New insights into episodic deformation at the Nankai trough subduction zone are provided by data from two Ocean Drilling Program borehole hydrologic observatories drilled into the Philippine Sea plate (Site 1173) and the seaward part of the Nankai accretionary prism off southwestern Japan (Site 808), and from an array of high-sensitivity borehole accelerometer and velocity seismometers on Shikoku Island (Hi-net). Fluid pressures monitored at multiple levels in each of the offshore boreholes document steady secular trends that indicate contraction of the crust and sedimentary section at the incoming plate site, and relaxation of the accretionary prism toe. The rate of strain (ca. 10⁻⁶ yr⁻¹) inferred from the rates of pressure rise at Site 1173 (up to 5 kPa yr⁻¹) is similar to that which would be produced by plate convergence if convergent elastic strain were distributed over a region a few tens of km wide. Opposing these trends are transient pressure anomalies observed in late June/early July 2003 that indicate an episode of rapid relaxation of the incoming plate and contraction of the outer prism. Concurrent with these transients, a swarm of very-low-frequency earthquakes occurred farther landward in the prism above the estimated seaward limit of the currently locked seismogenic portion of the subduction thrust. Reverse-fault mechanisms determined for one event of this swarm and several of another also indicate transient contraction of the prism. We suggest that both the earthquakes and the pressure transients are the consequences of an aseismic slip dislocation that initiated on about June 26, 2003, at or near the up-dip limit of the locked portion of the subduction thrust, and propagated seaward over the course of about 10 days to the accretionary prism toe along the decollement separating the prism and underthrust section. Deformational events like this may serve incrementally to relieve stress locally along the subduction thrust and to load neighboring areas. In addition to demonstrating that the prism is far from inactive during interseismic intervals, the observations may also provide a small-amplitude analog for strain and hydrologic response at the time of great subduction earthquakes.