SUBDUCTING-PLATE TOPOGRAPHY AND NUCLEATION OF GREAT AND GIANT EARTHQUAKES ALONG THE SOUTH AMERICA TRENCH

Earthquakes of M_w≥8.5 along the South America trench commonly have epicenters close to the intersection between the trench and the oceanic fracture zones and associated ridges on the subducting Nazca plate. The age difference in the ocean floor across fracture zones results in a bathymetric step. This step is preserved in the shallow part of the subducting plate, forming a lateral ramp that spans the full width of the subduction thrust seismogenic zone. Lateral ramps on subduction thrusts, like their equivalents on smaller thrusts in orogenic belts, are regions of increased mechanical coupling and thus likely nucleation sites for very large thrust events. Three such ramps along the South America trench that have the potential to nucleate M_w≥8.5 earthquakes have not been involved in such events for at least 100-500 years. Only a paleoseismological record significantly longer than the historically available 500 years and adequate geodetic observations would tell whether the typical long-term behavior of these ramps is aseismic creep, or infrequent giant earthquakes.