Evidence for serpentinization of the forearc mantle wedge along the Nicoya Peninsula, Costa Rica

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.