A new paper showing evidence for repeated rupture of La Laja fault in the late Pleistocene and Holocene in San Juan Argentina

Inferring a Thrust‐Related Earthquake History from Secondary Faulting: A Long Rupture Record of La Laja Fault, San Juan, Argentina

Thomas K. Rockwella, Daniel E. Ragonab, Andrew J. Meigsc, Lewis A. Owend, Carlos H. Costae, and Emilio A. Ahumadae

aDepartment of Geological Sciences, San Diego State University, 5500 Campanile Drive, San Diego, California 92182‐102
bBP p.l.c., 1 St James’s Square, London, SW1Y 4PD, United Kingdom
cCollege of Earth, Ocean, and Atmospheric Sciences, 104 CEOAS Administration Hall, Oregon State University, Corvallis, Oregon 97331‐5506
dDepartment of Geology, University of Cincinnati, Cincinnati, Ohio 45221
eDepartamento de Geología, Universidad Nacional de San Luis, 5700 San Luis, Argentina

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.

 

BSSAThomas K. Rockwell, Daniel E. Ragona, Andrew J. Meigs, Lewis A. Owen, Carlos H. Costa, and Emilio A. Ahumada, Inferring a Thrust‐Related Earthquake History from Secondary Faulting: A Long Rupture Record of La Laja Fault, San Juan, Argentina, Bulletin of the Seismological Society of America, First published on December 17, 2013, doi:10.1785/0120110080

 

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