It looks to me like primarily a corrosion failure. I've seen where tanks that were held at nearly a constant level would could have a band of corrosion at the interface.
Usually, vacuum or wind blow-in would be a large area on one side of the tank, not a band around it like that. Usually, seismic buckling would be at the bottom.
Looks like a riveted tank to me. I remember one guy saying the last tank he had seen riveted was in the 1950's (in the US) and it was done that way to match an existing tank. However, that transition could have varied depending on local labor and practices.