February 28, 2011

Tsunami misconceptions

Posted by Austin Elliott

Exactly one year ago today we (some of us) sat glued to our TVs as the tsunami generated by the gargantuan 8.8 Chilean earthquake barreled into bays around the Pacific Ocean. This is what we saw:



Many people found that anti-climactic, especially in comparison to this famous video of the 2004 Indian Ocean tsunami pouring into Patong Beach, or this dramatic one of the ’04 tsunami crashing ashore at another resort hotel on Phuket, or this disturbing video of the same surge tearing through a hotel restaurant. I, however, found the Hawaii video marvelously awesome, and rather less distressing to boot. I prefer videos of 2010’s rather “anti-climactic” and largely forgotten tsunami because their lack of flashy, splashy drama allows a more nuanced view that reveals the impressive power underlying these waves.

Tsunamis tend to be conceptualized incorrectly by newcasters, artists, and even eye-witnesses. They differ substantially from regular ocean waves, and are thus not well characterized simply as giant versions of their every-day cousins. I myself had difficulty imagining their nature until the eye-opening collection of tourists’ videos emerged documenting the tremendous 2004 Indian Ocean tsunami. Since then YouTube has enabled us to witness nearly every major tsunami that has crashed ashore somewhere in the world, and the videos lend substantial aid to our imaginations in understanding what a tsunami really is.

A tsunami occurs when a large portion of the ocean floor moves–imagine the water in a bathtub when you shift your body from one side to the other. Although submarine landslides may generate tsunamis, more commonly the giant displacements caused by undersea earthquakes heave immense volumes of water upward to spawn these waves. When a fault slips beneath the ocean it lifts a huge column of water by a few to a few tens of meters. This water then has to settle its newly gained gravitational potential and equilibrate back to global sea level. To move back downward, some of it must be displaced outward to account for the new protrusion of land caused by the earthquake below. The energy of the displaced water is transmitted rapidly through the ocean, shoving other water out of the way laterally and rolling along as a large area of elevated sea level.

Whereas regular, wind-driven ocean waves are merely surface disturbances, excited by moving air pushing gently over large areas of the ocean and extending no deeper than a few meters, tsunamis involve motion of the entire water column, top to bottom. As all that moving water approaches a shallowing shore (and especially one where the coast line funnels it to a point), it piles up, growing taller and slower until the leading edge of it begins to crest like an ordinary wave. The difference is the volume of water involved. Because tsunamis are generated by the rapid uplift of entire swaths of the ocean floor, their areal extent is huge. Behind even a modest-height crest are literally kilometers of water at that same elevation, ready to pour up onto shore. In effect we perceive tsunamis as large areas of elevated sea level, like a rapid shift in the tide, not like a steep short wave that merely reaches great heights. Once a tsunami hits, it continues pouring inland, as you can see in all of these videos, until the trough of the multi-kilometer-wavelength wave arrives and the water can finally drain back out to sea.

In the milder videos from the much smaller Chilean tsunami, this effect is very apparent, especially in time lapse. The relative calm with which the tsunami arrives belies the massive force driving it as this humongous ripple inexorably laps up over shorelines around the globe.

As the Chilean tsunami arrived at the northern California coast it merely appeared as a rapid decrease in the period of the tides, with an ebb and a rise accentuated by the narrow geometry of the harbor:



The rapid rise and dramatic drawback is even apparent along the wide, flat beach at Santa Monica Pier as the tsunami sweeps past Los Angeles:




Tsunamis are not predominantly impressive nor dangerous because of their height, but because of their width, or wavelength.

There you have it. I encourage you all to peruse NOAA’s National Center for Tsunami Research website (also in sidebar at right) for some great graphics and model animations of recent and historic tsunamis, like this map of modeled wave amplitude in the Feb 27-28 Chile tsunami last year.


Measured and modeled tsunami wave heights from the 2010 M8.8 Chile earthquake, from NOAA / PMEL / Center for Tsunami Research.