Seismotectonics and relative plate motions in the Scotia Sea region

Abstract

Source parameter inversion of P and SH waveforms and amplitudes for 20 shallow earthquakes using a singular value decomposition technique provides important constraints on the tectonics and relative plate motions of the Scotia Sea region. The derived focal mechanisms show both thrust and strike‐slip faulting along the North Scotia Ridge and normal and strike‐slip faulting along the South Scotia Ridge. Two thrust faulting mechanisms south of Tierra del Fuego indicate diffuse convergence of the Antarctic plate along the western margin of the Scotia plate. Earthquakes are consistent with active subduction and back arc spreading along the South Shetland Trench and Bransfield Strait. The depths of two earthquakes (35 km and 55 km) and one shallow thrust faulting event suggest continued subduction along the South Shetland Trench, but the underthrusting is largely aseismic due to the young plate age and slow subduction rate. The magnitude and depth of normal faulting earthquakes along Bransfield Strait and the South Scotia Ridge are suggestive of diffuse extension rather than typical organized mid‐ocean spreading, which is generally associated with smaller and shallower earthquakes. Eight strike‐slip and thrust faulting events provide well‐constrained slip vectors showing Scotia plate motion relative to the surrounding larger plates. Using a four‐plate model (Scotia (SC), South Sandwich (SS), Antarctica (AN) and South America (SA)) and a fixed SA‐AN Euler vector, we inverted these data together with published centroid moment tensor slip vectors along the South Sandwich Trench and published marine magnetic data from the SS back arc spreading center to derive the first quantitative plate motion model for the Scotia region. The SC‐SA Euler pole is located in the South Atlantic and shows large latitudinal uncertainties. The SC‐AN pole is better constrained and located in the Weddell Sea. These results predict left‐lateral strike‐slip motion with a component of compression along the North Scotia Ridge, left‐lateral strike‐slip with a component of extension along the South Scotia Ridge, and east‐west compression in Drake Passage. Although no rate data have been published connecting the Scotia plate with the global plate system, Euler vector magnitudes are obtained through closure. The results, though poorly constrained, suggest relative motion rates of 0.5 cm/yr along the North Scotia Ridge and 1.0 cm/yr along the South Scotia Ridge and in Drake Passage.