Elastic and Viscoelastic Strain Loading Models of the Wasatch Fault, Utah, From GPS Measurements and Comparisons With Geological Observations Abstract

abstract

  • The development of geodetic methods to measure ground movements using high precision GPS offers a new tool to deduce contemporary crustal deformation and fault-loading rates. Geologic evidence such as paleoearthquakes, on the other hand, provide in-plane, near-vertical displacements of a normal fault that may be used to imply long-term horizontal surface displacement and compare with the geodetic observations. We measured contemporary crustal deformation of the 370 km-long Wasatch fault zone, Utah, using continuous and campaign GPS since 1992. The overall GPS observations indicate an averaged horizontal extensional strain rate of 24±6 nstrain/yr across a 65-km wide area spanning the Wasatch fault, corresponding to a horizontal displacement rate of 1.6±0.4 mm/yr. Employing a simple-shear rupture model for a dip-slip normal fault, where the hanging-wall is deformed by simple shear with the inclination of the shear plane equal to the dip of the antithetic fault, we converted the horizontal displacement rate to in-plane near-vertical fault slip rate. Results based on our preferred dip angles for the Wasatch and its antithetic faults implied that the contemporary geodetic strain-loading rate (1.6±0.4 mm/yr in the horizontal direction) is consistent within the 95% confident-interval with the Holocene-average geological strain-release rate (1.7±0.5 mm/yr in vertical) in the brittle part of the crust. Other possible fault-geometry models, however, suggest different results: lower dips for fault and antithetic shear planes result in lower strain-loading rate, thus lower earthquake occurrence rate, at present than in the Holocene period, but steeper dips imply the opposite. Based on these constraints, we suggest that the comparison between geologically and geodetically implied horizontal extension rates across a normal fault like the Wasatch should be done with the knowledge of fault rupturing models. This requires a working model of fault dip and how the hanging-wall beds were deformed (or sheared) during earthquakes to estimate surface horizontal extension from vertical fault displacement.

publication date

  • 2006

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