Three spectators were injured after part of a stand collapsed during England's Twenty20 defeat by West Indies at Chester-le-Street.
Durham said the fans fell when "a small section of the North-East Terrace flooring became unstable" at about 21:30 BST, during England's innings.
Part of the stand, which Durham said passed safety inspections before the match, was evacuated.
West Indies won by 21 runs as England were bowled out for 155.
Pictures appeared to show a hole in the floor of the stand.