Gravity and Seismicity of Asymmetric Active Regions

Abstract

ISLAND arcs and active continental margins have recently been collectively called the asymmetric active regions¹. The characteristics of these regions are well known². They include a negative isostatic gravity anomaly over a trench, or between the trench and the continent or island chain, and a positive isostatic gravity anomaly, adjoining the negative anomaly on the inner side of the region. Vening Meinesz³ attributed the negative anomaly to downbuckling of the crust which displaced heavier rocks in the mantle. He found the positive anomalies difficult to explain, but suggested that “parts of them may be explained as the effect of lifting by the belts of negative anomalies of the adjoining areas”. More recently, the penetration of sub-crustal material into the crust or elevation of the crust–mantle interface has been suggested as the cause of the positive anomalies⁴.