A positive anomaly in the frequency-magnitude distribution (b-value) is detected at approximately 90-100 km depth in two subduction zones. The b-value in this anomalous zone is 40% higher than in adjacent volumes. We use regional catalogs with a magnitude of completeness of 2.6 (Central Alaska and Cook Inlet), 2.0 (Shumagin Islands), and 3.5 (New Zealand). To resolve the b-value as a function of space in more detail, we project all earthquakes onto planes perpendicular to the strike of the slab. These cross-section views of the b-value distribution locate the high b-value anomalies at a depth of 90-100 km, on the upper surface of the Wadati-Benioff Zone. At these depths, slab dehydration may increase pore pressure, thus lowering the effective stress and increasing the b-value. Increased pore pressure would also lower the liquidus in the overlying asthenosphere, giving rise to the volcanism that occurs directly above this zone. [References: 25]
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