Abstract

The Ancash earthquake of 10 November 1946 (Mw = 6.8) produced surface faulting along the Quiches normal-slip fault. Field studies reported discontinuous NW-SE to NNW-SSE-striking seismic scarps that displace Quaternary glacial moraines and outwash, as well as Mesozoic sedimentary rocks. Structural analyses show that the 1946 normal displacement was accompanied by a small left-lateral slip component. Morphological analysis of the fault scarp and observations in a trench excavated across the scarp show evidence of a single normal-fault displacement before the 1946 seismic event that offset late Pleistocene glacial moraines. This indicates an approximate recurrence interval of more than 13,000 ± 1,000 years for the major events, suggesting a rate of net vertical displacement on the fault of less than 0.25 mm/yr. Kinematic analysis along the Quiches fault, in comparison with other regional studies, suggests that the state of stress in the High Andes is regionally and locally homogeneous with only minor changes in the stress directions adjacent to major structures such as the Marañon Thrust and Fold Belt. The minimum horizontal stress (σ3 axis) is oriented N10-25°E, the intermediate horizontal stress (σ2 axis) is oriented N100-115°E, and the maximum stress (σ1 axis) is vertical. We suggest that the 60°SW-dipping Quiches surface fault plane is connected at depth to a low-angle-dipping fault plane. This seismic fault is a reactivated thrust fault of the Tertiary Marañon Thrust and Fold Belt.

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