Understanding the earthquake potential of blind thrust faults (faults that do not reach the Earth?s surface) has major seismic hazard implications for heavily populated areas such as Los Angeles and Seattle in the United States as well as other population centers within seismically active regions around the World. Key questions regarding the long-term behavior of destructive earthquake faults include what are the size and frequency of earthquakes and how do those vary in time and space. Better knowledge of the timing and sizes of past earthquakes is essential information used to forecast future events, to characterize local and regional seismic hazards, and to inform the public and emergency planners. Heavily populated areas, however, are difficult to study because concrete, buildings, and development obscure the expression of active faults. In this study, an active fault system near San Juan Argentina, which bears structural similarities to blind faults within metropolitan regions in the United States, but has the advantage that the area is largely undisturbed, is being investigated to better understand the surficial expression and earthquake history of several active blind thrust faults. This project is focused on dating of a large suite of samples collected from river terraces and alluvium deformed and faulted by the earthquake faults. From these new data, the timing of the past five large earthquakes on the blind fault system will be established. The new age data will allow comparison with the earthquake history of nearby fault systems documented in earlier work, faults that were responsible for damaging earthquakes in 1944 and 1977 in the region. This project will test whether earthquake frequency on individual faults is quasi-periodic (and therefore predictable), whether these fault systems produce earthquakes that cluster in time, and test whether some past earthquake may have ruptured a larger source region or have been confined to the same source dimensions as the historical events. Like the results from an earlier phase of this project, the Argentinean collaborators and others will use the new results from this project to constrain seismic hazard, inform earthquake engineers, and educate the public about earthquake hazards.
