Significant earthquakes generate two distinct types of seismic waves. The first, the Primary waves, are a fast moving back and forth movement (Similar to blast waves). Seconds after these waves have passed comes the slower moving Secondary waves that deliver an undulating up and down shake. Rigid construction that doesn't allow for some movement often doesn't far well and comes apart at the joints when subjected to prolonged S wave movement.
That would be your roof falling down. Without the roof to provide downward pressure on the walls, they collapse soon afterwards. It is important to remember, the earthquakes we have experienced in the DR over the past years have been relatively small in intensity and very short in duration. Large earthquakes can shake the ground for minutes at a time. I think I read the Fukushima earthquake lasted for longer than 3 minutes. That duration of shaking in this country will turn many concrete structures to dust. There does not appear to be any bedrock to build on here. Just ancient coral and caleche <sp?> The ground just might sink out from under buildings.
The classification scale used for earthquakes is not linear. A magnitude 6 earthquake is 10 times more powerful than a magnitude 5, a magnitude 7 is 100 times more powerful than magnitude 5 and a mag. 8 is 1000 times more powerful than a mag. 5 - truly apocalyptic type forces involved. There comes a point when construction codes don't really matter. When intensity and duration pass a certain point, everything comes down. I am not an engineer, but I doubt concrete block with rebar will fare well in a big shake.
I live next to an empty field. When I figure out why my world is moving, I'm out of the house towards the field as fast as my unsteady legs can take me assuming the field hasn't liquefied into a temporary swamp or is sliding g away from me faster than I can run.