S wave -- a shadow zone appears when the velocity suddenly decreases 103-180 degrees from the epicenter. Describe the differences between P and S waves. P waves expand and contract, can move through liquid solid and gas. S wave moves up and down or side to side moves through only solids.
Optimal conditions to see the northern lights in Iceland is when skies are clear and dark (away from the city lights, in cold temperature and ideally from mid-September to mid-April.
In addition, the best sightings are when there is a large amount of solar activity. Generally, it is held that prime seasons for viewing the northern lights are early autumn and spring. You could see them at any time between September and March but you need to be in the right place.