Abstract

Trenches excavated across the surface rupture of the 15 January 1944 Mw 7 San Juan, Argentina, earthquake show evidence for repeated rupture of La Laja fault in the late Pleistocene and Holocene. The 1944 rupture extended for about 7 km striking ∼N35°E, and dipping 42° E parallel to Neogene bedding, with a maximum east‐side‐up vertical displacement of 30 cm. We interpret nine discrete colluvial wedges, each capped by a soil, as evidence for nine surface ruptures in the past ∼32–35  ka. Our topographic profile of the T3 terrace surface projected across the fault suggests these nine events collectively produced ∼10.3  m of dip displacement. We used radiocarbon and optically stimulated luminescence dating to define the timing of past events, yielding an average recurrence interval of 3.9±0.3  ka. The average estimate for displacement per event is about 1.1 m, compared with the 0.5 m of dip slip that occurred in 1944. Using a fault‐related fold model that infers ∼28  m of shortening based on the axial surface migration for the T3 terrace, along with the ∼35  ka age of the terrace, yields a shortening rate of ∼0.8  mm/yr by the 1944 source thrust fault, which is about 20% of the Global Positioning System—constrained total shortening rate across the Precordillera. Using this shortening rate and an assumed fault dip of 30° implies a fault slip rate of about 1.1  mm/yr, which combined with the average recurrence interval, implies ∼4  m of average slip per event on the causative fault at depth.

Online Material: Unit descriptions from the trenches, stepwise reconstruction from trench 1, and annotated photographs of the La Laja region.

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