<p>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.</p>
<p>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.</p>

CC BY-SA 4.0
2023-08-05T05:00:20.440Z
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 :-)
JeopardyTempest
CC BY-SA 4.0
2023-08-08T14:16:28.990Z
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.
Rafael Garcia
CC BY-SA 4.0
2023-08-09T02:06:51.697Z
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 :-)
JeopardyTempest
CC BY-SA 4.0
2023-08-10T09:07:22.787Z
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.
Rafael Garcia
CC BY-SA 4.0
2023-08-10T09:46:46.963Z
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 :-)
JeopardyTempest
CC BY-SA 4.0
2023-08-19T07:41:13.417Z
Particles is the right terminology in Physics.  All waves in Physics are classed according to the Direction of vibration of their Particles, being transverse or longitudinal (or both which is the case here) except waves on spherical surfaces like cilinders or balls . These are classed according to the number of variables on their equations, so actually it's their equations which are classed.
Rafael Garcia
CC BY-SA 4.0
2023-08-04T15:26:23.377Z
2023-08-04T15:26:23.377Z
Rafael Garcia
<p>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. </p>

<p>The particles in some surface waves, including <a href="http://en.wikipedia.org/wiki/Wind_wave">wind waves</a> and <a href="http://en.wikipedia.org/wiki/Rayleigh_wave">Rayleigh waves</a> (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 <a href="http://www.acs.psu.edu/drussell/demos/waves/wavemotion.html">this animated comparison</a> for Rayleigh waves). The 'backwards' motion in the trough results in the <a href="http://en.wikipedia.org/wiki/Tsunami#Drawback">drawback</a> you are asking about.</p>

CC BY-SA 4.0
2023-07-29T11:23:34.010Z
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?
A. Zadeh
CC BY-SA 3.0
2014-04-23T00:05:27.737Z
2014-04-23T00:05:27.737Z
Matt Hall
<p>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. </p>

<p><img src="https://i.sstatic.net/OFN7s.jpg" alt="enter image description here"></p>

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

<ocean><tsunami>
Why does the shoreline sometimes recede prior to a Tsunami?
Kenshin
# Why does the shoreline sometimes recede prior to a Tsunami?

(Asked by: username_0 on 2014-04-22)

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.

![enter image description here](https://i.sstatic.net/OFN7s.jpg)

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

**Tags:** ocean, tsunami


---

## 2 answers

### Answer by username_3 on 2014-04-23. Score: 12 | Accepted answer

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](http://en.wikipedia.org/wiki/Wind_wave) and [Rayleigh waves](http://en.wikipedia.org/wiki/Rayleigh_wave) (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](http://www.acs.psu.edu/drussell/demos/waves/wavemotion.html) for Rayleigh waves). The 'backwards' motion in the trough results in the [drawback](http://en.wikipedia.org/wiki/Tsunami#Drawback) you are asking about.

> **Comments:**
> **username_4**: Excellent description username_3. 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?  

---

### Answer by username_1 on 2023-08-04. Score: -1

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.

> **Comments:**
> **username_2**: 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 :-)  
> **username_1**: 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.  
> **username_2**: 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 :-)  
> **username_1**: 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.  
> **username_2**: 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 :-)  
> **username_1**: Particles is the right terminology in Physics.  All waves in Physics are classed according to the Direction of vibration of their Particles, being transverse or longitudinal (or both which is the case here) except waves on spherical surfaces like cilinders or balls . These are classed according to the number of variables on their equations, so actually it's their equations which are classed.  

---

