Moderate and large earthquakes in the South Sandwich Arc: Indicators of tectonic variation along a subduction zone

Abstract

Moderate and large earthquakes of the South Sandwich region that occurred between 1929 and 1963 were relocated. Variations in the long‐term seismicity along the length of the South Sandwich arc are observed to correlate with changes in the age of the subducted lithosphere and with the position of a fracture zone inferred to be on the incoming plate. The northern portion of the arc generated several events of moderate size (6 ≤ Ms < 7) but no large events (Ms ≥ 7) in the zone of underthrusting during this century. In this region, relatively old lithosphere (∼70 m.y. old) is being subducted underneath a young plate (∼7 m.y. old). The central portion of the arc produced almost no events of magnitude 6 or larger during the period studied. This section of the arc is located across from the position of an inferred fracture zone on the plate that is being subducted. The southern portion of the South Sandwich arc is being uplifted in response to the subduction of buoyant lithosphere generated at an adjacent spreading center. This is consistent with the model of ridge‐trench convergence proposed by DeLong et al. Several events of moderate and large size occurred in both the zone of underthrusting and the area seaward of the trench in this region. The low level of seismicity of the South Sandwich arc, when compared to other island arcs of similar convergence rates (7–9 cm/yr), indicates that aseismic slip may account for a substantial portion of the convergence in this subduction zone. This paper also reports fault plane solutions for three earthquakes of the region.