إِذَا زُلْزِلَتِ الْأَرْضُ زِلْزَالَهَا
When the earth is shaken in its great shaking. (99:1)
Zulzilat is a passive perfect verb; al-arḍu is its passive subject, and zilzālahā is a cognate accusative (mafʿūl muṭlaq). Bringing the verbal noun after its own verb intensifies the presentation of the event. The possessive pronoun points back to the earth and helps convey an extraordinary, distinctive convulsion.
The passive construction does not imply ignorance of the ultimate agent. Here it places the shocking occurrence and its effect on the earth in the foreground. That rhetorical function follows from the complete eschatological context.