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It is well known among regular beach goers that a sudden shoreline drawback is often a warning sign for an impending Tsunami. My understanding of Tsunamis is they they form as a result of the seafloor abruptly changing, causing a local vertical displacement of water at the site of above the disruption, which initiates the wave.

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How does this process ultimately result in the shoreline often receding prior to the Tsunami reaching the coast?

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2 Answers 2

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It has nothing to do with the geological cause of the tsunami. Instead, it's a result of the way waves propagate. You can see the same effect on ordinary wind-generated ocean waves — the waterline draws back before each wave peak arrives and washes up the beach. Tsunamis are much bigger waves, in terms of both amplitude and wavelength, so the effect is more dramatic.

The particles in some surface waves, including wind waves and Rayleigh waves (a component of what is often called ground roll), have in a circular or elliptical motion — in the case of a wind wave the motion is clockwise if the wave is traveling from left to right (see this animated comparison for Rayleigh waves). The 'backwards' motion in the trough results in the drawback you are asking about.

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  • $\begingroup$ Excellent description Matt. Couldn't be explained any better. I had taught vibrations & worked with guided waves and yet couldn't figure out the reason behind the initial water recession (I confess). My prior thought was that the type of the fault, normal or compression, was the reason! However, there's still one ambiguity for me. Apparently, the leading edge of the wave reaching the coast isn't always a trough (receding water)? right? is it due to the slope of the coastal ground? i.e. wave breaking? $\endgroup$
    – A. Zadeh
    Commented Jul 29, 2023 at 11:23
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Because water particles move in eliptical or circular movement and since the wavelength of a tsunami is rather long (up to 500km) you see the lower part or the eliptical movement disappear on sand as it is above the sea level, which is the SWL (Still Water Level) of any wave.

All waves in water do this and we can easily observe the back and forth movement of water waves, but tsunami have much longer wavelengths, that's why.

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  • $\begingroup$ I'm not sure you got your meaning across well in this answer, it sounds confusing (as indicated by someone's vote). Perhaps reconsider the wording if English isn't your primary language, as you sound in others spots to be knowledgeable on oceanographic topics and such :-) $\endgroup$ Commented Aug 5, 2023 at 5:00
  • $\begingroup$ When refracting, water waves increase amplitude but decrease wavelength while reducing speed... up to a ratio of 1:7 when they break and release energy. A tsunami with 500km wavelength will refract till its amplitude is about 25m ... that means its wavelength will be 7 X bigger 175m... so the shoreline will be reduced and exposing 175m of extra sand. $\endgroup$ Commented Aug 8, 2023 at 14:16
  • $\begingroup$ What I don't get then in rereading your answer is "water particles move in elliptical or circular movement". Are you saying all water molecules are continuing traveling in circular paths everywhere, and that somehow contributes to the wave shape??? I think that's the confusing part :-) $\endgroup$ Commented Aug 9, 2023 at 2:06
  • $\begingroup$ Yes. The movement of water particles is circular or eliptical... a 2D propagation. They move up and down, back and forth. Longitudinal and transverse at the same time. The up and down movement you notice very easily right? But the Longitudinal movement you don't... however you can feel the Longitudinal movement when you are inside the water and get carried back and forth by the water. $\endgroup$ Commented Aug 10, 2023 at 9:07
  • $\begingroup$ Perhaps it's best to use a different word than particles? Because it's not the molecular level, but the larger scale you're talking then? (And I'd also argue against you agreeing with my word "everywhere"... certainly there's circulations [a better word!?] in waves and such, and much of near shore water is moving in that pattern... but in the larger scale, there's also currents and such that aren't particularly circular [even as they taken as a whole do of course eventually complete a circuit and a vertical cycle]). Just trying to help make it an answer others understand :-) $\endgroup$ Commented Aug 10, 2023 at 9:46

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