Vympel

However, you're making the assumption that the concussion was caused by the impact of the beam weapon. Perfect example of the logical fallacy cum hoc ergo propter hoc. The concussion could have been caused by a number of things, not the least of which is avalanches caused by beam strikes on ice. It could be something as simple as footsteps from AT-ATs who were coming too close. They ARE rather big and heavy, after all.

If it was the mere footsteps of AT-ATs then it should be a steady rocking that doesn't stop, not to mention it should be getting worse as it gets closer. The impacts are intermittent. For that reason alone it doesn't work, and it still doesn't explain Han saying the command centre had been hit. The only reasonable explanation is that they're being struck by weapons fire. The only reasonable source of that weapons fire is AT-ATs.

EDIT: the novel:

THE Rebel command center, its walls and ceiling still shaking and cracking under the force of the battle on the surface, was attempting to operate amid the destruction. Pipes, torn apart by the blasting, belched sprays of scalding steam. The white floors were littered with broken pieces of machinery and chunks of ice were scattered everywhere. Except for the distant rumblings of laser fire, the command center was forebodingly quiet.

 

There were still Rebel personnel on duty, including Princess Leia, who watched the images on the few still-functioning console screens. She wanted to be certain that the last of the transport ships had slipped past the Imperial armada and were approaching their rendezvous point in space.

 

Han Solo rushed into the command center, dodging great sections of the ice ceiling that came plunging down at him. One great chunk was followed by an avalanche of ice that poured onto the floor near the entrance to the chamber. Undaunted, Han hurried to the control board where Leia stood beside See Threepio.

 

'I heard the command center was hit.' Han appeared concerned. 'Are you all right?'

 

The princess nodded. She was surprised to see him there where the danger was severest.

 

'Come on,' he urged before she could reply. 'You've got to get to your ship.'

As for seeing the effects of what hit the base - we never directly saw the base get hit in the first place. So there's no contradictory visuals. Note this does not mean I think they were firing KT beams at the base or whatever, I'm just pointing out an instance that clearly points to pretty powerful weapons generally speaking.

Echo Base looked more like it was in a cavern than a mountain. Not to mention that it was made of ice, not rock.

No, Echo Base was clearly under a rock mountain/hill whatever you want to call it. We can see the rock that makes up the mountain in external shots of the base.

AT-AT's must have powerful cannons to cause damage to the interior of that base - as is clearly implied by the film with the base being rocked by explosions, and character testimony: "I heard the command centre had been hit!".