Rate-and-state friction (RSF) has proved remarkably versatile in explaining how faults store and release elastic strain, yet its applicability to complex natural fault zones remains uncertain. The 2018 collapse of Kīlauea caldera provides a unique opportunity to test fault constitutive behavior at kilometer scale, across successive deformation cycles and under highly repeatable loading. Exploring the admissible RSF parameter space with a three-dimensional slider-block model, we show that RSF fails to reconcile the observations: the best-fitting models release strain almost entirely coseismically and underpredict the observed 1-3 m of inter-collapse subsidence by an order of magnitude. Neither spatial frictional heterogeneity nor dynamic weakening resolves this trade-off. Introducing a finite-thickness, viscoplastic shear zone that partitions strain between localized slip and distributed yielding reproduces the observed coseismic and inter-collapse deformation within a single self-consistent model. Distributed off-fault yielding is therefore a first-order control on how strain partitions between seismic and aseismic slip.