Recurrent Holocene Movement on the Susitna Glacier Thrust Fault: The Structure that Initiated the Mw 7.9 Denali Fault Earthquake, Central Alaska

We conducted a trench investigation and analyzed pre-and postearthquake topography to determine the timing and size of prehistoric surface ruptures on the Susitna Glacier fault (SGF), the thrust fault that initiated the 2002 M-w 7.9 Denali fault earthquake sequence in central Alaska. In two of our three hand-excavated trenches, we found clear evidence for a single pre-2002 earthquake (penultimate earthquake [ PE]) and determined an age of 2210 +/- 420 cal. B. P. (2 sigma) for this event. We used structure-from-motion software to create a pre-2002-earthquake digital surface model (DSM) from 1: 62,800-scale aerial photography taken in 1980 and compared this DSM with postearthquake 5-m/pixel Interferometric Synthetic Aperature Radar topography taken in 2010. Topographic profiles measured from the pre-earthquake DSM show features that we interpret as fault and fold scarps. These landforms were about the same size as those formed in 2002, so we infer that the PE was similar in size to the initial (M-w 7.2) subevent of the 2002 sequence. A recurrence interval of 2270 yrs and dip slip of similar to 4.8 m yield a single-interval slip rate of similar to 1.8 mm/yr. The lack of evidence for pre-PE deformation indicates probable episodic (clustering) behavior on the SGF that may be related to strain migration among other similarly oriented thrust faults that together accommodate shortening south of the Denali fault. We suspect that slip-partitioned thrust-triggered earthquakes may be a common occurrence on the Denali fault system, but documenting the frequency of such events will be very difficult, given the lack of long-term paleoseismic records, the number of potential thrust-earthquake sources, and the pervasive glacial erosion in the region.