Ten Thousand Years of Paleo‐Earthquakes Record of the Magallanes‐Fagnano Plate Boundary Fault in Tierra del Fuego, Argentina

Abstract

The Magallanes‐Fagnano Fault is an active left‐lateral strike‐slip fault that cuts across Tierra del Fuego, forming the boundary between the South American and the Scotia plates. This fault may trigger strong earthquakes, as documented by the occurrence of two Mw ≥ 7.5 in December 1949. However, this region is characterized by one of the shortest historical archives in the world and by a growing population. The geological record is therefore needed in order to characterize the seismic information over a longer time scale and to improve the seismic hazard assessment. We conducted extensive field work, neotectonic mapping and excavated two paleoseismic trenches across one of the sharpest tectonic scarps in the Eastern onshore portion of the fault. Using scarp‐derived colluvial wedges, cross‐cutting relations, and 28 radiocarbon samples, we document evidences of at least six paleo‐earthquakes during the Holocene. Paleoseismic record is particularly accurate for the last two thousand years, for which period we determine an average recurrence interval of 1080 ± 150 years. For repeated earthquakes of same magnitude as the 1949 event, this recurrence interval is compatible both with the known geodetic and geomorphic slip rates. However, “paired earthquakes” in less than one or two centuries may also occur, suggesting that fault behavior could be characterized by irregular seismic cycles.