Just some back of the brain cavity thoughts here...

All materials are flexible. There are several parameters that limit flexibility before failure.

Most people intuitively think that a material at a scale which they can hold in their hand that is inflexible can't flex a great deal over larger sizes. Take a steel beam. Pretty stiff, eh? Now make one a hundred feet long and attach it at one end and see how much it flexes while you spring board off the end.

Concrete (in my parking lot it was asphalt, far more flexible) across a large expanse is relatively thin, and therefore more capable of flexure without brittle failure. Parking lots are only a few inches think. Come to think of it, next time you are in a multi-level parking garage, look at how THIN the floors are. 4 inches is typically what I see, albeit rebar strengthened.

Now, what's the wavelengths involved in quake motions? They're low hertz stuff, right? And what's the speed? Simple math gives you wavelength. Given a velocity of 2km/s and a frequency of 20hz results in a wavelength of 100 meters. I would think a concrete slab a few inches thick is capable of flexing even an inch or two across a span of 100 meters. (don't ya love my mixing of metric and imperial units? Kinda like my car I was working on yesterday)

Of course, lower frequency and/or higher velocity results in even longer wavelengths.

Brian