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.
One of best places in Iceland to see the Northern lights is in fact just around the Reykjavik area. In fact on a really clear night the northern lights can be seen from the middle of downtown Reykjavik. For more intensive displays however, travelling outside of the city and suburban areas is recommended.