  Radioactivity induced by cosmic muon spallation is a dominant source of
backgrounds for $\mathcal{O}(10)~$MeV neutrino interactions in water Cherenkov
detectors. In particular, it is crucial to reduce backgrounds to measure the
solar neutrino spectrum and find neutrino interactions from distant supernovae.
In this paper we introduce new techniques to locate muon-induced hadronic
showers and efficiently reject spallation backgrounds. Applying these
techniques to the solar neutrino analysis with an exposure of
$2790\times22.5$~kton.day increases the signal efficiency by $12.6\%$,
approximately corresponding to an additional year of detector running.
Furthermore, we present the first spallation simulation at SK, where we model
hadronic interactions using FLUKA. The agreement between the isotope yields and
shower pattern in this simulation and in the data gives confidence in the
accuracy of this simulation, and thus opens the door to use it to optimize muon
spallation removal in new data with gadolinium-enhanced neutron capture
detection.
