这种不寻常的现象在这个地区是罕见的。

This unusual ___occurrence is rare in this area.

这个地区地震的发生频率近年来有所上升。

The ___occurrence of earthquakes in this region has risen in recent years.

这种鸟类在该地区的出现非常罕见。

The ___occurrence of this bird species in the area is extremely rare.