The Earth Story

An island expands and shrinks

On December 25, 2004, the island at the center of this photograph was only as big as the area covered by trees.

The next day, the island was this size.

This island sits off the coast of Sumatra above the Sumatran-Andaman subduction zone. On December 26, 2004 that fault broke, leading to the disastrous 2004 tsunami and producing one of the largest earthquakes in recorded history.

The energy released in an earthquake is stored in rocks over hundreds of years. As tectonic plates move, they grind against each other and the rocks begin bending. In a subduction zone, these forces bend some parts of the plate downward and push other parts upward.

When the fault finally breaks, all the land that was bent downward suddenly pops loose and snaps back. That snapping releases the energy we feel as an earthquake and releases the land back to its original position. When the December 26 earthquake happened, this island and many around it popped up, suddenly exposing areas that were formerly submerged, often damaging or killing coral heads that grew in the shallow water during the hundreds of years between earthquakes. This island more than doubled in area in an instant.

If you came back to this island today, plant life would probably have started growing in the areas that were exposed in 2004, but the island would also be slightly smaller. It’s been a decade since the 2004 earthquake and the relentless motion of tectonic plates has continued. It will take centuries for the island to submerge to the same degree as in 2004, but every single day the plates move just a little and the land exposed in 2004 will sink again in response.

-JBB

Image credit:  https://earthobservatory.sg/project/sumatran-paleoseismic-study

Recurrence

See the layers of sediments in the first of these 2 photos? They were deposited on a memorably bad day. These sediments come from a cave found on the edge of the island of Sumatra, and all of that sand and silt above the scouring layer was deposited on December 26, 2004; the date of the great Sumatran Earthquake and tsunami.

Those sediments were deposited by the tsunami, but they’re not the only tsunami remnants in this cave. This cave sits very near the shoreline and until recent uplift it was actually underwater, so it has taken multiple hits from tsunami waves over the last few thousand years. This cave was discovered a few years ago by scientists from Nanyang Technological University in Singapore and using results from this cave they have constrained the behavior of the Sumatran megathrust going back nearly 10,000 years.

The modern tsunami wave has an erosional scour at its base and the sediments just below it are 2900 years old – probably the date the cave was uplifted out of the water. The tsunami waves in 2004 entered the cave in pulses and dropped sand in layers on the surface. After trenching at this site, the scientists found 11 similar sand deposits in the sequence at this site. Each of them has thin layers of fine marine silt and clay in-between, showing that the sequence was a rapid deposition of sand followed by a long period of slow sedimentation.

Each of these 11 sand deposits represents a tsunami wave. The scientists carbon dated the layer at the bottom of their trench and found it to be 7400 years old, giving a recurrence interval of 450 years between 7400 and 2900 years ago.

That average would seem to be a statement of how often the fault breaks, but the scientists looking at the layers also found that average number to be almost meaningless – the fault doesn’t care about the average. They found that several thin layers of sand were packed close together, with as many as 3 smaller tsunamis within a 100 year period and as long as 2200 years of no sand deposits after one of the largest tsunami waves.

There are other geologic records around the Indian Ocean like this showing repeating tsunami waves, but none has as many waves recorded as this site and none of them are detailed enough to show the clustering. This site indicates that the Sumatran fault sometimes breaks in small earthquakes that trigger small tsunami waves and then occasionally breaks in a rupture like 2004 where it produces a major wave. Although there are only a couple examples in this cave, it is after big events like the 2004 quake where the fault is quiet for over a thousand years; so one possible interpretation is that the Sumatran fault may take centuries or millennia to produce major waves again.

This behavior is similar to the behavior observed at the Cascadia Megathrust off of Oregon, Washington, and British Columbia. Evidence from landslides and turbidites off the coast suggests that the fault there may break in a rupture that triggers a magnitude 8 earthquake and tsunami waves, but then occasionally the entire fault breaks, triggering a massive magnitude 9+ quake and much larger wave (https://tmblr.co/Zyv2Js29RKOmF).

There are often press reports saying a fault is “overdue” based on analyses of how often the fault moves on average, but these types of studies show that type of calculation just doesn’t express the way major faults move. The size of the quakes and size of the fault that breaks changes over time, and the exact future behavior is extremely difficult to predict.

The Sumatran fault is guaranteed to produce another large quake and tsunami. It could be thousands of years, or it could literally be less than 100 years. The best guess from this cave is that the huge 2004 rupture means it will be millennia before it goes again, but that's not the lesson of this cave. There are areas in the cave with 3 quakes in less than 100 years, and we don’t know for certain there won’t be another one soon.

