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 - A maximum rupture model for the central and southern Cascadia subduction zone—reassessing ages for coastal evidence of megathrust earthquakes and tsunamis JF - Quaternary Science Reviews Y1 - 2021 A1 - Nelson, Alan R. A1 - DuRoss, Christopher B. A1 - Witter, Robert C. A1 - Kelsey, Harvey M. A1 - Engelhart, Simon E. A1 - Mahan, Shannon A. A1 - Gray, Harrison J. A1 - Hawkes, Andrea D. A1 - Horton, Benjamin P. A1 - Padgett, Jason S. AB - A new history of great earthquakes (and their tsunamis) for the central and southern Cascadia subduction zone shows more frequent (17 in the past 6700 yr) megathrust ruptures than previous coastal chronologies. The history is based on along-strike correlations of Bayesian age models derived from evaluation of 554 radiocarbon ages that date earthquake evidence at 14 coastal sites. We reconstruct a history that accounts for all dated stratigraphic evidence with the fewest possible ruptures by evaluating the sequence of age models for earthquake or tsunami contacts at each site, comparing the degree of temporal overlap of correlated site age models, considering evidence for closely spaced earthquakes at four sites, and hypothesizing only maximum-length megathrust ruptures. For the past 6700 yr, recurrence for all earthquakes is 370e420 yr. But correlations suggest that ruptures at-1.5 ka and-1.1 ka were of limited extent (<400 km). If so, post-3-ka recurrence for ruptures extending throughout central and southern Cascadia is 510e540 yr. But the range in the times between earthquakes is large: two instances may be-50 yr, whereas the longest are-550 and-850 yr. The closely spaced ruptures about 1.6 ka may illustrate a pattern common at subduction zones of a long gap ending with a great earthquake rupturing much of the subduction zone, shortly followed by a rupture of more limited extent. The ruptures of limited extent support the continued inclusion of magnitude-8 earthquakes, with longer ruptures near magnitude 9, in assessments of seismic hazard in the region. VL - 261 UR - https://apps.webofknowledge.com/InboundService.do?product=WOS&Func=Frame&DestFail=http%3A%2F%2Fwww.webofknowledge.com&SrcApp=search&SrcAuth=Alerting&SID=6DMZZppDlBSzMpeHDG2&customersID=Alerting&mode=FullRecord&IsProductCode=Yes&AlertId=4d48b20a-7d27-4fa2- ER - TY - CHAP T1 - Chapter 30 - Radiocarbon dating of tsunami and storm deposits T2 - Geological Records of Tsunamis and Other Extreme Waves Y1 - 2020 A1 - Kelsey, Harvey M. A1 - Witter, Robert C. ED - Engel, Max ED - Pilarczyk, Jessica ED - May, Simon Matthias ED - Brill, Dominik ED - Garrett, Ed KW - Dating coastal storm deposits KW - Dating tsunami deposits KW - Modeling approaches for dating tsunami deposits KW - Radiocarbon dating KW - Sampling tsunami deposits KW - Tsunami Deposits KW - Tsunami sand AB - Radiocarbon age determinations can be an expedient and accurate means to assign age to deposits of tsunami or storm origin. Essential to the process of incorporating radiocarbon age determinations in tsunami or coastal storm investigations is an awareness on the part of the investigator that a sample will always return an age from a laboratory, but only carefully selected samples inform deposit age. Samples that inform deposit age are of two fundamentally different sample types, in-growth-position samples and detrital samples. For both in-growth-position samples and detrital samples, stratigraphic context is the critical information needed to evaluate how well sample age can constrain deposit age. Well constrained deposit ages require bracketing samples collected to provide both maximum and minimum limiting ages for the deposit(s) of interest. Therefore, sampling should be carried out with the intention of multiple sample submissions for age in order to optimize the potential for acquiring closely limiting ages. If there are multiple age determinations within a stratigraphic sequence that contains tsunami or storm deposits, then the calibrated radiocarbon ages can be, and should be, framed within a Bayesian model structure to better constrain deposit ages. Such models can be further improved by the incorporation of independent stratigraphic age information. JF - Geological Records of Tsunamis and Other Extreme Waves PB - Elsevier SN - 978-0-12-815686-5 UR - https://www.sciencedirect.com/science/article/pii/B9780128156865000304 