imperceptibly

Earthquakes typically occur along the jigsaw-puzzle piecesof Earth's crust, called plates, which move relative to one another, mostof the time at an imperceptibly slow pace.

地震一般发生在犬牙交错的一块块地壳（板块）间。 这些板块相向运动，大部分时间我们察觉不到它们的移动。