# Tsunami causes 100-foot-high waves? **URL:** \ **Category:** Factual Questions\ **Created:** [January 17, 2017, 12:34am UTC](https://boards.straightdope.com/t/tsunami-causes-100-foot-high-waves/777359 "2017-01-17T00:34:57Z")\ **Posts on this page:** 15\ **Page:** 1
**Author:** ![Velocity](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/velocity/32/18006_2.png) [@Velocity](https://boards.straightdope.com/u/Velocity)\ **Post date:** [January 17, 2017, 12:34am UTC](https://boards.straightdope.com/t/tsunami-causes-100-foot-high-waves/777359/1 "2017-01-17T00:34:57Z")
I read about how tsunamis cause massively high, towering waves, but the YouTube footage of tsunamis - such as the March 2011 Tohoku quake - shows merely a lot of seawater relatively gently flooding over the Japanese coast as it arrives at shore. Did the 100-foot high waves dissipate at sea before hitting land? ---
**Author:** ![Shagnasty](https://avatars.discourse-cdn.com/v4/letter/s/9dc877/32.png) [@Shagnasty](https://boards.straightdope.com/u/Shagnasty)\ **Post date:** [January 17, 2017, 12:49am UTC](https://boards.straightdope.com/t/tsunami-causes-100-foot-high-waves/777359/2 "2017-01-17T00:49:18Z")
There is a misconception that tsunamis are giant waves that can be spotted far out at sea. They aren’t. In fact, ocean going ships and submarines won’t even notice them as they pass. Instead, they are a giant wall of energy that only becomes apparent when they reach land. The English term “tidal wave” is more descriptive because they are like a huge tidal surge rather than a rogue wave. However, there is nothing gentle about a large one. The entire ocean recedes and then rises much higher than normal to swallow up everything close to sea level. [- YouTube](https://www.youtube.com/watch?v=j0YOXVlPUu4) (skip to about the 6 minute mark for the devastation). ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [January 17, 2017, 12:59am UTC](https://boards.straightdope.com/t/tsunami-causes-100-foot-high-waves/777359/3 "2017-01-17T00:59:20Z")
Tsunamis make really big waves, but they aren’t big breaking waves like some people picture. They are very long period waves, so they look more like a really big tide that keeps coming in. That’s why they are sometimes called tidal waves, though scientists don’t like that term since there’s nothing tidal about their formation. Instead of being over in a few seconds like a typical breaking wave, the period of a tsunami can be several minutes to more than an hour. They can also easily be a lot more than 100 feet. The record height for a tsunami is 1720 feet. See here for details: > **[World's Biggest Tsunami | 1720 feet tall - Lituya Bay, Alaska](http://geology.com/records/biggest-tsunami.shtml)** > > The tallest wave ever recorded was a local tsunami, triggered by an earthquake and rockfall, in Lituya Bay, Alaska on July 9, 1958. The wave crashed against the opposite shoreline and ran upslope to an elevation of 1720 feet, removing trees and... ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [January 17, 2017, 1:05am UTC](https://boards.straightdope.com/t/tsunami-causes-100-foot-high-waves/777359/4 "2017-01-17T01:05:17Z")
Right. Out at sea, tsunamis are typically only a few feet high, but can be 100 miles in length, very different from a normal surface wave. At sea they travel at high speeds; 500 miles per hour is not atypical, but even at that speed, it can take a long time, 15-30 minutes, for the water to rise and fall as the wave passes by, even though it only rises a few feet. The wave is therefore barely noticeable at sea. When it approaches shore, the shallower waters near land force the wave to slow down, the leading edge slowing first since it reaches the shallower water first. This causes the water to start “piling up”, since the trailing part of the wave is traveling faster than the leading part. This reduces the wavelength and greatly increases the height. Thus the Japanese name “tsunami”, which means “harbor wave”, since the wave is only noticeable when it approaches land. ---
