Thrust faults are a major seismic hazard worldwide. Seismic hazard assessment for thrust faults depends on understanding the timing, frequency, and size of past earthquakes. Thrust fault earthquakes are often accompanied by folding in the shallow subsurface. Folds are the cumulative product of repeated earthquakes. Thus, knowledge of how much, with what frequency, and with what geometry folds grow in individual earthquakes is essential for determining how fold growth relates to earthquake size on thrust faults. In two areas in western Argentina, growth strata and geomorphic surfaces contain information about folding and represent two repositories of the earthquake history of the associated thrust faults. Structural, geomorphic, and paleoseismologic study of the two areas is revealing a detailed record of coseismic and long-term deformation and providing new insight into the geomorphic and stratigraphic signature of folding on earthquake timescales. A field-calibrated model of the production of geomorphic surfaces or growth strata in an individual thrust earthquake is being developed based on the surface effects of a large and damaging earthquake in 1944. Moreover, the new data are contributing directly to the Argentinean national seismic hazard assessment program. The cities of Mendoza and San Juan have a combined population of more than 1.5 million people and are located near active earthquake faults with little constraints on their past activity or future potential for earthquakes. This study is providing a template for the recognition of paleoearthquakes from folds and a new method of thrust earthquake hazard recognition. This project is being supported by the Geosciences Directorate and the Office of International Science and Engineering.
