Tsunamigenic Landslide Deposits Imaged beneath Port Valdez, Alaska

One of the highest tsunami wave runups ever documented (>60 m) occurred in Port Valdez following the 1964 M9.2 Alaska earthquake. The tsunami was primarily generated by submarine slope failures seismically triggered by the earthquake. Debris lobes emanating from the fjord-head delta in eastern Port Valdez and debris flow lobes incorporating large, coherent blocks in western Port Valdez, off of the Shoup Glacier moraine, were imaged on high resolution mini-sparker profiles collected in late summer 2007.

In addition to the near-surface debris lobe deposits directly attributable to the 1964 earthquake, up to 5 additional sets of debris lobe deposits, beneath parallel-layered reflectors were also imaged; these deposits are inferred to be paleo-tsunami deposits. Assuming that the first set of debris flow deposits, imaged beneath the 1964 lobes, was deposited at the time of the penultimate megathrust earthquake (dated at 913-808 yrs b.p., Carver and Pflaker, in press), we calculated a sediment accumulation rate of about 2 cm/yr for the inter-lobe deposits. This rate is comparable to that determined for post-1964 deposits using ¹³⁷Cs peaks in gravity cores from Port Valdez.

Deposits attributed to submarine failures triggered by the 1964 and penultimate events have a similar distribution across the entire fjord. However, earlier events are not present in western Port Valdez, suggesting that failures related to the Shoup Glacier moraine did not occur until more recently. In addition, the oldest debris flow lobes tend to be thinner and have thinner sedimentary sequences between the lobes than the younger flows. This may be the result of shorter recurrence intervals between megathrust earthquakes and perhaps differences in sediment input at the fjord head. Interestingly, beneath the 6 debris lobes that we have mapped, there is up to 100 m of layered sediment that shows no evident disturbance by submarine slope failures.