Basement structure of the Northern Foothills Fold and Thrust Belt and southern Fairbanks and Salcha, Alaska Seismic zones and their relation to large (M ≥ 6) earthquakes

Abstract

We used seismic, gravity and aeromagnetic information to better define the basement structure of the southern Fairbanks and Salcha seismic zones and the Northern Foothills Fold and Thrust Belt (NFFTB), regions that accommodate deformation in interior Alaska north of the Denali fault. At least five earthquakes of M ≥ 6 occurred within this region between 1929 and 1947. Relocated epicenters of two foreshocks and the October 16, 1947 (Mw 7.2) mainshock suggest they occurred in a topographically low area covered with recent alluvial deposits located ∼20 km north of the northernmost mapped thrusts of the NFFTB, although the largest aftershock of the sequence occurred within the exposed NFFTB. The three largest events of the 1947 sequence had similar waveforms, indicating similar mechanisms. Relocations of the 1929 events, also with waveforms similar to the 1947 sequence, indicate they likely occurred along thrust faults associated with the exposed NFFTB. In contrast, the 1937 sequence (mainshock of Mw 7.3) involved strike-slip rupture along the Salcha seismic zone. We constructed and modeled three gravity/aeromagnetic profiles extending across the NFFTB to the northwestern edge of the Tanana Basin that support the presence of subsurface thrusting north to 64.4°N, coincident with the absence of the Totatlanika schist at this latitude. A third profile extending across the Salcha seismic zone suggests reactivation of high angle faults within the Yukon-Tanana crystalline complex. The extension of thrust faulting ∼20–30 km north of the exposed NFFTB has important implications for the seismic hazards of the Fairbanks region.