The present invention is a seismometer/velocimeter, and can be also made to function as an accelerometer. The invention comprises an in-plane suspension geometry combined with a transverse periodic-sensing-array position transducer. The invention can incorporate a feedback actuator of magnetic design, incorporating fixed magnets and planar coils (25) on the surface of the proof mass (8) allowing for much lower noise than an equivalent electrostatic actuator without requiring high voltages.