An effective and accurate study of the slow dynamic behavior of a site or a structure requires the implementation of a distributed monitoring system characterized by a large number of broadband inertial sensors in synergy with FEA (Finite Elements Analysis) technique for its modeling. The sensors are most critical part of this approach are the sensors, that must be sensitive enough not only to measure the effect of critical events (e.g. earthquakes, site subsidence) on the site or structure, but mainly to continuously monitor the effects of continuous natural events (microseism, night and day insulation, etc.) and/or anthropic actions (gas and oil extraction, city car traffic, etc.), which may mine in the long term their stability. Among the possible different sensors architectures, the UNISA Folded Pendulum technological platform is very effective for the implementation of large band inertial sensors (horizontal, vertical and angular), well described by the Extended Folded Pendulum Model (EFPM), developed for a quantitative description of the dynamical behavior of a folded pendulum generically oriented in space. In this paper, after the presentation of the EPFM model, we present monolithic implementations of inertial mechanical seismometers/accelerometers based on the UNISA Folded Pendulum mechanical configuration, optimized for low frequency characterization of large structures, and characterized by large measurement band 10−7 Hz ÷ 1 kHz, sensitivity down to 10−15m/√Hz, low weight (down to 40 g), size (<5 cm), directivity > 104.