HR: 11:05h
AN: T22C-04    [Abstracts]
TI: The Relationship Between the Surface Expression of Blind Thrust Faults and Crustal-Scale Deformation in the Eastern Precordillera, San Juan, Argentina
AU: * Schiffman, C R
EM: schiffmc@geo.oregonstate.edu
AF: Oregon State University, Department of Geosciences, Corvallis, OR 97333 United States
AU: Meigs, A J
EM: meigsa@geo.oregonstate.edu
AF: Oregon State University, Department of Geosciences, Corvallis, OR 97333 United States
AB: Large earthquakes (M w 6.5+) are often accompanied by surface rupture that has a predictable relationship with the magnitude. However, in large thrust earthquakes that have a deep (30+ km) hypocenter or fault tip, coseismic surface deformation is expressed primarily by folding rather than as rupture along the fault surface. Knowledge of source characteristics and surficial geology are required to characterize the relationship between earthquake fault slip and coseismic folding. By fully identifying and characterizing the fault plane of the M w 7.4 earthquake that occurred in 1944 in the eastern Precordillera of the Andes, destroying the city of San Juan in northwestern Argentina, we seek to relate active folding in the near-surface structures to the blind-thrust fault at depth. Coseismic deformation associated with the 1944 earthquake are secondary fault-related folding features, and there is a large discrepancy between the amount of surface rupture and the magnitude. Subtle fold-related clues at the surface represent the only potential for recognition of the occurrence of past earthquakes. This two-part study employs seismology and structural mapping to provide a new image of the Eastern Precordillera at the crustal scale. Source parameter inversion of teleseismic seismograms from the 1944 event place the hypocenter on a west-dipping plane approximately 30 km deep, which has previously been defined by microseismicity, as opposed to a surface-rupturing event in the Neogene sedimentary strata. Preliminary results from field mapping show two types of folding due to a west-dipping thrust fault with a tip at 5 km depth: a broad long-wavelength fold (~8 km) in deformed strath terraces cut into previously deformed bedrock, and short wavelength folding and faulting in the bedrock in the form of reactivation of older thrust planes. As of now, we cannot uniquely tie any one of these surficial structure to the thrust fault at depth because the pre-existing deformation in the bedrock acts as a filter, and the deformation is distributed between both the bedrock and the terraces. A further complication is how the deformation is distributed over time: both the terraces and the reactivated flexural slip faults show differential amounts of slip over time with respect to terraces of different ages. The most recent deformation is located in a narrow band in the southeast. Hopefully our results will help lead to a method for relating secondary surficial information on a blind thrust fault to conclusive data on paleoseismicity. Results indicate that the Eastern Precordillera has been active throughout the Holocene due to the differential deformation in terraces of varying ages, and probably presents a greater seismic hazard than previously believed.
DE: 7215 Earthquake source observations (1240)
DE: 8123 Dynamics: seismotectonics
DE: 9360 South America
SC: Tectonophysics [T]
MN: Fall Meeting 2005