Securing heavy steel shelving unit (sitting on floor) to walls (earthquake prep)
I'm not an engineer, but I've built and destroyed a great many things in nearly half a century of living. Here's what I'd do.

Fasten a continuous 2x6 to the wall behind the top of the shelving units. It should extend past each unit to the nearest stud, and it should be fastened to each available stud with at least two fine-thread construction screws.
Secure each upright in the shelving units to the 2x6 with 5/16&quot; eye bolts and a loop of 3/16&quot; chain. Use threaded chain links to close the deal. They don't need to be tight. In an earthquake everything's moving somewhat anyway.

Plan view:
 _^_______________^_______________^______________^_____  &lt;-- screw locations
|______________________________________________________| &lt;-- 2x6 board
     *   *                                 *   * &lt;---------- chain
     *|-|*                                 *|-|* &lt;---------- shelving upright
     *****                                 *****