While this was originally a calc to determine the new tiers for Mortal Kombat God Tiers, this is something that can be applied to any feat of shaking the entire Earth. So, without any more trouble, let's dive in.
This method assumes that all they're doing is causing the Earth to quake via sheer brute Force. This is what is usually used for the standard Earthquake feat, but, if there's sufficient evidence they're also moving the plates via magic or sheer rule of cool, you can move to the next section.
Either way, first we'll need to determine the kind of magnitude needed to cause the entire Earth to quake. We'll assume that it feels like a Magnitude 4 across the world, just standard noticable shaking with no real damage.
To find how strong of an impact it truly was, you use this equation:
(Magnitude at distance) + 6.399 + 1.66×log((distance from epicenter in KM/110)×((2×π)/360)) = Richter Magnitude of Earthquake
In our case, it would be, using half of the Circumference of earth,
(4)+6.399+1.66×log((20037.5÷110)×((2×π)÷360)) = Magnitude 11.2328648415393
Now, we take the magnitude and use the formula for a joulecount from said magnitude listed in Earthquake Calculations
10^(1.5*(11.2328648415393)+4.8) is 4.459613919339E21 Joules, 1.06587330768147 Teratons, Small Country level
Total Seismic Energy
This method is only applied when you have proof you're causing it through manually moving the plates, as elaborated above. We'll use the same magnitude, 11.2328648415393, but, we'll use a different formula.
This time, we'll use 10^(1.5×11.2328648415393)×10^9.091 to get...
8.715599440089E25 Joules, 20.8307826005974 Petatons, Multi Continent level
If said quakes are shaking the world, but, said world isn't Earth, then you need a specific calculation for that verse. For example, Kirito shook the underworld, but due to the underworld not quite being Earth, a new calculation using official maps to determine radius had to be made.
Also note that the above is just a baseline, assuming the quakes are noticable across the world, but don't really cause any damage. You will usually use the above values if there is a statement of world shaking, but it is unknown the exact sort of magnitude that was displayed. If there is evidence for more intense shaking that indeed causes incredible damage, you can use the charts below for reference:
List of all other magnitudes of Earthquakes for reference
|Magnitude 1||1.410253747010E17||33.7058734945178 Megatons||City level|
|Magnitude 2||4.459613919340E18||1.06587330768175 Gigatons||Large Mountain level|
|Magnitude 3||1.410253747010E20||33.7058734945178 Gigatons||Island level|
|Magnitude 4||4.459613919339E21||1.06587330768147 Teratons||Small Country level|
|Magnitude 5||1.410253747010E23||33.7058734945178 Teratons||Country level|
|Magnitude 6||4.459613919340E24||1.06587330768175 Petatons||Continent level|
|Magnitude 7||1.410253747010E26||33.7058734945178 Petatons||Multi Continent level|
|Magnitude 8||4.459613919340E27||1.06587330768175 Exatons||Multi Continent level|
|Magnitude 9||1.410253747010E29||33.7058734945178 Exatons||Moon level|
|Magnitude 10||4.459613919340E30||1.06587330768175 Zettatons||Small Planet level|
Total Seismic Energy
|Magnitude 1||2.756114540437E21||658.727184617103 Gigatons||Large Island level+|
|Magnitude 2||8.715599440092E22||20.8307826006027 Teratons||Country level|
|Magnitude 3||2.756114540437E24||658.727184617103 Teratons||Large Country level+|
|Magnitude 4||8.715599440089E25||20.8307826005974 Petatons||Multi Continent level|
|Magnitude 5||2.756114540437E27||658.727184617103 Petatons||Multi Continent level|
|Magnitude 6||8.715599440092E28||20.8307826006027 Exatons||Multi Continent level+|
|Magnitude 7||2.756114540437E30||658.727184617103 Exatons||Small Planet level|
|Magnitude 8||8.715599440092E31||20.8307826006027 Zettatons||Small Planet level|
Any further is Planet level, IE, the Earthquake would literally cause the destruction of earth, and is thereby completely impossible, unless some unique circumstance arises in the specific work that would permit it. By which case, the calculation can be made with that specific contextualization.