Posts Tagged ‘evacuation routes’

Are Tsunamis Really Scary?

I readily admit tsunamis scare me the most. At the coast, I constantly identify evacuation routes. Every time I hike along a spit (up to 5 miles long), I feel I am taking my life in my hands, leaving high ground behind. I even passed up the opportunity to buy a beach house. So let’s look at tsunamis. Are my fears based in fact?

The Japanese word, tsunami, means “harbor wave”. The Japanese should know – they’ve listed over 300 tsunamis in the last 1400 years (talk about record keeping!). By the way, “tidal wave” is now a no-no word – tides play no part in these waves. In the 2011 Japan earthquake and tsunami, almost 20,000 people died (many of whom were never found), in the most prepared country on earth. 230,000 people died in the 2004 Indian Ocean tsunami. As long as people pack more houses and hotels on the coast, tsunami deaths will increase.2942294092_95359c6903

 What Makes a Tsunami?

Any sudden large displacement of ocean water can start a wave. Earthquakes are the major culprit, but only the subduction kind, primarily in the Pacific Ring of Fire (including Oregon!). In a subduction fault, the ocean plate gradually subducts (gets pushed) under the continental plate. When an earthquake finally happens, the ocean plate bounces back up, shoving a lot of water up with it (besides tsunamis, sea levels often rise permanently– good-bye beach front property!).

Displaced water ripples toward shore, much like ripples from tossing a rock, except involving enormous amounts of water. Over deep ocean, the ripple is small, although travelling at 500 miles an hour. A boat could glide over it and never notice (so much for the Poseidon Adventure movie of my youth). In one direction, the tsunami reaches the nearest shore quickly. In the other directions, the teletsunami (or distant tsunami) crosses the ocean, arriving 6-15 hours later.

When the ocean floor rises to meet the coast, everything changes. The shallows slow down the water in front, while the faster water in back shoves forward, creating a surge or wave. Do you imagine giant surfing waves? A tsunami might crest or rise up, or it might just surge forward, but it never looks like a big curling wave, except in movies.  The tsunami surges on shore and rolls on , and on, and on, before finally receding. Just like ripples, waves recur over 12-24 hours, and the first wave is often not the biggest. The stories of the ocean pulling out and exposing all the sea life just before the wave rushes in? A drawdown happens when the valley of the wave reaches shore before the crest. If you see one, you’re in trouble, but don’t count on it as a warning.

Diagram of tsunami wave formation

Diagram of tsunami wave formation

 How Big Can Tsunamis Be?

In the 2011 Japan tsunami, run-up (the highest point a tsunami reaches above normal tide) varied from 16 feet to almost 130 feet, with the norm around 50 feet – taller than many 4 story buildings. Inundation (the area covered by water) extended inland for 6 miles some places. The earthquake also generated a teletsunami, with a height of 7-11 feet in Hawaii, and 8 feet in California and Chile – causing millions in damages, and killing anyone stupid enough to be on the beach. Most tsunamis are only inches to 20 feet high. The 2011 Japan and 2004 Indian Ocean tsunamis were the rare ones that occur a few times a century (we hope).

Learning From Destruction

Geologists discovered a lot about “how far and how high” from the Japan earthquake, requiring all   tsunami inundation and evacuation maps to be redrawn. Obviously distance is a factor. A tsunami generated a short distance off the coast is larger than one crossing the ocean, losing energy along the way. A town straight in the tsunami path takes a greater beating than one further down the coast.

Pay attention to coast geography. Low flat coastlines create cresting tsunamis that surge miles inland. Sharp slopes and narrow river canyons focus the wave, with violent high runups. Harbors get massive surge without a wave. Below a cliff, the water just slowly rises. The ocean floor also matters. In Crescent City, California, the “tsunami capitol of the continental United States”, the shoreline “catches” teletsunami waves and swings them around into their harbor. Islands and ocean ridges can deflect tsunamis. Teletsunami prediction is now quite good, with ocean floor maps,  tsunami sensors scattered around the Pacific and complicated mathematical modeling procedures. For local earthquakes, there’s simply not enough time.