**Author:** ![levdrakon](https://avatars.discourse-cdn.com/v4/letter/l/49beb7/32.png) [@levdrakon](https://boards.straightdope.com/u/levdrakon)\ **Post date:** [January 17, 2017, 1:31am UTC](https://boards.straightdope.com/t/tsunami-causes-100-foot-high-waves/777359/5 "2017-01-17T01:31:59Z")
I suspect people sometimes confuse tsunamis with rogue waves, which have been recorded pretty close to 100 ft. Most people I think have at least seen the famous Japanese print, [The Great Wave off Kanagawa](https://en.wikipedia.org/wiki/The_Great_Wave_off_Kanagawa) by the artist Katsushika Hokusai, often mistakenly thought to be of a tsunami, but really a rogue wave. ---
**Author:** ![Cugel](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cugel/32/1199_2.png) [@Cugel](https://boards.straightdope.com/u/Cugel)\ **Post date:** [January 17, 2017, 5:07am UTC](https://boards.straightdope.com/t/tsunami-causes-100-foot-high-waves/777359/6 "2017-01-17T05:07:49Z")
Have a look at this [![]( " - YouTube") ](https://www.youtube.com/watch?v=zygE87F4qDI) ---
**Author:** ![Velocity](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/velocity/32/18006_2.png) [@Velocity](https://boards.straightdope.com/u/Velocity)\ **Post date:** [January 17, 2017, 6:53am UTC](https://boards.straightdope.com/t/tsunami-causes-100-foot-high-waves/777359/7 "2017-01-17T06:53:58Z")
OK, thanks for the replies…so the “height” is not what we usually think of as “height?” It’s like amplitude, added up? So a tsunami can appear to be only 2-3 feet tall, but if it’s a very long wave, it adds up at all of its segments and becomes officially recorded as a 100 foot high wave? ---
**Author:** ![Saffer](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saffer/32/6440_2.png) [@Saffer](https://boards.straightdope.com/u/Saffer)\ **Post date:** [January 17, 2017, 8:13am UTC](https://boards.straightdope.com/t/tsunami-causes-100-foot-high-waves/777359/8 "2017-01-17T08:13:50Z")
> [@Velocity](#): > > OK, thanks for the replies…so the “height” is not what we usually think of as “height?” It’s like amplitude, added up? So a tsunami can appear to be only 2-3 feet tall, but if it’s a very long wave, it adds up at all of its segments and becomes officially recorded as a 100 foot high wave? Do you have a cite for a tsunami being recorded as 100 feet high? I don’t think I have even seen this. Sent from my iPhone using Tapatalk ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [January 17, 2017, 8:23am UTC](https://boards.straightdope.com/t/tsunami-causes-100-foot-high-waves/777359/9 "2017-01-17T08:23:24Z")
> [@Saffer](#): > > Do you have a cite for a tsunami being recorded as 100 feet high? I don’t think I have even seen this. Yeah, it’s difficult for us to judge how “something you’ve read” is wrong or possibly correct. Just like with tides geography will make a big difference to how high a tsunami will be when it inundates the shore, and unlike the tide the energy of a tsunami varies a whole lot. Send a high volume tsunami in a narrowing inlet and it definitely could reach a 100 feet. ---
**Author:** ![GreenWyvern](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/greenwyvern/32/2834_2.png) [@GreenWyvern](https://boards.straightdope.com/u/GreenWyvern)\ **Post date:** [January 17, 2017, 9:27am UTC](https://boards.straightdope.com/t/tsunami-causes-100-foot-high-waves/777359/10 "2017-01-17T09:27:56Z")
According to this article, the largest known tsunami occurred in 1958 in Lituya Bay, Alaska - a narrow inlet. “The force of the wave removed all trees and vegetation from elevations as high as 1720 feet (524 meters) above sea level.” > **[World's Biggest Tsunami | 1720 feet tall - Lituya Bay, Alaska](http://geology.com/records/biggest-tsunami.shtml)** > > The tallest wave ever recorded was a local tsunami, triggered by an earthquake and rockfall, in Lituya Bay, Alaska on July 9, 1958. The wave crashed against the opposite shoreline and ran upslope to an elevation of 1720 feet, removing trees and... There are some videos, photos and personal accounts on the page. ---
