Designing Rules For Causing An Earthquake

[Squire Doodad]

So at some point in the MitD threads, we had a discussion about whether or not there were rules for making an earthquake through sheer might instead of casting the spell. Long story short, we didn't find anything from before 2004, but just for the heck of it I decided to write some. I don't have any experience writing homebrewed rules, but I have been around here for over a year, so hopefully this at least makes sense.

Here's what I wrote.

"Any creature capable of hitting the ground can create an earthquake. Creating an earthquake involves an intentional, meaningful strike on the ground for most creatures, but it is not required as any sufficiently big creature could create one just by walking.
A creature capable of creating an earthquake by hitting the ground can make any earthquake equal to or lower than the maximum they are capable of causing, or Earthquake Level. The effects of decimal values are not significant at low levels, but are instead subject to the digression of the GM.
A character whose strength is between two Earthquake Levels can attempt to create an earthquake between those levels (a 3.4 earthquake at 27 strength, for example) by rolling an additional Strength Check with a DC of (5+5*Earthquake Level). The DC on the Strength Check is reduced by two for each size category above Large.
The minimum Strength required to create an earthquake at a given Richter level starts at 17 for a category 2 or below. It then increases at a rate of (3+1.25 * Earthquake Level), rounded up. The Strength Requirement for a given Minimum Earthquake Level is decreased by 1 for each size category above Large.
Strength Requirements for Earthquake Levels without modifiers:
Richter 1: 17
Richter 2: 17
Richter 3: 24
Richter 4: 32
Richter 5: 42
Richter 6: 53
Richter 7: 64
Richter 8: 77
Richter 9: 92"

Does this seem like an interesting idea? I don't have any details on what kind of action it would take, or what effects the earthquake would have (I think there are rules for earthquakes in general), so this is a bit incomplete at the moment.

[DracoDei]

While less "physics" and more "quasi-magical martial arts", I did make something about earthquakes, not to mention the officially published Earthstrike Quake on page 82 of "Tome of Battle: The Book of Nine Swords". That is also the book that created the sub-system that I was working with to make my relevant stuff.

You want the "Quake" series (https://forums.giantitp.com/showthread.php?t=582822), not the "Faultline" series, so scroll down a bit to get to the first one. Note that your options for how wide an arc it can cover and how large the radius (or length for a line) can be increase with maneuver level.

[Squire Doodad]

While less "physics" and more "quasi-magical martial arts", I did make something about earthquakes, not to mention the officially published Earthstrike Quake on page 82 of "Tome of Battle: The Book of Nine Swords". That is also the book that created the sub-system that I was working with to make my relevant stuff.

You want the "Quake" series (https://forums.giantitp.com/showthread.php?t=582822), not the "Faultline" series, so scroll down a bit to get to the first one. Note that your options for how wide an arc it can cover and how large the radius (or length for a line) can be increase with maneuver level.

Physically as in "taking your physical body and slamming it into the ground". The laws of physics are largely irrelevant here, though I imagine anything with 92 STR can, in fact, cause some shaking by slapping the ground.

I was actually thinking of those martial art characters in stories where they strike the ground and make a gaping chasm, so "magic martial arts" works just fine, thanks for the link!

Would it make sense for triggering an earthquake to be at base a full-round action for creatures at Huge or smaller, and then a standard action for creatures at Gargantuan and larger? After all, a DnD Tarasque can probably make the ground shake with little effort.

[aimlessPolymath]

Some considerations for stride and for deliberate strikes:
-The Richter scale is a logarithmic one, with each additional point representing approximately 30 times (10^1.5) the energy of the one below it.
-From this (https://www.volcanodiscovery.com/earthquakes/energy.html)page, a 100 kg person falling 2 m produces a -1.0 magnitude earthquake.
-Because gravitational potential energy is linear with height and mass, we can roughly estimate that a multiplier to mass or height produces a comparable change in energy.
-I, a human, lift my foot approximately 10 cm per stride. (https://jeb.biologists.org/content/jexbio/early/2016/07/28/jeb.137356.full.pdf) This is 1/20 the 2 m drop, which means roughly a -2 (-1.88) magnitude earthquake. A stomp can roughly quadruple your effective weight (https://physics.stackexchange.com/questions/280563/how-does-one-typically-determine-the-energy-and-force-in-a-foot-stomp) for a reasonably strong person, and covers about 30 cm; this is worth about 3/4 of an additional point of magnitude (log base 30 of 12).
-Going up a size category increases my mass by a factor of 8, and my stomp length doubles; this is worth about 4/5 of a point of magnitude, since (8*2)^(5/4) = 32.
-For high-strength creatures, for every 10 points of Strength, your lifting ability quadruples (https://www.d20srd.org/srd/carryingCapacity.htm). So a +5 bonus to the Strength check from those 10 points is worth about 2/5 of a point of magnitude, more or less, since 4^(5/2) = 32. Note that I'm using this as an estimate of how force scales with strength, not lifting capacity as a direct measure of impact strength.

