This paper describes a mechanical horizontal monolithic sensor for geophysical applications developed at the
University of Salerno. The instrument is basically a monolithic tunable folded pendulum, shaped with precision
machining and electric-discharge-machining, that can be used both as seismometer and, in a force-feedback
configuration, as accelerometer. It is a very compact instrument, very sensitive in the low-frequency seismic noise
band, with a very good immunity to environmental noises, whose main characteristics are the tunability of the
resonance frequency and the integrated laser optical readout, consisting of an optical lever and an interferometer.
The theoretical sensitivity curves are in a very good agreement with the measurements. A direct comparison of
its performances with the STS-2 ones shows that better performances have been reached with the interferometric
readout (≈ 10-12 m/
Hz in the band 10-1 ÷ 10Hz as seismometer. Finally, the first results as accelerometer
(force feed-back configuration) are also presented and discussed.