Simulating a seismic load using an external force in Swedge is straightforward. If you have a vertical seismic coefficient of 0.2, simply multiply this by the weight of the wedge and add this value as an external force with the appropriate trend and plunge. For a vertical seismic force, use a 90 degree plunge and zero trend.
Two models are attached to this article which illustrate how to apply a vertical seismic load. The first model uses a vertical force, while the second uses a seismic load.