MAPPING THE B-VALUE ANOMALY AT 100 KM DEPTH IN THE ALASKA AND NEW-ZEALAND SUBDUCTION ZONES

A positive anomaly in the frequency-magnitude distribution (b-value) i s detected at approximately 90-100 km depth in two subduction zones. T he b-value in this anomalous zone is 40% higher than in adjacent volum es. 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 Z ealand). 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 locat e the high b-value anomalies at a depth of 90-100 km, on the upper sur face of the Wadati-Benioff Zone. At these depths, slab dehydration may increase pore pressure, thus lowering the effective stress and increa sing the b-value. Increased pore pressure would also lower the liquidu s in the overlying asthenosphere, giving rise to the volcanism that oc curs directly above this zone.