Characterizing the hydration state of the forearc mantle wedge yields valuable information on frictional stability at the downdip edge of subduction megathrusts. Simultaneous inversion of P- and S-wave arrival times collected as part of the Costa Rica Seismogenic Zone Experiment yields 1D and 3D P- and S-wave velocity models (VP and VS) for the Nicoya Peninsula segment of the Middle America Trench. Nicoya Peninsula 1D velocity models show similar velocity gradients to country-wide 1D velocity models from 5--30 km depth but diverge at expected Moho depths due to proximity to the subducting Cocos plate. 3D VP values range from 7.2--7.6 km/sec in the forearc mantle wedge. Receiver functions computed at Global Seismic Network station JTS in northwestern Costa Rica confirm these VP values, yield VP/VS of ~1.85, and place the continental Moho at 36 ¿ 4 km depth. VP and VP/VS are consistent with 15--25% serpentinization of the forearc mantle wedge.