Evidence From Seismology for Water in the Upper Mantle Transition Zone

We discuss two types of seismological observation that may provide evidence for water in the transition zone. First, we review constraints from (global) seismic tomography with P and S wave travel time data. For several subduction systems (e.g., western Pacific) it has been observed that P and S wavespeeds in the mantle wedge are anomalously slow to depths well into the transition zone. In regions where there is evidence for stagnant slabs (e.g., beneath Philippine Sea plate and SE Asia) the anomalous wavespeeds persist far away from the zone of active subduction. These regions are also marked by anomalous Vp/Vs ratios. These observations suggest the presence of compositional variations and, most likely, the presence of water that is recycled from the surface by subduction processes. Second, we present results from high resolution transition zone imaging with generalized Radon transform of the broadband wavefield containing SS precursors. We study the mantle beneath the central Pacific, around and west of Hawaii. In addition to pronounced, laterally continuous interfaces near 410, 520, and 660 km depth the images also reveal structure between 800-1000 km and near 370 km depth. These scatter zones may not form laterally continuous global interfaces but they may indicate hitherto unknown, or poorly understood, changes in composition or mineralogy. The complexity near the top of the transition zone is reminiscent of evidence from (long period) ScS reverberations for silicate melt atop the 410 km discontinuity beneath SE Asia (Revenaugh and Sipkin, 1994). This may suggest that silicate melt and, hence, water is present in the mantle transition zone beneath parts of the Pacific far away from present-day subduction.