Neotectonics around Fairbanks, Alaska: Identifying and characterizing active faults

Rachel Frohman
The University of Alaska Fairbanks, Department of Geology and Geophysics Fairbanks, Alaska, United States
[email protected]

The neotectonic framework of interior Alaska is defined by a series of linear, northeast-trending seismic zones that are characterized by diffuse seismicity and multiple magnitude 7 earthquakes. Seismic focal mechanisms indicate dominantly left-lateral strike-slip motion within these zones. Despite the abundant seismicity, the seismogenic faults have not previously been characterized in detail, mostly because of the lack of bedrock exposures. We used crustal earthquake hypocenters, DEM’s, and geological maps to better constrain the traces and dips of these faults. This revealed that the previously identified Fairbanks seismic zone actually consists of several linear seismic zones that correspond closely with mapped faults or topographic lows. We investigated several quarries that expose mapped faults to gain a better understanding of fault orientation, slip direction and sense, and paleostress orientation in the Fairbanks seismic zone. Faults are mostly near-vertical and locally define flower-like structures. Slip-sense indicators reveal left-lateral strike-slip motion, but commonly with a dip-slip component that may be either down to northwest or southeast. The faults are mostly normal-left-lateral, locally nearly pure normal, and rarely reverse-left-lateral. Geospatial analyses of DEM’s combined with evaluation of Quaternary geologic and topographic maps are currently in progress and will be used to analyze geomorphic anomalies that may reflect young deformation, including wind gaps, barbed drainages, and asymmetrical stream valleys. Results so far show that surface evidence exists to characterize active faults despite poor exposure and subdued topography, and that the faults have a dip-slip component, probably dominantly normal, that is not evident in earthquake focal mechanisms.

AAPG Search and Discovery Article #90157©2012 AAPG Foundation 2012 Grants-in-Aid Projects