[ckissick]

I'm a soils engineer so I might be of some help. The post moved because of a lack of passive resistance in the soil to counteract the moment arm caused by the tension in the wires at the top of the post. It doesn't look like there's room for a guy wire.

You just need deeper piers. They need to be deep enough to mobilize passive resistance in the soil. The upper 12" or so of soil is typically ignored since the soil deforms upward due to a lack of confining pressure from above. The depth depends on the strength of the soil (which translates to passive resistance) and the force from the wires. Wind load would be low since the tarp is horizontal.

How deep are the piers right now? I bet if you go 5 feet deep that will be enough. 16" inch diameter should be enough too.