 Tsunami Inundation Maps and Evacuation Routes

iPad tsunami times from Japan to Chile.

iPad tsunami times from Japan to Chile.

Without a family geologist, the rest of us need an inundation map to visit the coast. What areas are at risk? How far inland or up must we go? How can we get out of danger quickly? Luckily, maps are easily found. Phone books at the beach, state geology web sites, and even a free app for my phone called TsunamiEvac NW (Android and iPhone) all have tsunami maps, showing inundation levels for both distant and local tsunamis.  I guarantee the app will definitely help my beach paranoia.

 East Coast Tsunamis

Guess what? Tsunamis happen on the East Coast as well, just not as often. Smaller off-shore earthquakes trigger ocean landslides which in turn cause tsunamis. The coasts of Newfoundland and the Carolinas are prone to these, with tsunamis up and down the Eastern seaboard. Since 1774, east coast tsunamis have killed 334 vs. 403 killed on the West Coast. The last major Newfoundland tsunami was 1929 – no time at all from a geologic standpoint. The Caribbean subduction zone and the Gibraltar fault near Portugal also send distant tsunamis to America. East Coast shorelines tend to be flat, so even smaller tsunamis can cause high casualties and damages, particularly because no one on the East prepares. No matter where you live, don’t forget holidays – tsunamis are a risk in many favorite vacation spots.

Tsunami Preparation – What Can You Do?

If caught in a tsunami, you are probably a goner. Between strong currents and massive debris, very few people survive the water. Your only choice is to get out before the tsunami arrives. In the Pacific Northwest, we are high on the waiting list for a Cascadia earthquake. We anticipate a twin for 2011 Japan = subduction zone earthquake, severity ~9.0, extending 600 miles north to south, giant tsunami within minutes, and a similar coastline.

Washington State Tsunami Evacuation Route Sign

Washington State Tsunami Evacuation Route Sign

My paranoia isn’t paranoia – it’s smart behavior. When Cascadia hits, we’ll know instantly by the ground shaking. As soon as it stops, move to high ground ASAP. Don’t stop for supplies (except a prepared go-bag). Most importantly, don’t be a tourist and stay to watch –  tsunami videos from 2004 Thailand show how that turns out! You may have only 10-15 minutes to get at least 100 feet above sea level (150 is better). Depending where you are, it could be a long walk. And I mean walk (or run) – roads could be impassable either because of earthquake damage or evacuation traffic. You’ll get farther faster on foot. By reading maps and planning evacuation in advance, I am ready to move. My shoes and go-bag are next to my bed every night on the coast.

What if a beach lacks accessible high ground? Vertical evacuation is the only choice – climb to the top of the highest building, and pray for a “normal” tsunami. Oregon is discussing construction of tsunami  evacuation structures at least 6 stories high  (imagine a summer beach town population in one building). Whether high ground or building, don’t go down until the official all-clear. Some tsunami victims evacuated safely from the first wave, then returned to homes or businesses only to be washed away by subsequent waves.

Local tsunami preparation is different, and must focus on community. Individuals just need evacuation plans and a go bag. However, coastal communities might be isolated for days, with harbors, roads, and bridges destroyed, and inland communities dealing with their own earthquake problems. Hopefully people with homes on high ground will open their doors to evacuees. Better yet, the community will have pre-established shelters on high ground with stocks of essentials, and other support including re-establishing contact with family members.

For a teletsunami, you should have an ample warning in any beach community, between alarms, radio, and

Tsunami damage Japan. Photo by Roberto De Vido.

Tsunami damage Japan. Photo by Roberto De Vido.

even door to door notification. You may have longer to pack up essential supplies, but don’t forget evacuation traffic jams if everyone tries to leave with their cars. You may get outside help sooner, but destruction will still be impressive. Just look at the tsunami maps and imagine everything destroyed in the inundation zone.

Now you know why I have successfully avoided the temptation of purchasing a beach house. If I can’t live near the water, why bother. If I do buy something near the water, it will just wash away in the tsunami – not a good long-term investment plan.

Stay safe,

Sheila Sund, M.D.

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