Water tanks and earthquakes

Well the tpye of soil, amount of water held by the soil and the amount of rock.

Take into consideration the if shaken enough the earth with become a soup or quick sand and swallow anything with weight or push it down hill.

Then consider the rocks that will burst the tanks.

The forces acting on the tank will be calculated the same as for any other structure. You are going to have vertical and horizontal compnents of acceleration and velocity related to the seismic wave function, site-specific soils, etc..

Rigidity plays a major role in an underground water tank, relating to earthquake stresses. Any reinforcement, would have to be considered as minimal, seeing as how powerful the p-, and s-waves are, and how long the earthquake event would be, is therefore impossible to calculate. It would be one case where over-engineering a tank, would not be a bad thing.