Beach Ridges, Swamps and Tsunami

Last year, geologist Katrin Monecke and co-authors published a paper in Nature that showed the last tsunamis in Aceh occurred in 1907, ~600 years ago, and ~1000 years ago (Monecke et al., A 1,000-year sediment record of tsunami recurrence in northern Sumatra, Nature 455, 30 October 2008).

One of the areas described in the paper is shown above. This is a landscape of beach ridges separated by low-lying, swampy swales. It is in these swales that the paleotsunami deposits are found. In March 2009, we went back into the field to collect data to help determine how these ridges were formed.

One of the interesting aspects of these deposits in this area is that the oldest deposits (from 1000 years ago) occur only in the swales furthest from the modern beach. So, where was the beach when these sediment were deposited? How much has the coastline grown outwards since their deposition?

To answer these questions, we will attempt to date the ridges using thermolumniescence. Some minerals, when exposed to light, store heat in their crystal structures. When these samples are heated again, they emit light (luminesce) in a way that is related to how long ago they were removed from the light source. Really smart people can then determine the age of a sample based on the light the mineral emit when heated. Pretty cool!

We collected several samples from a village north of Meulaboh, NAD (Aceh Province), to see if this method is feasible here. If so, a group of researchers from the Netherlands will come back and do a full dating campaign, and by determining the timing of ridge formation, we can possible determine how the ridge was formed.

When you see a repeating pattern in nature, such as the ridges pictured above, there is often a recurring event that is responsible for their formation. Terraces step down the mountains around Santa Cruz, California, that we formed by changes in sea level. Rings around corals in the islands offshore Sumatra are formed when earthquakes lift the coral partially out of the water, killing the top animals of the colony. Following the 2004 earthquake, the coastline in this area subsided several tens of centimeters, so it begs the question- Could subduction zone earthquakes somehow be responsible for beach ridge formation?

Doubt it.

James Goff, director of the Australian Tsunami Research Centre, showed that in New Zealand, beach ridges are formed when pulses of sediment are delivered to the coast following massive landsliding triggered by earthquakes. There was not widespread landsliding following the 2004 earthquake, so it is unlikely that there would be the sediment available to create another ridge. Furthermore, as the coast subsided, a lot of land was actually lost, presumably to the offshore. Please, prove me wrong- it would be really helpful if these ridges were actually related to the subduction zone earthquakes!

It might be that the beach ridges are formed following earthquakes on the Great Sumatra Fault- the big strike-slip fault that runs through the Sumatra highlands. If the dates from the ridges correlate with this earthquake history, then it is likely that landslides in the highlands provided the sediment for their formation, and that they are just a convenient "catcher's mit" for tsunami sediment.

Or, perhaps they were formed by volcanic activity, as suggested by Stanford geology student Mindi Summers in her senior thesis on beach ridges in New Zealand. Another day....

Offshore Landslides and Tsunami

Could this landslide from the continental slope offshore Oregon generate a tsunami?

The answer is simple. Maybe...

It depends on several factors. First off, is it big enough? Tsunamis are generated by rapide movements in lots of water, be it from earthquakes, asteroids, volcanoes or landslides. A pebble thrown into the ocean is certainly no asteroid.

And while even a small event can generate a very small tsunami, it depends on the location of whatever it was that moved the water as to whether the tsunami will be noticed or not. Landslides, volcanoes and even asteroids are "point-sources", while and earthquake moves a very large area of seafloor (hence a lot of water). Waves generated by point source attenuate or decay much faster than those generated by a lifting up a vast area of seafloor. In other words, it is very difficult for a tsunami generated by a landslide, volcano, or asteroid to make it all the way across an ocean.

Yet another complication, as with many things, is timing. Most very large tsunamis are very old- in nature, big events tend to happen infrequently. Should very large offshore landslides occur as frequently as large-magnitude earthquakes, the continental slope would quickly eat its way onto land! So, while technology has allowed us to image all sorts of amazing landslides on the world's continental slopes, many of which appear very young, they tend to be old, and pose little danger to communities on shore.

(One might ask then, when do they occur? Most offshore landslides occur either during sea level lowstands or during times of changing sea level. During glacial periods, sea level is lower, and rivers dump heaps of sediment directly on the continental slope, providing fuel for landslides. During highstands, like now, sediment is sequestered in estuaries and in foreland basins on the continental shelf. If earthquakes trigger landslides, then the hypothetical first earthquake after a sea level lowstand would shed all the sediment off, leaving nothing left to fail! But, this is just a digression by this marine geologist....)

