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Good question. Seismic forces are actually inertial forces generated within a body in response to seismic acceleration at ground. When seismic ground waves (PGA) accelerate a building (Sa; spectral acceleration), the inertia of a building resist these accelerations generating inertial forces in the building. Inertial forces are nothing strange but the usual F=ma i.e. the product of mass and acceleration. In softwares, the joints or nodes are loaded with this F based on the mass attributed to the tributary area of that particular node x accelration of that floor. Like this, these forces are applied at each level like static procedures.