Well:
Powering-up items ALL boosts power by 20% (except Wise Glasses and Muscle Band which boosts by 10% and Life Orb by 30%)
Base power of Earthquake is 100, Torterra's Earthquake is 150 (STAB), holding Soft Sand or Earth Plate 180, holding Life Orb 195, Muscle Band 165.
0% of all are 0 power
25% of 100 is 25
150 is 37,5 (38?)
165 is 41,25 (42?)
180 is 45
50% of 100 is 50
150 is 75
165 is 82,5 (83?)
180 is 90
200% of 100 is 200
150 is 300
165 is 330
180 is 360
400% of 100 is 400
150 is 600
165 is 660
180 is 720
____
BTW, most powerful attack without counting stats is Blast Burn:
150 Blast Burn
1,5 STAB
1,5 Sunny Day
1,5 Blaze
1,3 Life Orb
2 Critical Hit
4 on Parasect
*1,25 by Parasect's Dry Skin
It's base power is 6581,25 (6582?)