Learn Spanish (Latin America): En ese momento, el edificio estaba temblando. - At that moment, the building was shaking.

Estaba temblando is the past continuous. It uses the imperfect form of estar (estaba) plus the gerund temblando (the "-ing" form). We use this to emphasize that the shaking was actively in progress at that exact moment, whereas tembló (the preterite) would just state the completed fact that it shook.

"En ese momento" sounds like a specific point in time. Doesn't that usually trigger the preterite?

Often, yes! But here, en ese momento ("at that moment") is acting like a camera snapshot. When we look at that exact snapshot in time, the action of the building shaking had already started and hadn't finished yet. Whenever you describe what was actively happening at a specific moment in the past, you need the past continuous.

What is the base verb for temblando?

The infinitive is temblar (to shake or tremble). To make the gerund (the "-ing" form), you drop the -ar ending and add -ando, which gives you temblando.

Why is it "ese momento" and not "eso momento"?

Momento is a masculine noun, so it needs the masculine adjective ese (that). Eso is the neutral form of "that." You only use eso when you are talking about an abstract idea, a general situation, or an unknown object—it is never placed directly in front of a noun.