The seismic activity associated with the century's largest eruption and caldera collapse at Mount Katmai, Alaska, during June 1912 was poorly understood and is studied here in detail. The epicenter of the large shock of 10 June was long thought to be 140 km northeast of Mount Katmai, but it is relocated to be 12 km west of Mount Katmai, near the Novarupta dome. Origin times, surface-wave magnitudes Ms, and broadband body-wave magnitudes mB are assigned to 50 new shocks from instrumentally observed data of more than 60 worldwide Milne's and other seismological stations. Of 50 shocks detected, 14 have Ms = 6.0 and greater, and the largest is 7.0. Both the frequency-magnitude relation and the mB − Ms relation apparently exhibit normal appearance for the Alaskan region. The strong earthquakes were concentrated in the nights of 6 and 7 June. It is inferred that the substantial collapse of Mount Katmai took place in these two nights. The total energy released in seismic waves is 7.1 × 1015 J, corresponding to the energy of a single shock with Ms = 7.4. The total seismic moment is estimated to be 1.4 × 1020 Nm from Ms values. This total moment can be interpreted in terms of failures caused by the total volume change of 5 km3. This volume agrees well with the estimated volume of the caldera collapse, which presumably took place in response to transfer of supporting magma from beneath Mount Katmai toward Novarupta.