Most definitely. Huge 7.1 earthquake hit the Charleston area in 1886 so the area definitely has a history of seismicity. You should expect to find iso-seismal graphs for the area to help with the acceleration coefficient.
That said, I would lean on your geotechs to provide the information and let the structural engineer of record decide, base code, or proximity of the retaining wall to the public and or damage implications for the client decide whether to apply them or not.
I've not heard of a height requirement though I think most would say that less than 5' is not significant and likely as small block modular wall.
Again, as to applying the seismic load, you need to evaluate the code IBC or similar code and discuss with the client, unless it's on public land. Like many structures that are not frequented by the public or large numbers of people, the requiremnts can be relaxed if life safety is not an issue. However, if the wall supports a building corner and it later settles after a small tremor the owner isn't going to be happy.
There are some good references for seismic design of retaining walls. Among those are
Soil Dynamics by Shamshar Prakash.
Principles of Foundation Engineering by Das.
The examples aren't weighted down with theory and basically come down to applying a lateral pressure of the weight of the wall times the accel coeff. (for mass inertia of the wall) and to apply that resultant at 1/2 the height. There is also a Kae, which is the active earth pressure for the earthquake load case. You'll also need to consider bearing pressure and what might happen due to ground motion - should be discussed by your geotech.
good luck.
Regards,
Qshake
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