20 years ago today, Dec. 26 2004, The Boxing Day Tsunami claims 230,000 lives

On the morning of Sunday 26 December 2004 a massive magnitude 9.2 earthquake occurred off the west coast of northern Sumatra in Indonesia. The resulting tsunami affected countries right across the Indian Ocean Basin. The earthquake generated a powerful tsunami which moved through the Indian Ocean at more than 500 kilometers per hour. The maximum measured tsunami run-up was over 40m in height. The tsunami waves caused widespread death and injuries and devastated coastal areas, destroying towns, homes, infrastructure and the livelihoods of thousands of displaced people. The worst affected countries included India, Indonesia, Malaysia, Maldives, Myanmar, Sri Lanka, Seychelles, Thailand and Somalia. Other countries that absorbed some of the brunt of the tsunami were Bangladesh, Kenya and Australia. This tsunami has been claimed as one of the most devastating natural disasters in recent history. Over 230,000 people died in this event, 160,000 in Indonesia alone. Much of this event was captured on film and posted across the internet. I was glued to the computer (more than normal) watching footage of this awesome and terrifying event.

As a geologist and professor of Earth Science my interest in geophysical processes and hazards has always been keen and this particular event really changed the way my community and the world thought about and responded to tsunamis. Today my interest in natural hazards is still strong, and I thought I would share parts of my tsunami lecture as we reflect on the event and the many lives lost 20 years ago.

Most people I know don't think about tsunamis very often, unless they live at low elevation near shore, and even then it's not a very common concern. Of course, movies and media sometimes remind us about tsunamis, but usually in extremely exaggerated and terror inducing ways. I thought I would try and provide some science and history about tsunamis to the community. The above image is from the movie "The Day After Tomorrow" 20th Century Fox, 2004. It shows a large tsunami inundating New York City. This is obviously not a real event but not totally out of the bounds of reality either.

Part of my point is that tsunamis are rarely thought about except when we're reminded of them, much like today. This is a Google trends search for "tsunami" as the term. Going back to 2004 (as far as it would let me go back). Prior to the Boxing Day tsunami, there was very little interest, after which you can almost track tsunami events by people's search trend. I've identified several recent tsunamis on this plot. It seems like the more powerful and devastating the tsunami is the more interest it generates. You can also see a small uptick over the last few days as we approach the anniversary.

These are the death toll from recent tsunami events:
2004 - Indian Ocean: 230,000 people
2006 – South Java: 800 people
2007 – Solomon Islands 52 people
2007 – Chile 10 people
2009 – Samoa 189 people
2010 – Chile 550 people
2010 – Sumatra > 400
2011 – Suluwasi > 800
2011 - Japan: 18000 total (EQ and tsunami)
Sept. 2015 Chile 13 dead
2018 – Java Sumatra 426 people
2022 - Hunga Tonga-Hunga Ha'apai volcano (HTHH) eruption, 4 people

Before 2004 much of what we knew and were taught about tsunamis came from historical accounts and before and after photos. When I was an undergrad at a college in the Pacific Northwest these are some of the photos we used in our classes to learn about tsunamis

The first tsunami I ever learned about was from the 9.2 magnitude Good Friday earthquake that occurred in south central Alaska generated a large tsunami that devastated communities on the West Coast of the U.S. When I was an undergrad, this event and these photos were often used in classes and lectures and really guided what we knew about tsunamis. There were not many other examples to use in class that included videos or photos. That all changed Boxing Day 2004.

These images show Banda Aceh, Sumatra (very close to the source of the tsunami) before and after the tsunami wave hit. The amount of photos, video, and satellite imagery from the 2004 Indian Ocean (Boxing Day) tsunami changed the way we understood, perceived and learned about tsunamis.

Let's talk about the physics of ocean waves. The still water line represents a zero energy level when water is at equilibrium. The wave trough is the part of the wave that is displaced below the still water level. The crest is the part of the wave that is displaced above the still water line. Wave height is the vertical distance between crest and the adjoining trough. Wave length is the horizontal distance between two neighboring crests (or any equal point of the wave). Wave period (T) is the time for one wavelength to pass a fixed point. Wave frequency (f) = 1/T which is also the number of wave crests moving past a fixed point in one second. Ordinary ocean waves are generated by the wind and have a period of 5-20 seconds and a wavelength of about 100-200 meters.

