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.
Increasingly, tourists come in winter to enjoy the beauty of the northern lights. Safety while travelling is always crucial, especially in winter conditions. The web-site Safetravel is run by ICE-SAR in order to minimize the risk of accidents in travelling and tourism in Iceland. Thorough information on safe travelling can be found there.