The greatest uncertainty in the amount of water input into the Earth at subduction zones results from poor constraints on the degree of mantle serpentinization in the incoming plate. Recent studies suggest that the depth of serpentinization within the incoming plate mantle is likely controlled by the depth of extensional faulting caused by lithospheric bending at the outer rise and trench. We explore the maximum depth of extension within the incoming plate at Northwestern Pacific subduction zones in order to estimate the depth limit of serpentinization and to identify any significant variation between subduction zone segments. We relocate trench earthquakes to identify which events occurred within the incoming plate and determine accurate depths for 63 incoming plate earthquakes occurring during 1988-2011 by inverting teleseismic broadband P and SH waveforms. We observe that the top 10-15 km of the incoming plate mantle experiences extensional faulting at all of the subduction zones with a reasonable sample of earthquakes; 60% of the total number of extensional earthquakes occur at crustal depths or within the top 5 km of the incoming plate mantle, 80% occur above 10 km within the mantle, and 95% occur above 15 km. There is evidence for variation throughout the different regions of study, for example extensional earthquakes occur down to 20 km below the crust in the western Aleutians and Izu-Bonin. We propose that the incoming plate mantle is most strongly hydrated in the upper 5 km, and that partial serpentinization exists regionally within the incoming plate mantle to ∼15 km. Making reasonable assumptions about the degree of serpentinization and incorporating previous estimates of crustal water, we calculate that the total water carried into the Northern and Western Pacific subduction zones is generally higher than previous estimates, and is approximately , or per kilometer of subduction zone.