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This paper constructs regular integral models for certain Shimura varieties of symplectic and orthogonal type by performing explicit equivariant modifications of canonical local models. These models, defined over the reflex field's integers, exhibit special fibers with normal crossings and components of multiplicity one or two. The construction, achieved through successive blow-ups of Schubert varieties, proves Pappas's equivariant modification conjecture for maximal parahoric level in these cases.
Pappas's equivariant modification conjecture is proven for a broad class of Shimura varieties via explicit geometric constructions.
We construct regular integral models for a class of Shimura varieties of symplectic and orthogonal type with maximal parahoric level at an odd prime $p$. These models are defined over the ring of integers of the reflex field and have special fiber with normal crossings and irreducible components of multiplicity one or two. Our construction uses explicit equivariant modifications of the corresponding canonical local models for symplectic and split even orthogonal similitude groups, with the minuscule cocharacters corresponding to maximal isotropic Grassmannians. These modifications are obtained by successively blowing up Schubert varieties in the special fiber. This proves the equivariant modification conjecture of Pappas at maximal parahoric level in the above cases.
