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1. A composite sliding block for friction-type seismic isolators which are mounted between a superstructure and either a substructure or a foundation and which comprise two sliding surfaces, the friction-type seismic isolators further comprising two support plates, one of the two support plates having one of said two sliding surfaces at a lower side thereof and being connected to the superstructure at an upper side thereof and the other one of the two support plates having the other of said two sliding surfaces at an upper side thereof and being connected to the substructure or a foundation at a lower side thereof, wherein the composite sliding block is arranged between the two support plates and in sliding contact with each of the support plates, wherein the composite sliding block is formed by two contact components disposed one on top of the other, each contact component of the two contact components having an external side and an internal side, wherein said internal sides face each other, wherein through said external sides the sliding block exerts said sliding contact with each of the two support plates and through said internal sides facing each other the two contact components are omnidirectionally slidingly coupled to each other by means of a vertical male coupling projection of one of the two contact components inserted in a female coupling recess of the other contact component, the female coupling recess having a shape that matches the shape of the vertical male coupling projection and having a cross-sectional area that is larger than a cross-sectional area of the vertical male coupling projection, whereby there is an empty space surrounding the vertical male coupling projection within the female coupling recess, said empty space being occupied by an elastomeric seal capable of being laterally compressed by an effect of a relative displacement between the two contact components when the friction-type seismic isolator is subject to a lateral load, wherein the composite sliding block further comprises a sliding plate on the external side of each of the two contact components, wherein the sliding plate is accommodated in a corresponding niche on said external side of each of the two contact components, wherein each of the niches has a bottom and side walls and wherein in between each of the sliding plates and the bottom of each of the niches there is disposed, confined, an elastomeric compression support that is deformable with vertical loads exerted on the friction-type seismic isolator.
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