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Crustal flexure associated with normal faulting and implications for seismicity along the Wasatch Front, Utah

DEPARTMENT OF GEOLOGY AND GEOPHYSICS UNIVERSITY OF UTAH, SALT LAKE CITY, UTAH 84112

Abstract

Elastic and viscoelastic models of crustal flexure in Utah's Wasatch Front area predict stress fields which are generally consistent with the observed pattern of seismicity. The models consist of an elastic or viscoelastic layer overlying a fluid-like substratum. Normal faulting that penetrates the entire crust induces isostatic forces which produce upward flexure of the footwall side of the Wasatch Fault, thus uplifting the Wasatch Mountains. Using reasonable values for the flexural rigidity and assuming the end of the layer is constrained at the fault, shallow extensional stresses are induced in a 48- to 80-km-wide zone adjacent to the fault. If the end is unconstrained, shallow compressional stresses are induced in a broad area 60 to 100 km wide that peaks about 24 to 40 km from the main fault. Seismicity patterns as well as available focal mechanism are consistent with a model in which the East Cache Fault is in a constrained flexure mode; whereas, segments of the Wasatch Fault are in unconstrained flexure. For the elastic model, the magnitudes of the bending stresses for 300 to 490 meters (m) of uplift that can be produced by the isostatic forces alone are 310 to 510 bars. If the uplift is an order of magnitude greater, as indicated by geological studies, the bending stresses are in the kilobar range. The viscoelastic model yields more satisfactory predictions for some of the observed parameters and allows for dissipation of the bending stresses through relaxation without faulting. The time variation of the bending stresses could have a significant influence on the episodicity of faulting on the Wasatch Fault.

Although the details of the mechanism are still in doubt, this study suggests a possible causal relationship between major faulting on the Wasatch Fault and surrounding microseismicity. An understanding of this mechanism may someday lead to a means of forecasting major earthquakes on the Wasatch Fault.

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