A Torsion-Coupled Seismometer for Rotational Ground Motion with a Computed Angular Noise Floor

Rotational ground motion is the neglected half of seismology: conventional seismometers record translation, and rotation is recovered, if at all, by differencing separated instruments. A torsion pendulum records rotation directly, and in the framework used here it carries a second coupling, to torsion itself, through its suspension fibre. This paper specifies a torsion-coupled seismometer with a resonance of 0.080 hertz and an interferometric angular readout. Its computed angular noise floor is 1.80e-10 radians per root hertz, of which the thermal contribution is 1.78e-10; the equivalent rotation-rate floor is 9.03e-11 radians per second per root hertz; and a reference teleseismic rotational signal of 1e-7 radians is registered at a signal-to-noise ratio of 3937 in a 0.02-hertz bandwidth. SIMULATED, NOT MEASURED: all figures are computed from stated assumptions.