The largest wid generated wave ever recorded was 34 meters (112 feet). in 1933 the crew of the USS Ramapo calculate the wave height when the stern of the ship was in the wave trough and an observer on deck lined up the crows nest with the wave crest.

Tsunami waves are different from ordinary ocean waves. Tsunami waves are caused by a large, abrupt disturbance of the sea-surface. A tsunami wave is a fast moving wave (~800 kmh or 500 mph) that propagates through the entire column of water. Tsunami waves are characterized by very long wavelengths (100-500 km), very small wave heights ~1 meter, and long wave periods 10 minutes to two hours. A tsunami wave is considered a shallow water wave, so it's velocity is a function of the acceleration of gravity (g) and water depth. WAIT! WHAT!? How is a tsunami a shallow water wave. It sounds counter intuitive, right? OK but, a shallow water wave is defined as having a wavelength that is greater than 1/20th the depth of water. The average ocean depth is about 4000 meters and at it's deepest about 11,000 meters; the average wavelength of a tsunami is greater than 100 kilometers (greater than 100,000 meters). So, you can see that there is no way the wavelength of a tsunami will ever be less than 1/20th the depth of the ocean (not on this planet anyway). The physical characteristics (small wave height, long wavelength) of a tsunami wave make it virtually undetectable to ships at sea. One more thing, because the way in which a wave loses energy is inversely related to its wave length, tsunami waves not only propagate at high speeds, they also can travel great distances (across oceans) with little energy loss.

This plot is to provide a comparison of different ocean waves. The y-axis is logarithmic (each tick mark is 10 times greater than the previous tick mark). The x-axis shows wave period, which is the time for one wavelength to pass by a fixed point. At the top are the three primary generating forces of ocean waves: 1) Wind, 2) displacement of large volumes of water, 3) gravitational attraction between the sun and the moon.

So, ordinary ocean waves are wind driven. The particles move in a circular motion and the diameter of the orbital motion decreases with water depth. At a depth that is equal to half the wavelength orbital motion is essentially non existent. If you have ever snorkeled or done SCUBA you know that at a certain depth you no longer feel the motion of the ocean. In open ocean, ordinary waves travel in water depths that are far greater than 1/2 their wavelength. They are considered deep-water waves. However, as they approach shore the water depth decreases and that orbital motion begins to interact with the bottom (at some point they transition into shallow-water waves). This causes drag and slows the lower portion of the wave down causing the wavelength to decrease and the wave starts to pile up and grow taller. The upper part of the wave continues moving a faster rate than the bottom and eventually it pitches forward and breaks.

This is a NOAA vessel encountering large seas. Wind generated waves

And surfers love wind generated waves too.

Shallow-water waves, like tsunamis, propagate through water depths less than 1/20th of the wavelength. The orbitals are compressed into elongated ellipses. In deep seas energy loss is minimal because of the long wavelength, but in shallow water waves interact with the seafloor because their wave base is grater then the water depth. Remember, speed of tsunamis is controlled by water depth. As they approach shore, the energy remains the same which leads to shortening of the wavelength and growing of the wave height to several tens of meters.

Just to prevent any confusion. Tidal waves are also shallow water waves, but they are not tsunami waves. Tides are the most common and consistent waves to hit our beaches from day to day, usually twice a day are caused by gravitational forces felt by different parts of the Earth caused by the sun and moon. Of course, this is a discussion for another day.

This is an artists rendition of a tsunami but it is definitely not a tsunami. Except for the largest tsunamis, such as the 2004 Indian Ocean event, most tsunamis do not result in giant breaking waves (like normal surf waves at the beach that curl over as they approach shore). You should be able to answer those questions at this point.

The strength with which ordinary ocean waves arrive on the coast line depends entirely on the movement within the first few meters of water. Tsunamis transfer energy through the entire water column
As a tsunami leaves the deep water of the open ocean and travels into the shallower water near the coast, it transforms. A tsunami travels at a speed that is related to the water depth - hence, as the water depth decreases, the tsunami slows. Because of this shoaling effect, a tsunami, imperceptible at sea, may grow to be several meters or more in height near the coast. Runup may be variable given differences in coastline. When it finally reaches the coast, a tsunami may appear as a rapidly rising or falling tide, a series of breaking waves, or even a bore. A tsunami is capable of inundating, or flooding, hundreds of meters inland past the typical high-water level. The fast-moving water associated with the inundating tsunami can crush homes and other coastal structures. In coastal areas with little vertical relief the wave can penetrate inland for 10s of km.

