Tuesday, May 17, 2016 AGS Luncheon

"Evidence for frequent large tsunamis in the eastern Aleutians that span presently locked and creeping parts of the megathrust"

Noon Luncheon 11:30-1:00 pm

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LUNCHEON ABSTRACT

The 1957 Andreanof Islands earthquake (M 8.6) generated a destructive, Pacific-wide tsunami and produced a 1200-km-long aftershock zone along the Aleutian megathrust. At the eastern end of the 1957 rupture, Driftwood Bay (Umnak Island) and Stardust Bay (Sedanka Island) lie along presently locked and creeping parts of the megathrust, respectively. Both bays face the Aleutian trench and were inundated by the 1957 tsunami. Here we compare geological evidence for frequent, large tsunamis since ~2.2 ka at Driftwood Bay to published evidence for 6 tsunami deposits since ~1.6 ka at Stardust Bay (Witter et al., 2015), 200-km to the east.
Along the coast of Driftwood Bay, in a valley ringed by drift logs stranded up to 23 m above mean sea level (asl), we mapped 9 sand sheets up to 22.5 m asl that show features similar to modern tsunami deposits surveyed globally. Regional tide gages show that the sand sheets exceed the highest 100-yr storm surge heights. Cs-137 activity measured in sediment above and below the youngest sand sheet indicates a time consistent with sand deposition by the 1957 tsunami. Drift logs lying stratigraphically above the youngest sand sheet match eyewitness reports of 23-m high tsunami runup nearby in 1957. The 9 sand sheets and C-14 ages indicate that large tsunamis have struck Driftwood Bay every 270–290 yr on average since ~2.2 ka.
We use Bayesian age-depth models to compare tsunami deposit ages at Driftwood Bay and Stardust Bay. Among the youngest 6 sand sheets at both sites, different ages of two deposits suggest that at least twice in the past a tsunami inundated one site but not the other. However, statistically similar ages of 5 of the 6 tsunami deposits allow the interpretation that some deposits record the same tsunami or different tsunamis that occurred within several decades of one another. Our results question the notion that creeping parts of the Aleutian megathrust do not accrue strain and reflect low, long-term seismic and tsunami hazards.

SPEAKER BIOGRAPHY

Speaker: Dr. Rob Witter

Dr. Rob Witter is a research geologist at the USGS Alaska Science Center in Anchorage. He received his undergraduate degree from Whitman College, Walla Walla, Washington and his PhD from the University of Oregon in Eugene. Dr. Witter's research emphasizes paleoseismology of active faults with a particular focus on neotectonics of convergent margins. His research interests also include: examining the range of rupture variability during subduction zone earthquakes; using paleogeodesy to estimate the amount of vertical displacement caused by past earthquakes; investigating tsunami deposits to better characterize tsunami hazards; and improving public education in geologic hazards. Prior to joining the USGS, Dr. Witter designed and implemented the tsunami hazard mitigation program for the Oregon Department of Geology and Mineral Industries. He also worked as an earth science consultant in Walnut Creek California.

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