Mapping the B‐value anomaly at 100 km depth in the Alaska and New Zealand Subduction Zones

Abstract

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.