Seismicity, shear failure and modes of deformation in deep subduction zones

Abstract

Planes of shear failure in deep seismic zones are identified as being parallel to focal mechanism nodal planes of deep focus earthquakes. We relocate deep seismicity reported in the International Seismological Centre (ISC) catalogue for earthquakes deeper than 400 km in the Honshu, Bonin, Marianas, Java, Banda, and South America subduction zones using the joint hypocentral determination method. Recent events with Harvard centroid-moment tensor (CMT) solutions and nearby seismicity are viewed from the direction of the CMT null axis, comparing the two orthogonal nodal planes with the seismicity pattern. Planar features of seismicity parallel to CMT nodal planes are found in each deep seismic zone, and are identified as planes of shear failure. The sense of displacement on these planes is one of resistance to deeper penetration. Honshu and Bonin show imbrication and horizontal displacement of the deepest part of the seismogenic slab, while Marianas and Brazil show conjugate shear failure. Banda and Argentina show patterns of vertical and horizontal shear planes. For the Western Pacific slabs we find significant lateral displacements towards the south of the deepest part of the subduction flow. Tonga has been analyzed previously, showing the same pattern as identified here. In all Benioff zones, planes of shear failure are observed which act to shorten the slabs and displace matter horizontally, normal to the slab; moreover, differential faulting acts to rotate the slab of subducted lithosphere to a more horizontal posture, indicating that slabs experience resistance to penetration of the 670 km discontinuity.