Rather than measuring a recurrence interval, the lesson of this cave is that these megathrusts are very complicated and we’ve only seen the tiniest window of their history, so if the fault decided to release another quake and tsunami 20 years from now or 2000 years from now, humans in the area can’t be surprised. That lesson isn’t one for just this site either; it’s a lesson for faults around the world.

-JBB

Image credits and original paper: https://www.nature.com/articles/ncomms16019_ _

An island expands

On December 25, 2004, the island at the center of this photograph was only as big as the area covered by trees.

The next day, the island was this size.

This island sits off the coast of Sumatra above the Sumatran-Andaman subduction zone. On December 26, 2004 that fault broke, leading to the disastrous 2004 tsunami and producing one of the largest earthquakes in recorded history.

The energy released in an earthquake is stored in rocks over hundreds of years. As tectonic plates move, they grind against each other and the rocks begin bending. In a subduction zone, these forces bend some parts of the plate downward and push other parts upward.

When the fault finally breaks, all the land that was bent downward suddenly pops loose and snaps back. That snapping releases the energy we feel as an earthquake and releases the land back to its original position. When the December 26 earthquake happened, this island and many around it popped up, suddenly exposing areas that were formerly submerged. This island more than doubled in area in an instant.

If you came back to this island today, plant life would probably have started growing in the areas that were exposed in 2004, but the island would also be slightly smaller. It’s been a decade since the 2004 earthquake and the relentless motion of tectonic plates has continued. It will take centuries for the island to submerge to the same degree as in 2004, but every single day the plates move just a little and the land exposed in 2004 will sink again in response.

-JBB

Image credit: Caltech/TO/Kerry Sieh http://bit.ly/1ezV6Pe

Tsunami wave propagation

This video, produced by the Tectonics Observatory at Caltech, shows a simulation of how the 2004 Tsunami worked its way through the Indian Ocean. You can see which features cause it to bend, slow, or stop, and which features take direct hits. You can also see how there are peaks and troughs formed by the waves as they spread out from the source.

Simulations like these, conducted on modern supercomputers, allow scientists to understand how tsunamis move after they’re generated and allow policy-makers and emergency preparedness organizations a better understanding of how their risks are impacted by different geologic scenarios.

-JBB

Video credit: TO https://www.youtube.com/watch?v=N95rsXj0jDU

Read more: http://nctr.pmel.noaa.gov/propagation-database.html

On Tsunami Waves

This infographic illustrates the basic principles behind the formation and movement of tsunami waves – on the 10th anniversary of the disastrous 2004 Indian Ocean tsunami this seemed like a good reminder.

A tsunami wave forms when things move up or down beneath the ocean, most commonly when a fault breaks in a large earthquake. If either a thrust fault or a normal fault breaks, rocks will move up and down. Compared to the weight of the rocks in Earth’s crust, the weight of ocean waters on top are tiny, almost completely negligible. The crust thrusts the waters upwards or pulls the waters downward as the fault breaks, creating a wave. 

The wave, either up or down, propagates outward from that spot, carrying energy with it. When a thrust fault breaks, it’s kind of like a giant pile of water getting out of the way; it races out in all directions.

When the energy from the tsunami gets close to shore, the waves pile up, slowed at the base by the friction between the flowing water and the rising ocean floor. As the waves pile up, their height increases and the energy of the waves are concentrated, bringing them up against the shore with potentially huge forces. 

Sometimes the trough, the downward part of the wave, will arrive first, causing the ocean to seemingly retreat, but this doesn’t happen every time and it depends on the exact motion of the fault. 

There can be other ingredients in tsunami waves, including undersea landslides, bending of waves around coastlines, and amplification of wave heights at high tide, but those are the basic processes involved in an event like the 2004 Indian Ocean disaster.

