# How to Create Tidal Tsunamis

In my world, I have the problem of dealing with a world that doesn't experience seasons or significant latitudinal variation when it comes to climate (it is essentially all tropical rainforest), and this entire things just seems kinda boring to me, so I am wanting to figure out a way to my landmasses more varied. Now, my story takes place in a modified version of the Trappist-1 system where the primary difference, when it comes to the orbits of the planets, is that instead of there orbits being in a near perfect resonance with one another, they are in a perfect resonance with one another, so there are seven worlds that each make (from furthest out to furthest in) 2:3:4:6:9:15:24 orbits around their star over the same period of time. Now, if you look at a chart of the system in real life (and mine isn't much different), you'll see that the innermost planets orbit about as far away from their start as callisto orbits from Jupiter, and the entire system could fit well within the orbit of Mercury around our sun. Because of this systems small size, each of the planets have a relatively large tidal effect on every other planet in the solar system. Given this knowledge, I came up with this solution to my problem: make the intertidal zones interesting.

One of the things I wanted to put in this world, and the thing I am asking you guys to help me create, is a way to create regular tidal tsunami's on the third, fifth, and sixth world of the systems; though I really only intend the regularly cataclysmic Tsunami's to occur on the third world from Trappist-1, or Trappist-1d. What I need help with is to #1 figure out how I would need to set up the system to create the largest tsunami's possible on Trappist-1d while also ensuring that no more than three planets align at any given point in time and that trappist-1f and g never align with h at the same time, and #2 I need to know what kind of geography a region on Trappist-1d would have to have in order for these Tsunami's to occur, and finally #3 I'd have to know how often these Tsunami's occur.

It should be noted that since the tides are primarily created by other planets orbiting the same star, they never last for long and often won't last all day.

This next section is not the question, but some additional information related to my system that I hope will help you answer the question.

Firstly, the planets are not tidally locked in relation to their star like they probably are in real life. I know this is innaccurate, and there are many other innaccurate things about this story, but such a setup was necessary for the world I wanted to build, and there is an in-world explanation for it. The most important thing to note is that each of these planets rotate at the speed required to generate a solar day that is as long as the solar days of every other world. This means the planets in this system rotate faster the closer they are to the star, and due to the small size of their orbits the difference is significant. What is important to note, however, is that this day-night cycle is in resonance with the orbits of the other worlds, which ultimately means the rotations are still in a resonance, though the resonance is weaker. The day-night cycle on each world is completed 36 times before the planets complete their resonance cycle (the period of time it takes for them to repeat their position in relation to one another). This fundamentally means that in my world, the tides will practically repeat themselves every 36 in-world days (each day is nearly the length of an earth day), so if the orbits of the planets do produce regular tidal tsunami's, the regions that experience them will be experiencing them atleast every 36 days, if not more often. This also is a fantasy setting, so they do not have any significant technological ability to deal with these regular tsunami's. Ultimately, all I want to know is how I would need to do to make my world experience regular, predictable, tidal tsunami's.

Edit: How Often the Planets Align

Since the tides will be greatest when the planets align, just so you guys didn't have to do the math and could answer my question easier, I'll give you the information for when Trappist-1d (in it's perfectly circular orbit) aligns with the other planets so you can more easily help me figure out the miximum tidal forces which are exerted on the world. Now, I don't know exactly when each of these transits happen in relation to one another, but I do know how many occur during each 36 day resonance period.

Trappist-1d aligns with b: 15 times a cycle (every 2.4 days) c: 6 times a cycle (every 6 days) e: 3 times a cycle (every 12 days) f: 5 times a cycle (every 7.2 days g: 6 times a cycle (every 6 days) h: 7 times a cycle (every 5.1428571 days [5+1/7 days])

• That's a very long question filled with, to me, lots of information that I'm not sure I need. It was a painful read. What is a "tidal tsunami?" (Reference) A tidal wave is caused by tidal forces (e.g., gravity). A Tsunami is caused by earthquakes, volcanism, etc. Tsunamis were once misidentified as tidal waves. Which are you looking for?
– JBH
Commented Jan 23, 2022 at 22:27
• Are you thinking of something similar to Summertide's (Charles Sheffield) annual planetary-wide sweep? Available on internet archive free Commented Jan 24, 2022 at 2:19
• Underwater quakes/tremors can easily lead to tsunamis, and they would be fairly easy to create if you introduce instability in the planetary core. Commented Feb 9, 2022 at 6:21

Besides being far too long and containing far too much irrelevant information, this question seems to be confused about what a tsunami is. It would greatly benefit by editing to a much smaller size and completely dropping the term "tsunami", which refers to large waves created by earthquakes.

But even then, the actual question that would remain still wouldn't make a lot of sense, given the laws of physics:

Newton’s law of universal gravitation states that the gravitational attraction between two bodies is directly proportional to their masses

Tidal generating forces vary inversely as the cube of the distance from the tide generating object.
What Causes Tides - Tides and Water Levels: NOAA's National Ocean Service Education

The question nowhere gives specific examples of the distance and mass of the fictional planets. If it did, the obvious thing to do would be to plug those values in the above equation and see how large the tides would be.

If we assume the mass of the Moon is 1 unit, and the distance between the Earth and the Moon is 1 unit, the tidal forces would be:

Mass-of-nearby-planet ÷ (Distance-to-the-planet)³


all measured in Moon units.

If we consider the actual case of Earth and Venus:

Moon mass = 7.34767309 × 10²² kg
Venus mass = 4.867 × 10²⁴ kg
= 4.867 ÷ 7.347 × 10²
= 66 moon units

Moon distance = 3.6 × 10⁵ km
Venus distance = 3.8 × 10⁷ km
= 100 moon units (at closest approach)


Plugging those into the formula, at its closest approach the effect of Venus on Earth's tides is:

66 ÷ 100³
= .000066


That is, the tidal effect of the Moon is about 15,000 times as strong as Venus's.

One can only conclude that whatever the unspecified arrangement of the planets is, those so-called "tidal tsumanis" are going to be measured in hundredths of a millimetre, comparable to the thickness of a human hair.

• Firstly, I did give some indication of the distance (I noted it was a modified version of the Trappist-1 system, ie. it is the same system but modified to make math neater) so all I need is to convert the mass of the earth into moon units and convert the distances in the real world system to moon units and that will give me a basic idea of the tidal forces each planet experiences. I am accepting your answer because you gave me the tools to find it myself, though you did not give me the answer. Commented Nov 16, 2022 at 5:22