**Author:** ![MandaJo](https://avatars.discourse-cdn.com/v4/letter/m/ac91a4/32.png) [@MandaJo](https://boards.straightdope.com/u/MandaJo)\ **Post date:** [January 17, 2017, 10:51am UTC](https://boards.straightdope.com/t/tsunami-causes-100-foot-high-waves/777359/11 "2017-01-17T10:51:48Z")
> [@Velocity](#): > > OK, thanks for the replies…so the “height” is not what we usually think of as “height?” It’s like amplitude, added up? So a tsunami can appear to be only 2-3 feet tall, but if it’s a very long wave, it adds up at all of its segments and becomes officially recorded as a 100 foot high wave? I believe the total height is how deep the water gets on shore. But it takes a while to get that deep-it doesn’t crash down. ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [January 17, 2017, 10:58am UTC](https://boards.straightdope.com/t/tsunami-causes-100-foot-high-waves/777359/12 "2017-01-17T10:58:35Z")
If megatsunamis count - the variety created by things like major impacts and landmass collapses - then they can get as high as [three miles](https://en.wikipedia.org/wiki/Megatsunami#List_of_megatsunamis)tall. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [January 17, 2017, 11:13am UTC](https://boards.straightdope.com/t/tsunami-causes-100-foot-high-waves/777359/13 "2017-01-17T11:13:58Z")
> [@Velocity](#): > > So a tsunami can appear to be only 2-3 feet tall, but if it’s a very long wave, it adds up at all of its segments and becomes officially recorded as a 100 foot high wave? No. I highly doubt that. But we’ll be able to clear that up once you give some examples of 100-foot tsunamis you’ve read about. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [January 17, 2017, 11:36am UTC](https://boards.straightdope.com/t/tsunami-causes-100-foot-high-waves/777359/14 "2017-01-17T11:36:07Z")
> [@Velocity](#): > > OK, thanks for the replies…so the “height” is not what we usually think of as “height?” It’s like amplitude, added up? So a tsunami can appear to be only 2-3 feet tall, but if it’s a very long wave, it adds up at all of its segments and becomes officially recorded as a 100 foot high wave? No. Re-read **markn+**'s post above. The tsunami out at sea may be fairly small, but as it gets close to shore the height increases dramatically. For example, the 2009 Samoa earthquake’s tsunami measured 3 inches at the epicenter, in the ocean. At its highest point on the Samoan coast, it reached 46 feet. > **[2009 Samoa earthquake and tsunami](https://en.wikipedia.org/wiki/2009_Samoa_earthquake_and_tsunami)** > > The 2009 Samoa earthquake and tsunami took place on 29 September 2009 in the southern Pacific Ocean adjacent to the Kermadec-Tonga subduction zone. The submarine earthquake occurred in an extensional environment and had a moment magnitude of 8.1 and a maximum Mercalli intensity of VI (Strong). It was the largest earthquake of 2009. The earthquake initiated with a normal-faulting event with a magnitude of 8.1. Within two minutes of the earthquake rupture, two large magnitude 7.8 earthquakes occ The 2004 Indian Ocean earthquake’s tsunami measured approximately 33 meters (108 feet). Wikipedia doesn’t say exactly where that was measured. 2011 Tōhoku earthquake and tsunami (the one that led to all of the problems with the Fukushima Nuclear Power Plant) measured 133 feet at Miyako, in the Japanese prefecture of Iwate. Video of the event: [https://www.youtube.com/watch?v=VqIQFZZoVR4](https://www.youtube.com/watch?v=VqIQFZZoVR4) ---
**Author:** ![jonpluc](https://avatars.discourse-cdn.com/v4/letter/j/fbc32d/32.png) [@jonpluc](https://boards.straightdope.com/u/jonpluc)\ **Post date:** [January 17, 2017, 1:42pm UTC](https://boards.straightdope.com/t/tsunami-causes-100-foot-high-waves/777359/15 "2017-01-17T13:42:40Z")
This article says otherwise, it says the Alaska Tsunami was 100 feet high and that it caused damage at 1700 feet upslope. Thats not the same thing as saying a tsunami is 1700 feet high. [What Are the Biggest Waves in Recorded History? | Science| Smithsonian Magazine](http://www.smithsonianmag.com/science-nature/biggest-waves-recorded-history-180952432/) and [Scientist Confirm the Largest Wave Ever Recorded](http://www.popularmechanics.com/science/environment/a24326/worlds-largest-wave-recorded-near-scotland/)