Frost line depth in Marion County, OR
How deep the ground freezes in Marion County, OR
9 in. Dig to 12 in, the 12 in code minimum. Nearest NOAA station: Silver Creek Falls, 4 mi.
Your building department sets the legal frost line depth (IRC Table R301.2(1)); footings must go below it (R403.1.4.1). Call them before you dig.