Observations on earthquake stress axes and seismic morphology of deep slabs

Abstract

Focal mechanism solutions from centroid‐moment tensor (CMT) and other studies in the NW Pacific and Tonga–Kermadec regions are analysed in association with velocity images from tomographic inversions. Deep compression axes and fast velocity slab anomalies are usually in consistent alignment, even when the slab is contorted or flattened. The down‐dip direction for each event is given based on slab geometry interpreted from velocity anomalies and seismicity. The well‐known pattern of tension to compression down‐dip stress by Isacks & Molnar is seen and fitted with a tangent curve, which gives the location of the neutral down‐dip stress zone (NDSZ). An abnormal pattern of NDSZ exists beneath major junctions of the arcs. The NDSZ is generally in the depth range of 100–250 km in the central parts of the arcs, while it is deeper in the Mariana region. The depth seems to depend on the dip and topology of the slab, but the NDSZ probably starts at more or less the same subducted length along the slab. Copyright © 1990, Wiley Blackwell. All rights reserved