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.
Plan your trip from late September to late March, when the nights are long (between 6:00 p.m. and 1:00 a.m. is the peak time for sightings). Download the free VisitNorway.com Norway Lights App (for Apple, for Android and for Windows) to find out when and where you can see the northern lights.