[Squire Doodad]

Some considerations for stride and for deliberate strikes:
-The Richter scale is a logarithmic one, with each additional point representing approximately 30 times (10^1.5) the energy of the one below it.
-From this (https://www.volcanodiscovery.com/earthquakes/energy.html)page, a 100 kg person falling 2 m produces a -1.0 magnitude earthquake.
-Because gravitational potential energy is linear with height and mass, we can roughly estimate that a multiplier to mass or height produces a comparable change in energy.
-I, a human, lift my foot approximately 10 cm per stride. (https://jeb.biologists.org/content/jexbio/early/2016/07/28/jeb.137356.full.pdf) This is 1/20 the 2 m drop, which means roughly a -2 (-1.88) magnitude earthquake. A stomp can roughly quadruple your effective weight (https://physics.stackexchange.com/questions/280563/how-does-one-typically-determine-the-energy-and-force-in-a-foot-stomp) for a reasonably strong person, and covers about 30 cm; this is worth about 3/4 of an additional point of magnitude (log base 30 of 12).
-Going up a size category increases my mass by a factor of 8, and my stomp length doubles; this is worth about 4/5 of a point of magnitude, since (8*2)^(5/4) = 32.
-For high-strength creatures, for every 10 points of Strength, your lifting ability quadruples (https://www.d20srd.org/srd/carryingCapacity.htm). So a +5 bonus to the Strength check from those 10 points is worth about 2/5 of a point of magnitude, more or less, since 4^(5/2) = 32. Note that I'm using this as an estimate of how force scales with strength, not lifting capacity as a direct measure of impact strength.

The main thing I'm getting from this is that a size category is worth almost a magnitude each. Does that mean that each size category above Huge would let a creature do an additional point on the Richter scale?

So working off of 3.5: a Tarrasque, with a STR of 45, would normally be able to do a 5.0 quake; with this, it would be capable of doing a 7.0 one. Given that a 5 quake is damaging while a 7 is a major one, that does make sense for something that is colossal... but then, the Epic Ha-Naga has not even 30 STR but is Colossal and so would be able to pull off a 5.0 tremor just like a medium creature with the STR of a Tarrasque would. Does this sound balanced?


Maybe each size past Medium gives +4 (maxing at +16 to a Colossal; it negates the constant in the 3+1.25*x) towards the scale, and then the requirements should be reduced a bit. Say, lumping in Richter 3 with the first two since that's the first one people can feel and therefore that the player can use in combat, and/or reducing the base to 15 so it can be accessed by players more easily.

[aimlessPolymath]

With some fudged numbers, my math says this:
A Medium creature naturally creates a -2 magnitude earthquake as they walk, and can stomp with Str 10(more or less) to increase that by 1 point.
Each increased size category adds a point of magnitude to both of those numbers.
Each 10 additional points of strength (or equivalent +5 bonus to ability checks) add 1/2 a point of magnitude when you stomp.

One thing not mentioned in these calculations is the effect of having the earthquake epicenter so much closer to the surface; while this doesn't change the magnitude of energy, the intensity experienced will be much higher.

[Squire Doodad]

So, I made a revised ruleset.
The main differences are size categories give a +4 per category above Medium, and I lowered the starting point to 13 so as to make it more accessible to Medium creatures. Also some fine-tuning. I'm not sure how many rounds it should take for a tremor to start, so I put "up to 3 rounds" (Delayed Fireball is up to 5) as a reasonable sounding stand in.


Making a tremor

Any creature capable of hitting the ground can create an earthquake. Creating an earthquake involves an intentional, meaningful strike on the ground for most creatures, but bigger creatures may do it just through their mighty strides. Creatures in a gaseous form cannot create an earthquake by striking the ground.

Earthquakes are not instantaneous and take place from 1 to 3 rounds after they are created, at the digression of the user.

A creature can create an earthquake of power lower than or equal to their Earthquake Level (this uses the Richter scale; see below). These are abstractions, not actually producing the quake on the surface; making a "Level 4 Earthquake" means "making an earthquake that feels like a naturally occurring earthquake that hits a 4 on the Richter scale". The effects of decimal values are not significant at low levels. Their use, if any, is subject to the digression of the GM.

A character whose strength is between two Earthquake Levels can attempt to create an earthquake between those levels (a 3.4 earthquake at 27 strength, for example) by rolling an additional Strength Check with a DC of (5+3*Earthquake Level). The DC on the Strength Check is reduced by two for each size category above Medium.

The act of creating an earthquake requires a full turn action for Large or smaller creatures. Huge and larger creatures can create an earthquake once per round as a standard action.

Scaling

The minimum Strength required to create an earthquake at a given Richter level starts at 15 for a category 2 or below. It then increases at a rate of (3+1.25 * Earthquake Level), rounded up.

Any creature of a size category above Medium can automatically create Richter 1 and 2 tremors, and get +4 to their Earthquake Level for each size category above Medium.
Any creature with a feat or other ability giving it affinity to or significantly above-average knowledge of the earth gets a +4 to their Earthquake Level.
For example, a Gargantuan creature with a Strength score of 26 would be able to create a Richter 3 quake from their Strength score, but because of the +12 from the size category can create a Richter 5 quake (38 points on the scale).


Strength Requirements for Earthquake Levels without modifiers:
Richter 1: 13
Richter 2: 13
Richter 3: 20
Richter 4: 28
Richter 5: 38
Richter 6: 49
Richter 7: 61
Richter 8: 74
Richter 9: 89