Abstract

Small tsunamis caused by a ship and a supercell thunderstorm that occurred on the coast of Norway on 30 June 2019 are compared. Free waves are generated in either case, at shallow depth changes of the ocean area or fjord. The generation occurs at subcritical speed and is similar in the two cases. How shorter waves appear in the generation process are discussed. The ship moving in Oslofjorden causes upstream waves where a leading part is followed by shorter waves of essential extension and amplitude. The latter, of period of one third of the main waves, are caused by the short bow and stern. The fast moving thunderstorm generated a free wave trough that was reflected from the coast 22 minutes prior to arrival of the forced wave. This period triggered resonant oscillations in fjords and caused damage. Reflected free waves had period just over 30 minutes.