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Do We Have to Worry About Tsunamis and Seiches

June 30, 2020

The National Weather Service has studied every kind of wave and has tracked tsunamis and possible tsunamis, those generated by earthquakes and those induced by volcanic activity. Tsunamis are giant waves caused by movement of plates in the earth or underwater eruptions under the surface of the sea. The Weather Service tells us that anywhere there is water there can be a tsunami. Tsunami is the Japanese word for harbor wave.
The National Weather Service has also studied seiches. A seiche is the sloshing of water in an enclosed or almost enclosed body of water that can produce tsunami-like waves. The Good Friday earthquake in Alaska in 1964 produced tsunamis in Alaska and caused seiches in such distant places as Lo
uisiana. This same earthquake  got the water in New Jersey swimming pools sloshing back and forth.
Old timers in Alaska remember the 9.2 Alaska quake that shook the ground in King Cove, six hundred miles south of Anchorage. Anchorage and Kodiak had major damage to buildings, paved streets, sidewalks and water and sewer mains that were not built to earthquake zone standards. In Port Valdez an underwater landslide collapsed the harbor and the docks and destroyed the boats docked there. A 27 foot tsunami destroyed Chenega, a native village, killing 23 of the 68 village residents. Tsunamis also affected other communities in Alaska, in British Columbia, Oregon and California. What the old timers still talk about in King Cove, were the tides that followed the big earthquake. At King Cove the high and low tide cycles came more frequently and were much higher and much lower  than they normally are, but there was never one large tsunami wave.
There were two types of tsunamis produced by the Alaska earthquake. Over twenty countries saw tsunami waves caused by the Alaska Good Friday quake, including Japan, Mexico and New Zealand. The largest tsunami wave was 220 feet in Shoup Bay, Alaska.  Of the 131 people who died, only nine died as a result of the earthquake. The others died as a result of the tsunami waves.
In 1960 the largest earthquake ever recorded anywhere, 9.5, hit Chile. The earthquake and tsunamis that followed caused millions in property damage and killed over 2,000 people. The tsunamis that reached Hawaii and Japan killed 61 people.
The 8.9 earthquake in 2011 in Japan caused a 30 foot tsunami. The Richter magnitude usually has to read 7.5 for an earthquake to produce a destructive tsunami. A video taken from a Japanese hillside after the earthquake  shows people climbing the side of the hill trying to escape as the tsunami came in, picking up and carrying off houses and belongings as it tore through the village. Tsunamis can travel 500 to 600 miles per hour. The video can be seen at Livescience.com. It was part of an article, “No, you can’t outrun a tsunami.”
When tsunami-like waves crashed on the southern Massachusetts and New Jersey coasts in June of 2013, there were no earthquakes or landslides that could have been responsible. Under clear skies three people were injured when a six foot wave swept them off a Barnegat Inlet jetty into the water. Because the timing of the wave coincided with a severe weather pattern offshore (high waves were captured by NOAA’s coastal water level stations from Puerto Rico to New England), scientists at the time called the event a meteotsunami. It was also reported  by a Deep Ocean Assessment and Reporting of Tsunamis (DART) buoy 150 miles offshore. Meteotsunamis are caused by air pressure disturbances associated with fast moving weather systems. The waves these disturbances create can travel at the same speed as the weather system. Hitting shallow water causes it to slow down and increase in height and density.
Another meteotsunami occurred in Boothbay Harbor, Maine in October, 2008, when a series of waves up to 12 feet high flooded the harbor and emptied it  at least three times in a 15 minute period, damaging boats and shoreline infrastructure.
The National Weather Service described a seiche as a large slosh on a lake where water from one side of the lake is pushed to the other side and then returns. The oscillation of water levels continues for some time until the lake stabilizes. Seiches on the Great Lakes usually occur in response to strong, prolonged winds blowing across the lake’s surface, especially when the wind is aligned along the long axis of the lake. The Weather Service official definition of a seiche is “a standing wave oscillation of water in large lakes usually created by strong winds and/or a large barometric pressure gradient.” The enclosed or almost enclosed body of water the Weather Service later refers to can be a reservoir, swimming pool, bays, harbors and seas such as the Adriatic Sea which is bounded by Italy, Albania, Montenegro, Slovenia, Bosnia and Croatia but is not completely enclosed by land mass.
