Several thin layers of nonjuvenile lithic-rich tephra from Mount Katmai are intercalated with 1912 plinian pumice-fall deposits that originated at Novarupta 10 km away, demonstrating that caldera collapse at Katmai and eruption at Novarupta were synchronous. Emplaced by phreatic explosions and avalanche-generated dustclouds, each Katmai-derived layer is thickest and coarsest at the caldera rim and pinches out within 5-8 km. In contrast, thicknesses and clast sizes of plinian Layers A-G decrease radially away from Novarupta. The first Katmai-derived layer lies between Novarupta-derived Layers B2 and B3, indicating that collapse began after release of ~6.9 km3 of magma at Novarupta. The first major earthquake (MS=6.5) at 2356 h on 6 June, ~11 hours after the eruption began, may have marked the onset of collapse. The Katmai River landslide and several debris-flow deposits are also sandwiched within the plinian deposits, indicating that they too were emplaced during the eruption.