Relationship between bend‐faulting at trenches and intermediate‐depth seismicity

Abstract

We have studied faulting associated with bending of the incoming oceanic plate along segments of Middle America and Chile subduction zones and its relationship to intermediate‐depth intraslab seismicity and slab geometry. Multibeam bathymetry shows that bending‐related faulting forms patterns made of sets of faults with orientations ranging from parallel to almost perpendicular to the trench axis. These fault patterns may change along a single subduction zone within along‐strike distances of several hundred kilometers or less. Where available, near‐trench intraplate earthquakes show normal‐fault focal mechanisms consistent with mapped bending‐related normal faults. The strike of bending‐related faults in the incoming oceanic plate is remarkably similar to the strike of the nodal planes of intermediate‐depth earthquakes for each segment of the study areas. This similarity in strike is observed even for faults oriented very oblique to the trench and slab strikes. Thus, in the studied subduction zones, results strongly support that many intraslab earthquakes do not occur along the planes of maximum shear within the slab and that much intermediate‐depth seismicity occurs by reactivation of faults formed by plate bending near the trench. Furthermore, a qualitative relationship between trench faulting and intraslab seismicity is indicated by segments of the incoming plate with pervasive bend‐faulting that correspond to segments of the slabs with higher intermediate‐depth seismicity.