In the field of earthquake seismology, a new class of seismic wave has emerged that is associated with episodic tremor and slow-slip activity at tectonic plate boundaries. In order to detect and characterize these weak seismic waves, a high dynamic range optical sensor capable of remotely measuring minute displacements over a broad frequency range is being developed. The optical fiber Bragg grating sensor system is compact and inexpensive, and is well suited for collecting real-time data from remote sites deep underground within boreholes. A PZT mechanical vibration transducer simulates the small displacements created by a mechanical sensor’s response to the tremor. Low-frequency waveforms in the 1–20 Hz band are used to drive the PZT actuator. Stable detector output demonstrates optical sensor sensitivity to vibrational displacements less than 400 nm without using any digital filtering of the output signal.