The subduction plate interface along the Nicoya Peninsula, Costa Rica, generates damaging large (M-w > 7.5) earthquakes. We present hypocenters and 3-D seismic velocity models (V-P and V-P/V-S) calculated using simultaneous inversion of P- and S-wave arrival time data recorded from small magnitude, local earthquakes to elucidate seismogenic zone structure. In this region, interseismic cycle microseismicity does not uniquely define the potential rupture extent of large earthquakes. Plate interface microseismicity extends from 12 to 26 and from 17 to 28 km below sea level beneath the southern and northern Nicoya Peninsula, respectively. Microseismicity offset across the plate suture of East Pacific Rise-derived and Cocos-Nazca Spreading Center-derived oceanic lithosphere is similar to 5 km, revising earlier estimates suggesting similar to 10 km of offset. Interplate seismicity begins downdip of increased locking along the plate interface imaged using GPS and a region of low V-P along the plate interface. The downdip edge of plate interface microseismicity occurs updip of the oceanic slab and continental Moho intersection, possibly due to the onset of ductile behaviour. Slow forearc mantle wedge P-wave velocities suggest 20-30 per cent serpentinization across the Nicoya Peninsula region while calculated V-P/V-S values suggest 0-10 per cent serpentinization. Interpretation of V-P/V-S resolution at depth is complicated however due to ray path distribution. We posit that the forearc mantle wedge is regionally serpentinized but may still be able to sustain rupture during the largest seismogenic zone earthquakes.