Tsunami waves travel across the ocean at great speeds (200m/s=500mph) with very little attenuation. When they approach shore from the deep sea they encounter the Continental Slope (the sloping region), then hit the flat region, known as the continental shelf. The depth decreases Speed goes down, but waves are still coming every few minutes (the period) and they bunch up. Similar to traffic on the highway slowing down to rubberneck an accident. The wavelength decreases, which causes the wave height to increase. It eventually gets high enough that the wave breaks. Once it breaks it becomes a rushing wall of water forcing its way inland. Wave height on land can be greater than 30 meters. In some cases it may rise similar to a tide but in a much shorter amount of time: 15 minutes compared to 12 hours. In coastal areas with little vertical relief the wave can penetrate inland for 10s of km. There are typically 4-8 waves in a tsunami and the 1st wave is not necessarily the largest

One of the most diabolical features of a tsunami occurs when a tsunami wave trough arrives at shore, often what happens is called drawback. If the wave trough arrives first, drawdown or drawback occurs. This often catches people off guard as they go out to investigate the freshly exposed sea floor, collect shells and fish, etc. During the Sumatra EQ/Indian ocean tsunami there was drawback in some places up to 5km. However, in some cases the wave crest arrived first and there was no drawback and no warning. This seems to be what happened to the people in this photo. I don't know what was their fate or what became of them after this photo, but I have a feeling it wasn't a happy ending, although the NY Times reported that they all survived (Photo by AFP/Getty Images, accessed at PBS.com)

Where are sources that generate tsunamis? This map shows identified tsunamis since 2000 BC and the generating sources are identified in the key. Most occur in the Pacific Ocean associated with the "Ring of Fire" followed by the Indian Ocean. The Mediterranean has also had its share. Most deaths by tsunami occur very close to the generating source.

The most common source of tsunamis is shallow, undersea earthquakes. These are usually associated with converging plates and subduction zones tectonics. They are referred to as megathrust earthquakes. These massive ruptures along undersea thrust faults generate the largest and most destructive tsunamis as well.

Submarine landslides (occur under water) or sub-aerial landslides (fall into the water). These are less common but still occur. This happened in 1958 in Lituya Bay Alaska and generated a localized tsunami wave that reached 1720 feet above sea level.

Even less common than landslides but still realistic. This happened in 1883 during the eruption of Krakatoa, and more recently in 2018 with the eruption of Anak Krakatoa and 2022 with Hunga Tonga-Hunga Ha'apai volano

The granddaddy of tsunami generators is a meteorite impact. These are very rare, however, the Chicxulub meteorite (66 million years ago) is thought to have generated a tsunami over 100 meters high.

Nuclear testing like in this photo from the Bikini Atoll during the 1940's can also generate tsunamis. Hopefully, these types of tsunami generators are a thing of the past.

Meteotsunamis, as in meteorological, are generated by very low pressure and high winds and are more common than previously believed, especially in the Great Lakes. In 1954 a 22 foot wave arrived 4 hours after a major thunderstorm and killed 10 people in Chicago. It was previously classified as a seiche but scientists are now rethinking that. Researchers also believe there was a meteotsunami that occurred in the Persian Gulf in 2017.

So, to recap: A large disturbance, such as an earthquake or landslide, triggers a tsunami. The potential energy that results from pushing water above mean sea level is then transferred to horizontal propagation of the tsunami wave (kinetic energy). For the case shown above, the earthquake rupture occurred at the base of the continental slope in relatively deep water. Within several minutes of the earthquake, the initial tsunami (Panel 1) is split into a tsunami that travels out to the deep ocean (distant tsunami) and another tsunami that travels towards the nearby coast (local tsunami). The height above mean sea level of the two oppositely traveling tsunamis is approximately half that of the original tsunami (Panel 1). (This is somewhat modified in three dimensions, but the same idea holds.) The speed at which both tsunamis travel varies as the square root of the water depth multiplied by the acceleration of gravity. Therefore, the deep-ocean tsunami travels faster than the local tsunami near shore. Several things happen as the local tsunami travels over the continental slope. Most obvious is that the amplitude increases. In addition, the wavelength decreases. This results in steepening of the leading wave — an important control of wave runup at the coast. In the next panel note that the first part of the wave reaching the local shore is a trough, which will appear as the sea recedes far from shore. This is a common natural warning sign for tsunamis. Note also that the deep ocean tsunami has traveled much farther than the local tsunami because of the higher propagation speed. As the deep ocean tsunami approaches a distant shore, amplification and shortening of the wave will occur, just as with the local tsunami shown.Tsunami runup occurs when a peak in the tsunami wave travels from the near-shore region onto shore. Runup is a measurement of the height of the water onshore observed above a reference sea level.