Seiches are more likely to happen when the body of water is disturbed by wind, by atmosphere, seismic activity or by tsunamis.  Low, rhythmic seiches are usually found  on larger lakes and go unnoticed.  Because it is shallow and oriented in a northeast-southwest axis which matches the prevailing wind direction, Lake Erie can have extreme seiches that put the water level up to 16 feet higher from one end of the lake to the other.
There are also seiches generated by earthquakes – the tremors from quakes often match the frequencies of the small bodies of contained water, such as swimming pools. The Good Friday earthquake in Alaska was responsible for seiches in swimming pools as far south as Puerto Rico. The quake in Tibet in 1950 caused seiches in southern New England and in Norway.
There have been so many floods in Venice that the entrance to the Venetian Lagoon has been rebuilt with 79 separate 300 ton flaps that rise up between the Lagoon and the Adriatic Sea to prevent flooding. Seiches in the Adriatic are not the only cause of flooding. Six hundred years ago the City diverted four rivers that flowed into the lagoon. Erosion and lack of sand replenishment followed. From the 1920s to the 1970s fresh water was pumped  from an aquifer under the lagoon. The city is sinking and the sea level is rising. The flood gates that are still unfinished, that will cost at least $6.3 billion are seen as a way to buy time to find a better solution.
The gates fill with water and sit on the bottom until a high tide when, on a signal, air is pumped into the gates to force the water out, allowing the gates to come to the surface and protect the lagoon from the incoming high tide. After the high tide surge the gates fill with water and sink back to the bottom. Oceanographers looking at the MOSE gate system say the idea is an old one, conceived by 18th century Venetian engineers. The gates will protect Venice from major floods such as the 1966 flood, but are not geared to protect low lying areas like St. Mark’s Square. As global warming brings tides up higher, the gate system will become inadequate. The more the gates are used, wear and tear increases. The gates have already had structural problems which may shorten the life of a system not yet finished.
To prepare for future flooding, other countries have been working on their own strategies to save lives and reduce property damage. Villages in India had fewer casualties from the 2004  tsunami because they planted trees along the coastline. Japan has built seawalls and vertical platforms for people to stand on. They are finding that the less obstruction a wave has in its way, the less damage it does. They are incorporating larger windows and doorways in new building in coastal areas that offer the water a clearer path to go through a house.  Inside doorways and windows are aligned instead of staggered so waves don’t get locked in a house. This allows for cheaper, easier reconstruction after wave damage.
Warning systems have been in place and have significantly improved. Warnings for tsunamis are usually triggered by earthquakes, not all of which generate tsunamis, so there are a lot of false alarms. If the earthquake or underwater occurrence that precipitates the tsunami is close by, the first tsunami wave may come ashore before the system can predict it.
Because these waves are mostly in the Pacific Ocean, people who don’t live there don’t have an awareness of tsunami danger. Vacationing tourists may ignore signs that would protect them. Survivors of several earthquakes had some good general advice.

• Tsunamis often cause the ocean to withdraw from the shore, coming back with a sound survivors compared to a train or jet plane engine’s roar. If you  should see the ocean pull back, run as fast as you can to higher ground.

•  If you get caught by a wave try to find floating debris to help you stay afloat.

•  Don’t think about belongings - just move out of the path of the water.

Do we have to worry about tsunamis and seiches - probably not, but it’s not a bad thing to remember what key advice the survivors had and be aware of local weather reports  anytime you are on the water or in the water.


 

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