Here we present a fiber optic seismic waves sensor based on in-fiber Bragg gratings. Fiber Bragg Grating sensors have been demonstrated to have very high sensitivity to dynamical strain in the sub-micro-strain range and very extended dynamical response from static to very high frequency. The seismic sensing system is based on the integration of three FBGs dynamical strain sensors in a mechanical structure acting as an inverse pendulum. Polar symmetry of the mechanical system and 120° placement of the FBG sensors guarantee a directional capability of the seismic sensor. Design, manufacturing and preliminary dynamical testing of the seismic sensor are discussed.