The National Oceanic and Atmospheric Administration (NOAA) tsunami program is part of an international cooperative effort to save lives and protect property. The mission is to provide real time tsunami monitoring, ensure early detection of tsunamis and acquire data critical to real-time forecasts. It works through a system of anchored seismometers and pressure recorders on the sea floor. These sensors transmit information to a buoy floating at the surface, which then sends a signal to a satellite and then to a control center on land. All told it takes about an hour after data acquisition to model the wave and issue a warning.

This is a map of DART buoys. Most are installed by the U.S. and NOAA. Other countries are also involved. There are now about 40 buoys placed in regions with a history of destructive tsunamis.

These plots show modeled and actual data from the 2011 Japan tsunami. The red line is the modeled data and the black is the actual data. On both plots the x-axis shows time since the earthquake occurred. the y-axis shows the wave amplitude in meters recorded and modeled at the buoy location. The top plot is from a buoy just 450 nautical miles offshore of Tokyo. The bottom plot is from a buoy just 250 nautical miles offshore of Coos Bay, Oregon.

On the left is a map of tsunami generating sources. On the right is a map of population density. The most densely populated regions on earth are some of the lesser developed countries on the Pacific Rim. Risk calculates the probability of event happening. In terms of geology this is challenging because we have such a short snapshot of the geologic record relative to past tsunamis. Vulnerability, on the other hand examines who is actually going to be affected. It takes into consideration access to resources, information, capacity to resist, respond, cope, and recover. The demographics of human migration towards the coast increase our vulnerability for more destructive, more deadly, more costly, more devastating tsunamis. Nature operates on a geologic time scale but humans do not.

In the United States on of the most risky areas for a major tsunami is the Pacific North West. The Cascadia Subduction Zone has a geologic record of many previous megathrust earthquakes and local tsunamis. There is evidence in the cultural record as well both in written Japanse history and in Native American lore. The recurrence interval for another large earthquake could be as little as 300 years. The last time this happened was 1700.

The Cascadia subduction zone is considered an analog to the Sumatra subduction zone. The Sumatra earthquake started approximately 250 km south-southwest of Banda Aceh and rptured the entire section of the Sunda Arc between Sumatra and the Nicobar and Andaman Islands. There was over 1000 kilometers of fault rupture! The Cascadia zone is currently locked and loaded. It has been storing elastic energy for over 300 years and the next release could generate the next megathrust earthquake just over 100 kilometers offshore. If it ruptures it could essentially unzip the fault zone for hundreds of kilometers.

Today with improved understanding, monitoring, and warning systems it is unlikely that another tsunami this deadly will occur. However, If you live in a seismically active area and/or near the shore you should know what to do and where to go. Don't let this become a phobia, or neurosis that affects your day-to-day life, but just be aware or your surroundings and indicators that may signal something big. I hope you learned something or at least enjoyed it. Lastly, my background is in volcanic hazards, my interest in tsunamis was just something on the side. If any oceanographers or tsunami experts find any inaccuracies or have notes please feel free to let me know. Thanks again.

Edit update: thank you all for the positive feedback and for sharing stories, information, and experiences. I've made a few edits to fix some typos that were pointed out. I also want to address the deathtoll from the 2011 Tohoku earthquake and tsunami. That event (EQ & tsunami) is estimated to have taken approximately 18000 lives. I originally had 5000 as a conservative number as I was trying to distinguish between the earthquake and the tsunami, and only count those lost to the tsunami. However, I realize that it is not possible to say with certainty how many people parished in the earthquake and how many by the tsunami, so I have updated that figure as well.