Can someone tell me either the minimum frost depth of footers and footings for northern Mighigan?
OR
An online site where I might find this information?
Your best bet is to contact your local building inspector, or building permit office. They will be able to give you the answer to assure compliance with local building code.
Anytime I've had the same question for various areas around the country, the local inspectors have been very helpful.
If you need "actual" frost line local plumbers or dirt work companies may help.