Catalyzes the conversion of (2E,6E)-farnesyl diphosphate (FPP) to yield the bicyclic sesquiterpene selina-4(15),7(11)-diene via a 1,10-cyclization, which requires the abstraction of the pyrophosphate from FPP leading to a (E,E)-germacradienyl cation (PubMed:23307484, PubMed:24890698). The only accepted substrate is (2E,6E)-farnesyl diphosphate (FPP) (PubMed:23307484, PubMed:24890698). (2E,6E)-farnesyl diphosphate = diphosphate + selina-4(15),7(11)-diene Binds 3 Mg(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Monomer. The Asp-Asp-Xaa-Xaa-Xaa-Glu (DDXXXE) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family.