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.
Aurora-gazers have the best chances around the spring and fall equinox, so anytime around September 22, 2013 and March 20, 2014 are looking good, or anytime in between. Chase them on your own, or book a tour. I spent four nights in Iceland; four chances to see the northern lights.