Best bet would be packages such as Flac3D or Udec3D.
It depends what your model will consist of though - mining such as underground or open pit, building, tunnel etc.
The intigration of recorded seismic data into your model will be beneficial for the endresult.
Local site-effect from PPV's (peak particle velocities) is very important - must have some form of calibration here.
Not very easy though....

I've heard that Geo-Slope is beta testing their new QUAKE/W software, the following was resently posted to the discussion list, hope it helps.

Mo

---------- Original message ----------
We currently have our QUAKE/W software in limited public beta testing and hope to release it sometime in 2001. The following are a few highlights of the software.

QUAKE/W follows the Direct Step Time Integration Method. This method integrates at user-defined time steps over the entire time history to compute the motion arising from the inertial forces. QUAKE/W also computes excess pore-water pressures that may arise due to shaking. QUAKE/W supports linear analyses using linear-elastic properties and equivalent-linear analyses by varying the soil properties for successive iterations. Both the Damping Ratio and the G-Modulus can vary with cyclic strain for linear-elastic analyses. The variation of these properties is specified with a generalized function in the same way as other material properties.

QUAKE/W will also be integrated with other GEO-SLOPE products. For example, you will be able to use your QUAKE/W results in SLOPE/W to assess the stability of slopes and embankments.