SEPA Summary

The present day tectonics and seismological structure of Patagonia, Drake Passage, and the Antarctic Peninsula region are among the most poorly understood of any location. The geodynamic setting of this region offers a unique opportunity to study complex tectonic interactions, as illustrated by the recent cessation of volcanism in the South Shetland Islands and onset of rifting in the Bransfield Strait. The SEPA project is a deployment of broadband seismographs in the Patagonia and Antarctic Peninsula regions designed to answer the following questions:

• Is current subduction occurring beneath the South Shetland Islands, as evidenced by a descending slab and intermediate depth earthquakes?
• Is there evidence from seismic anisotropy for large scale mantle flow around the South American slab through the Drake Passage region?
• What is the seismological structure and earthquake activity of the active back-arc spreading center in the Bransfield Strait? Are active volcanoes present?
• What are the tectonics of Drake Passage, where plate motion models imply compression should be occurring?
• Where are the active faults in Patagonia, and what level of seismic hazard exists there?

The data will be used to study the seismicity and upper mantle velocity structure of several complicated tectonic regions in the area, including the South Shetland subduction zone, the Bransfield backarc rift, and diffuse plate boundaries in Patagonia, Drake Passage, and along the South Scotia Ridge. In particular, the operation of these stations over a longer time period will allow a better understanding of the seismicity of the South Shetland Trench, an unusual subduction zone showing very slow subduction of young lithosphere. These seismometers were also be used to record airgun shots during a University of Texas geophysical cruise in the Bransfield Strait in April, 2000. These data will provide important constraints on the crustal structure beneath the stations, and the improved structural models will enable implementation of more precise earthquake location procedures in support of a seismological understanding of the region.

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