This project provides funds for a two-year renewal of the St. Elias Erosion-tectonics Project (STEEP). STEEP is a 9 institution, multidisciplinary study of the St. Elias orogen in southern Alaska that involves researchers examining the system from the outcrop to lithosphere scale. To date, STEEP has produced 17 papers with another 9 submitted or nearing submission, sponsored 71 abstracts, will have matriculated 5 masters and 4 Doctoral students by Spring 2010, and fundamentally changed our understanding of Alaskan tectonics and the interaction of tectonics and climate in mountain building. The renewal funds will be used for: 1) final processing and interpretation of some key datasets that were not acquired until year 5 of the project including the marine seismic survey (ship delays) and reoccupation of key GPS sites (weather problems in 2008); and 2) a complete integration of results which was not possible until now due to these delays. A complete integration and synthesis of these superb datasets has the potential to be transformative in our understanding of how crustal structure and tectonic forces interact with Earth surface processes of glacial erosion and sedimentary transport to grow a mountain range and a massive continental shelf.
This research project dealt with the history of earthquakes along the plate boundary of southern Alaska, with fieldwork focused in the Katalla River Valley. The goal was to study this apparently anomalous region of the plate boundary's landscape where landforms indicate the possibility of net uplift related to earthquake activity. This behavior, if true, is anomalous compared to deformation along adjacent sections of the coast in the northeastern Gulf of Alaska, where uplift during great subduction zone earthquakes is followed by subsidence between earthquakes that results in net submergence relative to sea level. The presence of an area of net 'tectonic' uplift may therefore indicate the presence of subsidiary or secondary faults that may form a separate seismic source for earthquakes in addition to those that occur along the main subduction fault. This would have significant implications for estimating the earthquake and possibly tsunami hazards of the region. Fieldwork was carried out over a period of 11 days in the late summer of 2011, when a research team composed of members from the University of Utah, Durham University in the UK, and the Alaska Division of Geological and Geophysical Surveys visited the Katalla River Valley. R. Bruhn the grant’s Principal Investigator led the effort. Dr. Ian Shennan of Durham University provided expertise on Quaternary sampling and analysis of microscopic plant fossils. The research team mapped the geomorphology and collected samples of sediment from hand dug pits and cores. Special attention was given to identifying evidence for abrupt uplift during earthquakes, which is indicated by changes from marine to non-marine deposition, as well as evidence for coastal submergence caused when relative sea level rose between earthquakes. The timing of earthquake uplift was dated by radiocarbon methods, and the depositional environment of the deposits was determined by their composition and the presence of very small plants called diatoms. The type of diatoms in the sediments differs from non-marine through brackish to marine water. Results indicate that the Katalla Valley responds in a similar manner to adjacent sections of the coast during and following great magnitude (M9) earthquakes. That is, the coast rises abruptly during the earthquake, and then subsides in the interval before the next great earthquake. However, there is an anomalous uplift event circa 500 BP that does not correlate with timing of great magnitude earthquakes, and which may indicate the presence of a subsidiary or splay fault beneath the coast that ruptures, from time to time, independently of the subduction zone fault. The mean recurrence interval on the subduction fault is about 535 years, with the exception of the time between the penultimate event and the Mw 9.2 earthquake of 1964, which was about 833 years. The recurrence interval for the 500 BP - type earthquake localized in the Katalla Valley is not known, but may be greater than 4000 years. There is evidence for several meters of net uplift in the valley over the last 7000 years, which may also reflect local earthquake activity. The results of the study provided a new constraint on the recurrence interval between great subduction zone earthquakes in the northeastern Gulf of Alaska. This part of the study is published in the journal Quaternary Science Reviews, V. 84, 2013 (Authors I. Shennan, R. Bruhn, N. Barlow, K. Good, and E. Hocking). A separate publication on the long-term history of seismic behavior in the Katalla Valley is being prepared from the thesis of Ms. Kelly Good, who completed her MS degree at the University of Utah in the spring of 2013. Ms. Good is now gainfully employed in the geological consulting industry. The results of her thesis work indicate that a buried fault system may lie beneath the valley, and from time to time, generate earthquakes separate from those on the underlying subduction zone fault. This has important implications for the geological hazards of the Gulf of Alaska region, and may play a role in the generation of tsunami. Outreach from this project includes the following: 1) Training of an MS student in Geology and Geophysics (Ms. Kelly Good), 2) refining the earthquake history and seismic cycle of earthquakes in the Gulf of Alaska, which has significant implications for evaluating risk to people and structures caused by earthquake ground motion as well as both local and far traveled seismic sea waves (tsunami).