-JBB

Image credit: http://en.wikipedia.org/wiki/Tsunamis_in_lakes…

Read more: http://www.tsunami.noaa.gov/basics.html

Bending a wave One of the many scientifically interesting things about the 2004 tsunami was what happened on the island of Sri Lanka.  Sri Lanka sits just across the Indian Ocean from Sumatra and was right in the path of the tsunami; the eastern sides of the island took a direct hit and there were thousands of casualties. But that wasn’t the only part of the island hit; the waves actually hit the backside of the island as well. Tsunami waves behave like other wave types in that they don’t travel in straight lines. In the case of Sri Lanka, as the waves approached the island, the parts of the wave exposed to the coastline slowed down while the parts in deeper water continued traveling at their same pace. This caused the wave energy to bend around the southern edge of the island, slamming into the portions that didn’t face Sumatra.  On the southwestern coast of Sri Lanka, more than 1000 people were traveling on a loaded train between Colombo and Galle as the tsunami hit. That train was hit by waves that were over a meter taller than the top of the train. It was pushed inland, drowning many of the passengers as well as passers-by that climbed on top of the train hoping for safety. That disaster only occurred because of the way the tsunami wave bent around the island. The image showing Sri Lanka’s coastline actually captured the waves as they were moving. NASA’s Terra Satellite passed over the area just as the waves were approaching and the Multi-angle Imaging SpectroRadiometer (MISR) was able to detect the waves due to the changing angle of the ocean’s surface that also changed the angle that sunlight reflected back to the satellite. -JBB Image credits:  https://eosweb.larc.nasa.gov/sites/default/files/project/misr/gallery/tsunami_sri_lanka.jpg http://www.thelongacre.net/2011/03/tsunami-where-you-at.html Read more: http://www.bom.gov.au/tsunami/info/ http://bit.ly/13Bw2bs http://news.bbc.co.uk/2/hi/south_asia/4132247.stm http://sciencelearn.org.nz/Science-Stories/Tsunamis-and-Surf/Behaviour-of-waves

When the waters retreated Today is December 26, 2014. I had to write out the date because the day after Christmas is locked in my memory from what happened 10 years ago.  Just before 8:00 a.m. local time, the earth’s crust off the western coast of the island of Sumatra broke. The crack started near Sumatra and propagated a huge distance to the North, over 2000 kilometers. The ground shook violently and so much energy was released that the entire planet bent and swelled for days afterwards.  The incredible shaking was only the smallest part of the disaster. On the floor of the ocean where the crack formed, areas where the crack reached the surface were pushed upward, in some cases by as much as twenty meters. That rupture lifted up the ocean waters into a giant pile that the waters flowed away from as a series of giant waves – a tsunami.  A few minutes after the shaking stopped, this spot was hit by waves tens of meters high. The upper image shows Banda Aceh, the closest community to the quake’s epicenter, after the quake in 2004.  The tsunami wave was a slow-motion disaster. It moved across the Indian Ocean over several hours, striking people in Sri Lanka, Thailand, and India who received no warning. It even hit the continent of Africa with enough force to cause damage and deaths on the other side of the ocean. Over 225,000 people were killed, making the quake estimated to be the 5th deadliest earthquake in recorded human history. Much has changed in the 10 years since this disaster. As you see in the second photo, taken recently on the same spot, many communities have rebuilt from the damage. The debris and pollution spread by the tsunami has mostly washed away. That day likely still leaves scars on the survivors and memories of those who were lost, but the outward scars seen around the ocean in the following days have mostly healed. The earth science community and the world also were changed by the tsunami. In 2004, tsunami science was thought of as important, but there wasn’t a recent event to spur work in that field. By some estimates, since 2004 the number of researchers working on tsunamis has multiplied by a factor of 10. Those scientists have made some remarkable findings, such as ways to build structures able to survive the waves, predictions of tsunami heights depending on earthquake locations and energies, and how to interpret geologic records of tsunami as evidence of earthquakes across the ocean. But even with that progress, the overtopping of sea walls by the Japanese tsunami in 2011 still shows that we have work to do.  The most tangible evidence of a response to the 2004 quake can be found in tsunami preparation. In 2004, the only tsunami warning office in the world was the Pacific Tsunami Warning Center. It had a staff of 8, would close for part of the day, and had access to 6 tsunami-monitoring buoys. Today, the Pacific Tsunami Warning Center has a staff of 15, is open 24 hours a day, 7 days a week, and controls a network of 60 tsunami-monitoring buoys. The Pacific system also helped Australia, India, and Indonesia establish a warning system in the Indian Ocean, which opened in 2006 and is gradually improving its capabilities and being linked to at-risk coastal communities.  Had these systems been in place in 2004, they literally could have saved hundreds of thousands of lives. Perhaps that is one positive legacy of this disaster worth remembering today, 10 years later, as we look back on the recovery of those communities. Image credit: AFP/AP/Getty http://www.theguardian.com/world/gallery/2014/dec/11/then-and-now-the-aftermath-of-the-2004-indonesian-tsunami-in-pictures Read more: http://earthquake.usgs.gov/earthquakes/world/most_destructive.php http://www.nsf.gov/news/news_summ.jsp?cntn_id=106726 http://news.yahoo.com/scientists-strides-tsunami-warning-since-2004-155016481.html http://www.gns.cri.nz/Home/Our-Science/Natural-Hazards/Tsunami/2004-Boxing-Day-Tsunami http://www.bbc.com/news/world-asia-30572079 http://portal.unesco.org/en/ev.php-URL_ID=33442&URL_DO=DO_TOPIC&URL_SECTION=201.html http://soundwaves.usgs.gov/2005/02/