We demonstrated a multiplexed fiber Bragg grating (FBG) sensor with static strain resolution of 10 nanostrain (nε) for crustal strain monitoring. Each sensor unit consists of a pair of identical FBGs for strain sensing and reference, respectively. A narrow linewidth tunable laser is used to interrogate the FBGs, and a cross-correlation algorithm is incorporated to demodulate the wavelength difference induced by strain. When no strain is applied, an ultrahigh wavelength precision corresponding to strain resolution of 3.3 nε was obtained, indicating the ultimate resolution of the sensor system. With a variable strain applied by a piezo-stage, strain resolution of 17.6 nε was demonstrated. When the sensor is adopted for the in situ monitoring of crustal deformation, the strain induced by oceanic tide is clearly recorded with a resolution of 10 nε, providing a potential tool for the geophysical measurements.