The Collaborative International Dictionary of English v.0.48:

Seismic \Seis"mic\, Seismal \Seis"mal\, a. [Gr. seismo`s an
   earthquake, from sei`ein to shake.]
   Of or pertaining to an earthquake; caused by an earthquake.
   [1913 Webster]

   Seismic vertical, the point upon the earth's surface
      vertically over the center of effort or focal point whence
      the earthquake's impulse proceeds, or the vertical line
      connecting these two points.
      [1913 Webster]