions that a natural fault enjoys, which is why laboratory work ideally has to be integrated with other data, such as field observations of natural faults and theoretical modelling. But it is possible to perform experiments at quite realistic pressures and temperatures, and some deformation apparatus can deform fault zone samples over a very wide range of slip velocities – the sort you would expect during the seismic cycle along natural faults.

Laboratory experiments allow us to produce data on rock strength and fault zone behaviour that simply wouldn’t be possible by any other means. At the same time, it’s important to bear in mind that a small laboratory experiment might represent the behaviour of only a single point on a much larger fault surface – that’s where complementary field and geophysical observations come in. Looking at fault geometry and fault zone evolution over time helps put lab studies into context.

Admiring the inner workings of a fantastic fault on the Italian Island of Elba. (Credit: Bob Holdsworth, Durham University)

Admiring the inner workings of a fantastic fault on the Italian Island of Elba. (Credit: Bob Holdsworth, Durham University)

How does deformation differ across the world’s fault zones?

In the last few years, seismologists have detected different types of slip behaviour along active faults, from those that creep along steadily at rates of a few millimetres per year to those that fail catastrophically in earthquakes. In between there are other types of slip behaviour seen in seismological signals (such as seismic waves) from the deeper parts of fault zones like the San Andreas fault and the Alpine fault in New Zealand. Understanding the basic controls of this rich variety in fault slip behaviour is one of the key goals of modern earthquake science. Geologists studying exhumed fault zones have also long recognised that fault zone structure in the crust is very complex and highly dependent on factors like the type of host rock, level of exhumation, availability of fluids and so on. A future goal for geologists like me is trying to understand whether different types of fault slip behaviour recognised in modern-day seismic signals are preserved in the structure of fault zones in some way.

Is digital mapping an essential tool for the modern-day structural geologist? How does it help?

As in most branches of science, digital technologies are now increasingly used for geological mapping purposes, although a majority of universities still favour the traditional pen-and-paper approach to training students in geological mapping. In many mapping situations – ranging from reconnaissance-scale mapping to detailed surface topography measurements – there are a number of benefits to a digital approach, the main one being that field data are automatically located using GPS or laser-based technology. This opens up a range of possibilities to map and study 3D structures that would be difficult and time-consuming, if not impossible, using more traditional approaches.

I certainly think that digital technologies will play an increasingly important role in geological mapping and research in the future, and the technology will inevitably become more fit-for-purpose and affordable. I recently taught a short workshop on digital mapping to a group of 4th year students in Otago; after a few days they were all quite confident using handheld computers and GPS devices to map, and I think they were impressed by the ease with which the digital data could be analysed to better understand the inherent complexities of geological structure.

Finally, will you be working on faults in the future or moving on to pastures new?

I think there are a lot of important research problems to tackle in fault zones, so it’s likely I’ll continue working on faults in a broad sense in the future. The devastating Tohoku-oki earthquake and tsunami in Japan in 2011 highlighted that our understanding of fault zones is far from complete. My post-doc experience has convinced me that integrating field and experimental work is a promising way to understand fault zone processes. At the moment I’m particularly interested in the very-small scale frictional processes that occur on fault surfaces during earthquakes, and I’m currently using some of the analytical equipment here in Otago to study experimental samples that were deformed under earthquake-like conditions.

If you’d like to suggest a scientist for an interview, please contact Sara Mynott.

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