Slab is designed to take vertical loads only. Such a slab may be one-way, two-way, flat and ribbed. As earthquake is a horizontal load therefore in my knowledge there is no code that allows to design a slab using such load. But ability to resist horizontal load by slab-beam-column combination is far batter than slab-column combination. If in some way slab can be introduced with column and beam then certainly the rigidity of a building will be increased as lot to resist horizontal force.

while designing the structure we should be aware of the location in which zone the structure is falling (as per the codal provision) and also we have to study the soil parameters (Liquification action ). Proper ductility effect should be provided as per the analysis mostly the frame will have a sway action during the horizontal ground motion (earthquake) by providing confinement reinforcement at column beam junction as per IS 13920 we can control some extent of damage to the structure. Pl refer the papers published by Prof. Pradeep garu., IIIT., Hyderabad. or Prof Prof.CVSR Murthy garu., IIT., Hyderabad.

Because of the behavior of concrete is not uniform, then the answer must be given an exact approach through a numerical example. But in General, the behavior of slabs depends on: 1. the quality of concrete (slab, beams and columns); 2. Configuration and details of reinforcement is used (on a slab, beams, columns and joints); 3. cross section Rigidity of beams and columns. When concrete slabs designed relatively thick (e.g. 150 mm for a stretch of 6 meters) with negative reinforcement and positive reinforcement enough meeting and dimensions as well as the rigid beam reinforcement plates will then behave as a diaphragm with infinite stiffness. because of the behavior of concrete is not uniform, then the answer must be given an exact approach through a numerical example. But in General, the behavior of plates depends on: 1. the quality of concrete (slab, beams and columns); 2. Configuration and details of reinforcement is used (on a slab, beams, columns and join); 3. cross section Rigidity of beams and columns. When concrete slabs designed relatively thick (e.g. 150 mm for a stretch of 6 meters) with negative reinforcement and positive reinforcement enough meeting and dimensions as well as the rigid beam reinforcement plates will then behave as a diaphragm with infinite stiffness. So if the quality of the concrete the minimum eligible, configuration and details of the reinforcement also has been installed according to the pattern and distribution of force and moment (including those as a result of horizontal loads) and the stiffness of the cross section of the beams and columns is enough then the plate was basically can hold horizontal earthquake load with very good behaviour (according to the assumptions).

Slabs (especially the relatively thick, not shells) is primarily designed to withstand the elements of bending. So in principle slabs behave like beams. Its Response to earthquake load of course depends on the strength (details and configuration of reinforcement attached) and rigidity (span, the thickness and quality of the concrete). The slab itself will be able to withstand horizontal loads induced from beams and its rigidness relatively unchanged (assuming rigid diaphragm).

I feel the slabs which joins with the horizontal beams is a weak point where the slopes at the ends keep changing in addition to the slab in-plane inertia loads during an earthquake. The slope changes may be treated as additional end fixity loads and adequately designed. The slab weight should be optimized for least weight with proper quality control procedures during slab preparation and layup.