Catalyzes alpha-humulene and delta-cadinene, as well as beta-elemene, the thermal rearrangement product of germacrene A and several other bicyclic sesquiterpenes when incubated with (2E,6E)-farnesyl diphosphate. Binds 3 Mg(2+) or Mn(2+) ions per subunit. Belongs to the terpene synthase family. Tpsa subfamily.