The middle of the store. Often, these areas are absent of high piled racks. If shaking starts, sprint to an open area of the store. In Costco, this is the area where the books and clothing are located. Often there are doorways or arches to duck into, such as passages to restrooms or offices.
Doorways are excellent safety zones.
In fact, this will usually be near an exit to use after the shaking stops, so it's a good place to ride out a quake. Into a high piled rack. If you can't get away from the racks, climb into one. This will get you out of the way of falling objects. The rack most likely will not collapse because its load will be lightened when articles at the top spill off.