A series of unusual leaks on two natural gas pipelines running from Russia under the Baltic Sea to Germany triggered concerns about sabotage Tuesday, overshadowing the inauguration of a long-awaited pipeline that will bring Norwegian gas to Poland to bolster Europe’s energy independence from Moscow. Seismic stations Sweden, Norway and Finland registered two explosions Monday near the leaks. Bjorn Lund, a seismologist with Uppsala University who is part of Sweden’s national seismic network, said the first explosion was recorded in the early hours southeast of the Danish island of Bornholm. The latter and stronger blast that night was northeast of...