That depends on the strength of the Iron. Since GCI has no elongation (to speak of) The yield strength is the same as the tensile strength. For each class of iron there is a minimum spec given. I dont know the modern designations you can find a great paper on GCI here:
Gray and ductile iron castings handbook; including data on gray, ductile, white, and high alloy irons
•• By: Charles F Walton
•• Publisher: Cleveland, Gray and Ductile Iron Founders' Society, 1971
Wouldn't you want a material that has more forgiveness to it?
Something with a nice, stretchy quality. Do you live in an earthquake zone? Trick question. My physics professor used to say everywhere is an earthquake zone.