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Photograph by Eduardo Fierro, BFP Engineers.
This photograph appears on FEMA's (Federal Emergency Management Agency)website:
http://www.fema.gov/earthquake/fema-e-74-reducing-risks-nonstructural-earthquake-damage-31
FEMA Document:
FEMA E-74 Example 6.4.2.2 Flat Bottom Tanks and Vessels
Buckles that are much longer in the circumferential direction than in the axial direction are often caused by axial compression (overturning moment from earthquake lateral motion) combined with internal pressure (from fluid inside the tank).
The highest axial compressive resultant (compressive force per circumferential arc length) occurs at the bottom of the tank. However, the tank is built up of courses that are thickest at the bottom and become thinner in steps with increasing distance from the bottom. The "elephant foot” buckle about half way up the tank occurs at a level where the tank wall thickness steps down from one course to the next higher course.
(In this picture there also appears an “Elephant’s Foot” buckle at the bottom of the tank, a buckle that is mostly obscured by bushes in the foreground.)
There is an image in the “Compound Structures” slide show that shows similar “Elephant’s Foot” buckles at mid-height and bottom of a finite-element model of a liquid storage tank analyzed by Godoy, et al.
Buckles that are long in the circumferential direction occur when the shell wall buckles at a level of effective stress possibly well above the yield stress, or, for mild steel, after there has been significant plastic flow in the extreme fibers of the tank wall due to local bending.
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