If the Franchise Tax Board (FTB) is notified that the bank account consists only of Social Security earnings, your account would not be garnished. Social Security income is excluded from state tax collection activity. It's important to note that having Social Security income does not stop withholding orders for earnings from being issued if the taxpayer is already in active collections, and it also does not stop liens from being filed.
If you open a bank account and deposit your Social Security earnings, you should contact the FTB to let them know. Otherwise, when your bank account is located and there is a pending lien against you, the FTB will take actions to levy or attach the bank account as part of their collection activity, as long as due process has been served. You'd then have to show proof that the deposits are Social Security and therefore not garnishable.
It is important to note that if you deposit other income into your new bank account, the FTB can levy those non-Social Security amounts. That's why it is important to contact them and provide substantiation as to what deposits are related to Social Security.