Dominant fault plane orientations of intermediate-depth earthquakes beneath South America

Abstract

The South American subduction zone exhibits considerable variation: the subduction angle alternates between flat and steep; the subducting plate has complex structures; and arc volcanism in the overlying plate has gaps. I investigate the effect of these differences in incoming plate structure and slab geometry on intermediate-depth earthquakes, specifically their fault orientations and rupture characteristics, and find that slab geometry has the largest impact on fault orientation. I use rupture directivity to estimate rupture direction and rupture velocity and to distinguish the fault plane from the auxiliary plane of the focal mechanism. From analysis of 163 large (M w≥5.7) intermediate-depth (60-360km depth) earthquakes from along the length of South America, estimated rupture azimuths and plunges show no trends, appearing to be randomly distributed on the determined population of fault plane orientations, and a majority of earthquakes are made up of multiple subevents. As seen in other subduction zones, subduction segments descending at normal angles have predominantly subhorizontal faults. Flat slab segments also have a dominant fault orientation, but those earthquakes slip along the conjugate nodal plane of the focal mechanism. In strongly curved slab segments, such as at the downdip edge of flat segments where the slab resubducts, earthquakes may slip along either nodal plane orientation. While both fault orientations could be consistent with the reactivation of fossil outer rise faults, the fault orientations are also consistent with expectations for newly created faults in agreement with the ambient stress field. Fault reactivation alone does not explain why different fault orientations are active in segments with different geometries, so the preferred explanation for having regionally consistent fault orientations is that they minimize the total work of the system. The previously observed predominance of subhorizontal faults appears to be a consequence of slab geometry. Key Points Orientations of intermediate-depth faults correlate with slab geometry Faults are subhorizontal in normal-angle slabs and dip landward in flat slabs Earthquake rupture directions are randomly oriented ©2014. American Geophysical Union. All Rights Reserved.