Buckling occurs because there exists a partial vacuum caused by movement of the fluid inside the tank.
FROM:
http://lef.uprm.edu/Failure%20of%20two%20Tanks/Examples%20earthquakes.html
The webmaster writes: "There are two main modes of failure in tanks during earthquakes: elephant foot buckling [shown in previous slide] and buckling at the top of the shell [shown here]. Tanks are more vulnerable to earthquakes when they are full of liquid."
This research was supported by NSF-CCLI, DUE-0736828: "A Computer-Based Simulated Environment to Learn on Structural Failures in Engineering"
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