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

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 M-w >= 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. Plain Language Summary The Magallanes-Fagnano Fault is a main active fault in Tierra del Fuego, at the southern tip of South America. This remote region has one of the shortest historical seismic record in the world. However, this fault may trigger strong earthquakes as occurred in 1949 and the surrounding population has been massively growing since. We therefore studied the paleo-earthquake history of the fault to improve the seismic hazard assessment. We analyzed and dated the disrupted sediments in two trenches excavated across the fault to establish a paleo-earthquake calendar. We document evidences of at least six paleo-earthquakes during the last ten thousand years. Paleoseismic record is particularly accurate for the last two thousand years, for which period we determine a recurrence interval of 1080 +/- 150 years. However, paired major earthquakes in less than one or two centuries may also occur, suggesting that fault behavior could not be based on regular seismic cycles.

Automated summary

The Magallanes-Fagnano Fault is an active strike-slip fault separating the South American and Scotia plates. Our study reveals evidence of at least six earthquakes in the past 10,000 years, with an average recurrence interval of 1080 years in the last 2000 years. However, we also find that irregular seismic cycles may result in paired earthquakes occurring within one or two centuries.