Perhaps the last unconstrained variable (i.e. something we have no idea of!) is once a landslide begins, how fast does it actually fail? Most models that predict tsunamis assume that the landslides are frictionless- that is, once they get going, there is nothing slowing them down. This isn't likely the case, but nonetheless, landslides occur offshore, and they generate tsunamis.

There are two clear examples of offshore landslides that have generated tsunami. The first (that I know of) was 7,200 years ago offshore Norway called Storegga. Following the last sea level lowstand, glaciers provided sediment to the continental slope, and as the land rebounded from having the weight of the glaciers released, earthquakes fired off, shaking loose the sediment. The failure generated a tsunami that laid a deposit in many places surrounding Norway, including underneath the greens at the famous golf course at St. Andrews.

The second example was much more recent- in 1929 a magnitude 7.2 earthquake triggered a landslide offshore Newfoundland, Canada in the Grand Banks. We know there was a landslide because it severed submarine communication cables going between North American and England. Not long after the earthquake, a 7.5 m tsunami hit Newfoundland's Burin Peninsula, killing 28 people in Canada's worst earthquake-related disaster.

So, the most important thing we can do to plan for landslide generated tsunami is to constrain the properties that trigger landslides. What is the strength of the sediment in question? Has there been landslides in this region in the past? What is the frequency and magnitude of earthquakes in the region? Is there a weak layer that might provide a sliding surface for a failure?

To address these questions, geotechnical engineers, geologists, geophysicists and seismologists must come together to address this complicated, technical question. Not one of these scientists can address this working in a vacuum, nor does any one of these possess the necessary knowledge and skills to really be effective. Yet a community working together, making the necessary data and information freely available, can make advances towards protecting life and property in leaps and bounds. A kind of scientific open-sourcing.

Paleotsunami

When a tsunami comes on land, it usually brings a lot of sand with it, depositing it on land as the wave slows, then retreats back to the ocean. These 'tsunami deposits' are a powerful tool for determining the recurrence interval of tsunamis on a given coastline. Now, researchers are working on determining the size of the tsunami that left the deposit based on the distribution of sand grains of different sizes.

In the 1980s, researchers in the Pacific Northwest lead by Brian Atwater of the US Geological Survey, discovered sand layers buried in marshes up and down North America's west coast- from northern California to Vancouver Island. These deposits were lain by a massive tsunami that hit around 1700 AD according to carbon-14 dating, confirmed by a tsunami that hit Japan that year. This groundbreaking work brought to light a hazard that was previously unrecognized in this region.

Recent discoveries in Thailand (pictured above) and Aceh Province show that large tsunamis occur in this region every 300-600 years. There was one event that hit Aceh in 1907 that was not recorded in Thailand, but we do not know how large that event was. Our team is planning to go back to Aceh to collect more detailed sediment samples with the aim of calculating the magnitude of past tsunamis.

These data will be critical to those planning for future events. Right now, many planners and NGOs are planning for a 2004-scale event every 45 years. A magnitude 9.2 earthquake generating a basin-wide tsunami that kills 250,000 people every 45 years! Perhaps we should air on the side of caution here, but resources could and should be directed toward more critical needs that might be overlooked in the face of such a potential disaster in the relatively near future.

Aceh Tsunami Height

The tsunami that hit Aceh in 2004 was a BIG tsunami.

Wave heights averaged between 10-15 meters at the coast for close to 250 km of coastline between Banda Aceh and Meulaboh.

The highest water mark was at Leupong- 35 m. This is the number that people often quote as being the "size" of the tsunami.

As you can see from the picture, this is where the tsunami splashed against a cliff, scouring the soil and trees off to the bedrock. To the right of the picture, the "trim line" drops back down to the 15 m height that is common along the coastline.

The 2004 tsunami was a massive wave, both in its height on the coastlines of Aceh, Thailand, India, Sri Lanka, Somalia and the others, and by its basinwide nature. But it is also important to recognize that this is NOT the event we should be planning for. This event occurs once every 100-500 years. There has been 3 tsunamis averaging 3-8 m since 2004 that killed over 1,000 people. These events are not yet predictable, but simple coastal planning including early warning systems and healthy coastal ecosystems can help communities weather the storm.

