abstract The gyro seismometer, consisting of two counter-rotating pendulous Nordsieck gyroscopes, permits separation of tilts from horizontal motion. Sensitivity is limited by the accuracy with which the gyro orientation can be measured, ∼1 arcsecond rms in a 0.5 Hz bandwidth. This instrumental noise level is equivalent to ground tilts of ∼1 arcsecond and ground velocities of ∼0.1 cm/sec. Peak values in the instrumental pass band (0 to 0.5 Hz) of the tilt and horizontal velocity during the Borrego Mountain, California, earthquake of April 9, 1968, were 1 cm/sec and 10 arcseconds respectively. In La Jolla, 115 km from the epicenter, the static displacement and tilt were less than 1 cm and 0.1 arcsecond (5 × 10−7 radian).