All coastal regions which share an ocean or large lake with major fault lines are vulnerable to tsunami. Subduction earthquakes under the ocean floor send vast amounts of energy through the ocean, which can be converted into damaging wave energy. Even where there is no major fault line, a tsunami on one side of the ocean can be caused by a large landslide on the other side. These landslides have been documented to cause tsunami which are even higher than those which are caused by subduction earthquakes.
The 1958 megatsunami which occurred in Lituya Bay, Alaska, forced all the energy of a 7.5 magnitude earthquake, combined with a landslide of 40 million cubic yards and possibly sudden glacial lake drainage as well, into Gilbert Inlet, a long, narrow inlet where the water was only 11 yards deep at the entrance. The resulting tsunami reached a height of 492 feet. Because it was generated very close to land, there was almost no warning before the wave hit.
No current engineering can protect against a megatsunami such as this. However, the conditions which can create an extremely large tsunami are known. The most damaging tsunamis occur in coastal areas abutting shallow water or narrow inlets. This is because all the energy of the wave is concentrated into a smaller space, which causes the wave to grow higher. When all those conditions come together, as they do in Lituya Bay, it is best not to build in those regions at all.
Fortunately, the vast majority of tsunami are much smaller than the Lituya Bay metatsunami. Very few are higher than a few feet. While these are still very dangerous, there are steps a country can take to protect itself against most tsunami.
Early warning system
Tsunami warning systems rely on a combination of earthquake sensors and ocean buoys. Both sets of information are needed.
Japan has the most extensive earthquake sensor array in the world. Its Earthquake Early Warning System gave up to a minute’s warning before the 2011 Sendai earthquake hit. Taiwan, Mexico, and California are also protected by early earthquake warning systems.
These systems can detect earthquakes seconds before they reach the surface because the non-destructive p-waves arrive faster than the destructive s-waves. California’s Seismic Warning Systems estimates a difference of about 1 second for every 5 miles of distance to the hypocenter of the earthquake, including vertical distance.
Although an earthquake can be detected almost immediately upon striking and sometimes even a little before, current earthquake research is not up to predicting which earthquakes will result in devastating tsunamis and which ones will not. Thus, even though the information that an earthquake has struck can be spread around the world at the speed of light, no one can know at that point if a major tsunami will follow.
Most countries issue tsunami warnings immediately if the earthquake is deemed potentially capable of creating a tsunami, even though most of those earthquakes will not. This results in a lot of false alarms. However, the alternative is to wait until a tsunami actually develops. By then, it will be too late for the closest coastal regions.
Most major fault lines are so close to the coast that a long prediction time is currently impossible for all regions. However, with an adequate sensory network of earthquake sensors and ocean buoys, nearly all coastal regions will have at least a few minutes of warning after a tsunami-causing earthquake or landslide hits. The closest areas may have little or no warning at all, but more distant areas may have several hours of warning before the tsunami hits.
All Pacific Rim countries participate in a Pacific Ocean tsunami warning system. After the devastating Christmas Day 2004 Indian Ocean tsunami, an early warning system has been set up for the Indian Ocean as well. Other oceanic coastal regions are much more poorly covered.
Seawalls, floodgates, and other barriers
Fixed installations along the coast cannot be moved during a tsunami warning. Japan protects its vulnerable installations along the Pacific coast with seawalls or floodgates capable of protecting the coastline from tsunamis of up to 19 feet high, which is Japan’s highest tsunami warning level.
Most Japanese tsunami barriers are built to withstand tsunamis of up to 15 feet high. This covers the vast majority of tsunamis. Unfortunately, the 2011 tsunami which struck Sendai and the Fukushima reactors was up to 24 feet high.
Other barriers on or near the shore can also dissipate the force of a tsunami. Undersea coral reefs which come close to the surface of the sea can absorb much of the undersea force of a tsunami. Even shoreline forests, if they are large enough, can protect areas behind them.
It is a common myth that underwater canyons and other deep spots can break the force of an incoming tsunami. This myth is not true. Deep areas do nothing to dissipate a tsunami’s energy.
Many Pacific Rim countries have clearly-marked tsunami evacuation routes which lead to gathering places at elevations above where a tsunami is likely to reach. Some Pacific Rim countries also practice tsunami evacuation drills. However, most coastal countries only hold occasional voluntary tsunami evacuation drills, if any at all.
Once again, Japan sets the standard. All government, local authorities, emergency services, schools, and most large businesses in vulnerable regions have mandatory tsunami evacuation drills at regular intervals. As with seawalls, the standard to which Japanese tsunami evacuation drills are set is a 15 foot wave.
In an emergency
You cannot outrun a tsunami. Although they may seem to be arriving in slow motion, they hit shore at close to highway speeds.
Your best bet is to evacuate to higher ground, preferably along marked evacuation routes. However, if there is no time for that, you may be able to find sufficient shelter in time if you get to a multifloor concrete building and start climbing. You should be safe from nearly all tsunamis when you reach the third floor. Do not rely on a woodframe or brick wall structure, because the force of the tsunami can break these structures and sweep them away.
If the tsunami turns out to be unusually high, you will notice that the incoming sea level is still above your current location. In that case, keep climbing until the sea is no longer above you. The fifth floor should be above even the highest earthquake-generated ocean tsunamis.