Documentation of Seven Earthquakes over the Past similar to 7000 Years on the West-Central Denali Fault at the Nenana River, Alaska

Abstract

© 2018, Seismological Society of America. All rights reserved. The Denali fault in south-central Alaska is a major right-lateral strike-slip fault that parallels the Alaska Range for much of its length and represents the largest seismogenic source for interior Alaska. The fault system is over 1200 km in length, and identification of paleoseismic sites that preserve more than 2–3 paleoearthquakes has proven challenging due to its remote location and difficulty of access. In 2012 and 2015, we developed the Dead Mouse site, which provides the first long paleoearthquake record west of the 2002 Mw 7.9 Denali fault earthquake sequence rupture extent. This site is located on the west-central segment of the Denali fault near the southernmost intersection of the Parks Highway and the Nenana River. We hand-excavated three fault-perpendicular trenches and documented new evidence for seven surface-rupturing paleoearthquakes from deformation in the upper 2.5 m of stratigraphy. Evidence for these events includes offset units, filled fissures, upward fault terminations, and an angular unconformity. Chronological constraints from Bayesian sequence modeling of radiocarbon ages and one tentative tephra correlation indicate these seven earthquakes occurred at 388 cal B.P. (442–319; E1), 807 cal B.P. (853–764; E2), 1282 cal B.P. (1392–1160; E3), 2652 cal B.P. (2805–2460; E4), 3402 cal B.P. (3790–3010; E5*), 5673 cal B.P. (6676–4632; E6*), and 6987 cal B.P. (7281–6668; E7*). Although there are likely missing earthquakes in our chronology prior to E4, the intervals between E1 and E4 suggest significant variability in recurrence period at the Dead Mouse site. Additional paleoearthquake chronologies at neighboring sites are required to make reliable estimates of the spatial and temporal rupture history for the west-central Denali fault, but our data demonstrate the potential for recurrence periods as short as 300–600 yrs, well within range of the current open interval for the Denali fault at the Nenana River.