A novel fabrication procedure for implementation of suspended, interdigital capacitive sensor on silicon-based membranes is reported. The process employs a combination of anisotropic back-side micromachining with front-side vertical deep reactive ion etching of silicon. The formation of cavities underneath the interdigital structure beside the evolution of nano-grass on the back side of the sample, allows inclusion of liquid to add to the effective mass of the device and to realize a low frequency sensing device suitable for earthquake prediction. The etching of vertical pieces is feasible using a hydrogen-assisted reactive ion etching. Also the evolution of grass on the back and front sides of the sample can be controlled by etching parameters.