Mangroves

With all the best intentions, NGOs and governments planted thousands of mangrove in and around Banda Aceh. Mangroves are extraordinary ecosystems in that they provide a range of services, from nurseries for fisheries, silt filtration that keeps coral reefs healthy, wood for people, and buffers against storm waves and possibly tsunami. As such, they should be conserved wherever possible.

One issue surrounding mangroves is that they often come into direct conflict with other land uses. Aquaculture (fish and shrimp ponds) use intertidal zones where mangrove often live to produce fish for local and export products. This can often provide jobs for the local population. Studies have shown, however, that the majority of earnings from these businesses goes to larger companies outside the area (those with the resources to actually build the facilities), and that the food product are usually exported, adding little to the local food security.

So, the question in a place like Banda Aceh is this- After the entire intertidal system was wiped clean by the 2004 tsunami, should it be restored to wetlands and mangroves so that the region can benefit from the services they provide, or should the fish ponds be restored so that jobs and export income can come to the region?

I don't think there is a clear answer to this question. Many NGOs supported the mangrove replantings schemes like the failed one shown above, but did not consider how the changes in the environment would affect the success of the replanting scheme. Nor do they necessarily consider the population that needs to rebuild their livelihoods, be them environmentally sustainable or not.

One very helpful tool to address this question would be to place a value on the mangrove ecosystem and compare it to the income generated by aquaculture. This "ecosystem valuation" would provide numbers that NGOs and governments could sink their teeth into when considering coastal redevelopment policy. It would also provide critical information when reconsidering what types of coastal zone jobs should be invested in following a disaster of this scale and even smaller.

For more information on ecosystem valuation, check out The Natural Capital Project at Stanford University, http://www.naturalcapitalproject.org.

Vertical Evacuation in Padang, Sumatra

This is not Padang.

This is Banda Aceh, on the northern tip of Sumatra, Nanggroe Aceh Darussalam, Indonesia. Over 100,000 people died here during the 26 December 2004 tsunami. Aid money flooded the region after the tsunami, funding projects like the massive vertical evacuation structure pictured above. Such a structure is not only effective in the face of another tsunami, but acts as a constant reminder of the threat that exists offshore.

Large earthquakes and tsunamis hit the north Aceh coast in 1907, and before that some 350 years earlier. Large earthquakes occurred elsewhere on the northern part of the Sunda subduction zone in 1887, 1881 and 1941. In 1833, the section of the Sunda arc offshore Padang ruptured in a large magnitude earthquake, sending a tsunami toward the then sparsely populated Padang. Should the same event happen today, the effects could be catastrophic.

Padang is the largest city on Sumatra's west coast- population ~800,000 or 3 to 4 times that of pre-tsunami Banda Aceh. Like Banda Aceh, Padang occupies flat coastal plain that offers little refuge from an incoming tsunami. And unlike Banda Aceh which has (had) a couple of kilometers of wetlands with aquaculture prior to the tsunami, most of Padang's population lies within 3 km of the coast.

A group of students from Standford University's Structural (http://www.stanford.edu/group/strgeo/) and Earthquake Engineering (http://blume.stanford.edu/) are working with Geohazards International (http://www.geohaz.org) and the Stanford School of Earth Sciences (http://ses.stanford.edu) to design a vertical evacuation structure in Padang, Indonesia. These groups are partnering with a local NGO, KOGAMI (http://kogami.multiply.com/) who is working on educating people toward disaster preparedness. Their goal is to design a building that could withstand an earthquake and serve as a refuge in case of tsunami.

For more information on this project, contact project manager Veronica Cedillos at Geohazard International.

Welcome to The Tsunami Project

The mission of the Tsunami Project is to explore the contributions that earth scientists, civil engineers, ecologists working with social scientists, including economists, sociologists and anthropologists can make to tsunami hazard risk reduction. These academics will then collaborate with practitioners from NGOs, international agencies, and governments to institute thoughtful and sustainable coastal zone policies.

While the project will focus on risk reduction in areas exposed to tsunamis, many of the mitigation techniques are effective against cyclones and the slow-onset climate change factors. Only by inter-disciplinary cooperation can we reduce the impacts of disasters in the coastal zone.