。
She was trembling with fear.
Literal
She [topic-は] terrible-ness [from-で] was-trembling.
震える ('to tremble, shake, shiver') covers a range — physical shivering from cold, trembling from fear, shaking from anger. The cause marker で specifies what's causing the tremor — 寒さで震える ('shiver from cold'), 怒りで震える ('shake with rage'). The ~ていた frames it as a held condition: she was caught in the trembling, not just shaking once. 恐ろしさ again is the abstract noun from 恐ろしい.