Seismic and geodetic observations in subduction zone forearcs indicate that slow earthquakes, including episodic tremor and slip, recur at intervals of less than six months to more than two years(1,2). In Cascadia, slow slip is segmented along strike(3) and tremor data show a gradation from large, infrequent slip episodes to small, frequent slip events with increasing depth of the plate interface(4). Observations(5-7) and models(8,9) of slow slip and tremor require the presence of near-lithostatic pore-fluid pressures in slow-earthquake source regions; however, direct evidence of factors controlling the variability in recurrence times is elusive. Here we compile seismic data from subduction zone forearcs exhibiting recurring slow earthquakes and show that the average ratio of compressional (P)-wave velocity to shear (S)-wave velocity (v(P)/v(S)) of the overlying forearc crust ranges between 1.6 and 2.0 and is linearly related to the average recurrence time of slow earthquakes. In northern Cascadia, forearc v(P)/v(S) values decrease with increasing depth of the plate interface and with decreasing tremor-episode recurrence intervals. Low v(P)/v(S) values require a large addition of quartz in a mostly mafic forearc environment(10,11). We propose that silica enrichment varying from 5 per cent to 15 per cent by volume from slab-derived fluids and upward mineralization in quartz veins(12) can explain the range of observed v(P)/v(S) values as well as the downdip decrease in v(P)/v(S). The solubility of silica depends on temperature(13), and deposition prevails near the base of the forearc crust(11). We further propose that the strong temperature dependence of healing and permeability reduction in silica-rich fault gouge via dissolution-precipitation creep(14) can explain the reduction in tremor recurrence time with progressive silica enrichment. Lower gouge permeability at higher temperatures leads to faster fluid overpressure development and low effective fault-normal stress, and therefore shorter recurrence times. Our results also agree with numerical models of slip stabilization under fault zone dilatancy strengthening(15) caused by decreasing fluid pressure as pore space increases. This implies that temperature-dependent silica deposition, permeability reduction and fluid overpressure development control dilatancy and slow-earthquake behaviour.