Akiteru Takamori, Alessandro Bertolini, Erika D'Ambrosio, Riccardo DeSalvo, Virginio Sannibale

We are developing the seismic attenuation system (SAS) for advanced gravitational wave detectors such as LIGOII. The first aim of SAS is to provide isolation performance sufficient to drive the seismic attenuated mirror motion well below the thermal noise starting at a few Hz. The second aim is to reduce the residual RMS motion of the mirrors for easy locking.  To realize these aims, the SAS has a pre-isolation stage formed by an inverted pendulum for horizontal motions and a low frequency vertical isolation filter with geometrical anti-spring (GAS). A passive isolation chain of the pendula with 300 mHz GAS filters for the vertical direction will hang from the pre-isolator. Test masses will be suspended by a triple pendulum system attached to the bottom of the chain.  The pre-isolation stage will be basically a passive isolation system effective from about 100 mHz, and also provide the active damping of the SAS chain with LVDTs, accelerometers and actuators. We report on preliminary results of the experiment of the SAS prototype.