HOLOENCE COSEISMIC RUPTURE PATTERNS FROM GREAT EARTHQUAKES: SEWARD TO MIDDLETON ISLAND, ALASKA

Using tsunami run up, seismic reflection and bathymetric data, we identify and characterize rupture patterns of active megathrust faults over multiple earthquake cycles, including the 1964 M9.2 Great Alaska earthquake. Following the groundbreaking work of George Plafker, we characterize these coseismic ruptures that are rooted in splay faults that appear from Prince William Sound to Kodiak Island. Seismic and geochronology data suggest that the Prince William Sound asperity, located at the trailing edge of the subducted Yakutat terrane, has remained locked in place for thousands of earthquake cycles. We show that although these splay faults appear to have ruptured during most Holocene earthquakes, these faults produce complex rupture patterns that vary during consecutive earthquakes.

Based on estimated tsunami travel times from 1964, we identify thrust faults that produced 5-10 m wave heights in the coastal town of Seward and along the Kenai Peninsula. We identify the Hanning Bay, Patton Bay, Cape Cleare, and Middleton faults as local tsunami sources. These faults moved vertically upwards of 12 m during the 1964 earthquake, Offshore of Montague Island, the Cape Cleare and Patton Bay faults, with (post-glacial) sea floor scarps as large as 69 m and 25 m respectively, has accommodated the greatest vertical uplift during large Holocene earthquakes. Another highly active thrust fault, termed the Middleton fault, hosts a west-east 100 km long >20 m high sea floor scarp. Uplift along this fault is likely responsible for 3.5 m raised shoreline on Middleton Island from the 1964 earthquake and the total sea floor scarp height likely includes uplift from at least the prior 3 earthquakes that individually produced 6-9 m marine terraces on Middleton Island. Although rapid exhumation results from repeated coseismic slip on multiple faults, the spatial and temporal slip distribution on these faults is highly variable.