TY - JOUR T1 - Geomorphic expression and slip rate of the Fairweather fault, southeast Alaska, and evidence for predecessors of the 1958 rupture JF - Geosphere Y1 - 2021 A1 - Witter, Robert C. A1 - Bender, Adrian M. A1 - Scharer, Katherine M. A1 - DuRoss, Christopher B. A1 - Haeussler, Peter J. A1 - Lease, Richard O. AB - Active traces of the southern Fairweather fault were revealed by light detection and ranging (lidar) and show evidence for transpressional deformation between North America and the Yakutat block in southeast Alaska. We map the Holocene geomorphic expression of tectonic deformation along the southern 30 km of the Fairweather fault, which ruptured in the 1958 moment magnitude 7.8 earthquake. Digital maps of surficial geology, geomorphology, and active faults illustrate both strike-slip and dip-slip deformation styles within a 10°–30° double restraining bend where the southern Fairweather fault steps offshore to the Queen Charlotte fault. We measure offset landforms along the fault and calibrate legacy 14C data to reassess the rate of Holocene strike-slip motion (≥49 mm/yr), which corroborates published estimates that place most of the plate boundary motion on the Fairweather fault. Our slip-rate estimates allow a component of oblique-reverse motion to be accommodated by contractional structures west of the Fairweather fault consistent with geodetic block models. Stratigraphic and structural relations in hand-dug excavations across two active fault strands provide an incomplete paleoseismic record including evidence for up to six surface ruptures in the past 5600 years, and at least two to four events in the past 810 years. The incomplete record suggests an earthquake recurrence interval of ≥270 years—much longer than intervals <100 years implied by published slip rates and expected earthquake displacements. Our paleoseismic observations and map of active traces of the southern Fairweather fault illustrate the complexity of transpressional deformation and seismic potential along one of Earth's fastest strike-slip plate boundaries. VL - 17 SN - 1553-040X UR - https://doi.org/10.1130/GES02299.1 IS - 3 JO - Geosphere ER - TY - JOUR T1 - Unusually large tsunamis frequent a currently creeping part of the Aleutian megathrust JF - Geophysical Research Letters Y1 - 2016 A1 - Witter, Robert C. A1 - Carver, Gary A. A1 - Briggs, Richard W. A1 - Gelfenbaum, Guy A1 - Koehler, Richard D. A1 - La Selle, SeanPaul A1 - Bender, Adrian M. A1 - Engelhart, Simon E. A1 - Hemphill-Haley, Eileen A1 - Hill, Troy D. AB - Current models used to assess earthquake and tsunami hazards are inadequate where creep dominates a subduction megathrust. Here we report geological evidence for large tsunamis, occurring on average every 300–340 years, near the source areas of the 1946 and 1957 Aleutian tsunamis. These areas bookend a postulated seismic gap over 200 km long where modern geodetic measurements indicate that the megathrust is currently creeping. At Sedanka Island, evidence for large tsunamis includes six sand sheets that blanket a lowland facing the Pacific Ocean, rise to 15 m above mean sea level, contain marine diatoms, cap terraces, adjoin evidence for scour, and date from the past 1700 years. The youngest sheet and modern drift logs found as far as 800 m inland and >18 m elevation likely record the 1957 tsunami. Previously unrecognized tsunami sources coexist with a presently creeping megathrust along this part of the Aleutian Subduction Zone. VL - 43 UR - http://doi.wiley.com/10.1002/2015GL066083 IS - 1 ER -