Variations in the ring laser signal of the rotating Earth are caused by local rotations and by north–south tilts. Analytic expressions for the computation of synthetic seismograms for ring lasers based on normal mode summation are derived. We find that horizontal ring lasers are predominantly sensitive to torsional motion. Synthetic Sagnac signals are computed for recent earthquakes and for spherical Earth models in order to establish the general characteristics and amplitude of the expected ring laser signals. In the normal mode band north–south tilts typically amount to less than 10% of the total signal for a horizontal ring laser. The fact that no free oscillation spectra have so far been reported from ring lasers is consistent with amplitudes of our synthetic Sagnac signals and current noise levels of these sensors.