The inexorable, rapidly-rising wall of water of a tsunami is a terrifying, deadly sight. This is the disaster demystified, with all the science to help you survive.
Tsunami are the stuff of legends, a natural disaster steeped in stories and misinformation. Even their name is deceptive: tsunami translates as harbour-crossing wave, and yet the catastrophic event is far from that geographically constrained. Naming them tidal waves is even more bizarre: the moon and sun do create enormous, planet-circling waves that drive high and low tides, but those slow-moving behemoths are both completely unrelated and no where near as destructive as the surging power of a tsunami.
Only the French have it right, naming them raz-de-marée, a violent rising of the tide that poetically encompasses both their appearance and the threat they pose. Each crest resembles a surging wall of water more than an artistically curving peak. And each wave can flood far inland, trashing the coast by dragging, scouring, and overturning during waves’ advance and retreat.
Tsunami are generated by any vertical disruption of the ocean, including earthquakes, landslides, volcano collapses, or impact events. Image credit: Natural Resources Canada
A tsunami is a ripple of water generated by displacement, but writ large. Anything that disrupts the ocean—technically, a vertical displacement of the water column—can send waves racing around the world. We’re most accustomed to hearing about them in the context of subduction earthquakes, but that’s not the only trigger. Tsunami can be triggered by smaller events like a calving glacier shedding hunks of ice or a landslide, or something more substantial like the flank collapse of a volcanic island sending the entire side of the peak splashing into the sea, or even a meteor impact.
Water waves obey different equations of motion depending on if the water is shallow or deep compared to the wavelength of the wave. In open ocean, that distance between crests is hundreds of kilometers, much greater than the scant handful of kilometers depth of even the deepest recesses of the ocean. Consequently, the entire water column moves as the tsunami propagates, right down to the ocean floor. In some ways, this is fantastic: the fluid dynamics of shallow-water waves make it extremely easy to predict tsunami travel times, and thus when the waves will start arriving on distant shores. On the down side, it’s nearly impossible to predict just how big those waves will be.
Predicting the height of a tsunami before it starts coming on-shore is nearly impossible. A direct measurement of the vertical displacement of an earthquake fault snapping the seafloor upwards happens weeks after the disturbance, if ever, while detecting the few-meters height of a tsunami’s crest spread over the hundreds of kilometers wavelength is virtually impossible under the constant chaos of wind waves. Instead, models are calibrated once those first waves start reaching the shore and we gain a sense of how much of a monster a particular tsunami will be.
While we can barely see tsunami with our naked eyes in the open ocean (which is why that’s the safest place for a boat to be), we can still detect the passing waves by measuring changes in pressure. The National Oceanic and Atmospheric Administration maintains a series of buoys for just this purpose, the Deep-ocean Assessment and Reporting of Tsunamis (DART) network. It isn’t until a tsunami is recorded propagating through the open ocean that geophysicists know the event is going to an ocean-wide disaster instead of just a local event.
Tsunami shoal as they enter shallower water, with the wavelength and speed dropping while the wave height increases. Image credit: International Tsunami Information Center
As the tsunami enters shallower water, it shoals with the waves bunching up into steeper peaks that are closer together. The waves slow down from a racing jet airliner to more modest highway speeds. Only the period stays the same, the time between each crested peak or valley trough passing a fixed point.
Even once the waves start coming ashore, tsunami can still be devastatingly unpredictable. The trough might arrive first, sucking the water out to sea in a phenomena that has lured people to their deaths as they curiously explore the suddenly-exposed sea floor. The crest might arrive first, but that first wave isn’t necessarily the largest in the wave sequence. It may be just a precursory ripple, dwarfed by subsequent waves, or interference between advancing and retreating waves. Even quirks of shoreline and near-shore bathymetry (seafloor topography) can amplify the waves through reflection and refraction. Would-be rescuers who survive the first wave but come down before the all-clear can be swept away by subsequent waves even if they stay above the previous high-water mark.
The trough arrived first, and the fourth wave was the largest for the tsunami triggered by the recent Chilean earthquake, but at least it arrived at low tide. Image credit: William Barnhart with data from the UNESCO/IOC Tide Gauge Monitoring Facility
The time between waves—the period—is unique to each tsunami but can be as little as 10 minutes and as far apart as an hour. When this period aligns with the natural resonance frequency of a harbour, inlet, or fjord, the waves can build into a seiche, a tsunami amplified through constructive interference into even larger waves. While seiche can be adorable when generated by a child sloshing back and forth in a bathtub, or harmlessly surreal when triggered by an earthquake in a swimming pool, when it comes to tsunami they can amplify an already-horrifying disaster into a city-destroying catastrophe.
Even the smallest of tsunami can still be deadly. While people may scorn the anticlimactic arrival of a centimeters-high tsunami, the relentless surge of these smaller waves can set up ferociously strong currents. It’s near-inevitable that curious onlookers or misinformed daredevils will head to the coast to “watch the tsunami arrive,” then be so underwhelmed they fail to treat these tiny terrors with the respect they deserve. Wading into the water is a death sentence for even a strong swimmer as powerful currents sweep them out to sea with a force far beyond the most infamous of rip currents.
Damage in Rikuzentakata, Japan from the Tokohu earthquake and tsunami on March 11, 2011. Image credit: NASA/Earth Observatory
But you can survive.
The most important tactic for surviving a tsunami is to get to high ground the moment you think one is coming, and worry about if you were right or not later. If you’re near a coastline and feel severe shaking, see the seafloor exposed as water is sucked out of a harbour, or spot an oddly-solid looking wall of water slamming inland, it’s time to run. The first waves may start arriving within minutes, far too quickly for official warnings to be distributed.
Typical tsunami warning sign marking an evacuation route. Image credit: AP/Heri Juanda
If geology is denying you a handy cliff or nearby mountain, look for a human-constructed vertical refuge in the form of a sturdy, tall, solid building that will hopefully survive the scouring waves. Look for signs marked with the international symbol for tsunami (a deceptive cresting blue wave) to identify evacuation routes or designated vertical refuges. If absolutely all else fails, cling to something that floats and hope for the best.
When farther away from the triggering source, you might have no idea a tsunami is coming. Pay attention to your local alert system—coastal sirens, radio announcements, or even cellphone alerts—and follow instructions. Emergency managers walk a fine line between protecting their residents and avoiding false alarms, and will increase or decrease threat levels as more data comes in. Keep paying attention to see if you’re in an area on watch, advisory, or full-blown warning.
Once in a place of safety, stay put until getting an official all-clear: tsunami can last for hours.
Top image: Rahmatullah Lampuuk mosque in Lhoknga, Indonesia on January 30, 2005, a month after the Boxing Day tsunami. Credit: AP/Greg Baker