ER - TY - JOUR T1 - Differences in coastal subsidence in southern Oregon (USA) during at least six prehistoric megathrust earthquakes JF - Quaternary Science Reviews Y1 - 2016 A1 - Milker, Yvonne A1 - Nelson, Alan R. A1 - Horton, Benjamin P. A1 - Engelhart, Simon E. A1 - Bradley, Lee-Ann A1 - Witter, Robert C. KW - Cascadia subduction zone KW - Coseismic subsidence KW - Megathrust earthquakes KW - Paleoseismology KW - Salt-marsh foraminifera KW - Sea-level change KW - Transfer functions AB - Stratigraphic, sedimentologic (including CT 3D X-ray tomography scans), foraminiferal, and radiocarbon analyses show that at least six of seven abrupt peat-to-mud contacts in cores from a tidal marsh at Talbot Creek (South Slough, Coos Bay), record sudden subsidence (relative sea-level rise) during great megathrust earthquakes at the Cascadia subduction zone. Data for one contact are insufficient to infer whether or not it records a great earthquake—it may also have formed through local, non-seismic, hydrographic processes. To estimate the amount of subsidence marked by each contact, we expanded a previous regional modern foraminiferal dataset to 174 samples from six Oregon estuaries. Using a transfer function derived from the new dataset, estimates of coseismic subsidence across the six earthquake contacts vary from 0.31 m to 0.75 m. Comparison of subsidence estimates for three contacts in adjacent cores shows within-site differences of ≤0.10 m, about half the ±0.22 m error, although some estimates may be minimums due to uncertain ecological preferences for Balticammina pseudomacrescens in brackish environments and almost monospecific assemblages of Miliammina fusca on tidal flats. We also account for the influence of taphonomic processes, such as infiltration of mud with mixed foraminiferal assemblages into peat, on subsidence estimates. Comparisons of our subsidence estimates with values for correlative contacts at other Oregon sites suggest that some of our estimates are minimums and that Cascadia's megathrust earthquake ruptures have been heterogeneous over the past 3500 years. VL - 142 UR - https://doi.org/10.1016/j.quascirev.2016.04.017 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 - TY - JOUR T1 - Beach ridges as paleoseismic indicators of abrupt coastal subsidence during subduction zone earthquakes, and implications for Alaska-Aleutian subduction zone paleoseismology, southeast coast of the Kenai Peninsula, Alaska JF - Megathrust Earthquakes and Sea-level Change: a Tribute to George Plafker Y1 - 2015 A1 - Kelsey, Harvey M. A1 - Witter, Robert C. A1 - Engelhart, Simon E. A1 - Briggs, Richard A1 - Nelson, Alan A1 - Haeussler, Peter A1 - Corbett, D. Reide AB - The Kenai section of the eastern Alaska-Aleutian subduction zone straddles two areas of high slip in the 1964 great Alaska earthquake and is the least studied of the three megathrust segments (Kodiak, Kenai, Prince William Sound) that ruptured in 1964. Investigation of two coastal sites in the eastern part of the Kenai segment, on the southeast coast of the Kenai Peninsula, identified evidence for two subduction zone earthquakes that predate the 1964 earthquake. Both coastal sites provide paleoseismic data through inferred coseismic subsidence of wetlands and associated subsidence-induced erosion of beach ridges. At Verdant Cove, paleo-beach ridges record the paleoseismic history; whereas at Quicksand Cove, buried soils in drowned coastal wetlands are the primary indicators of paleoearthquake occurrence and age. The timing of submergence and death of trees mark the oldest earthquake at Verdant Cove that is consistent with the age of a well documented ∼900-year-ago subduction zone earthquake that ruptured the Prince William Sound segment of the megathrust to the east and the Kodiak segment to the west. Soils buried within the last 400–450 years mark the penultimate earthquake on the southeast coast of the Kenai Peninsula. The penultimate earthquake probably occurred before AD 1840 from its absence in Russian historical accounts. The penultimate subduction zone earthquake on the Kenai segment did not rupture in conjunction with the Prince William Sound to the northeast. Therefore the Kenai segment, which is presently creeping, can rupture independently of the adjacent Prince William Sound segment that is presently locked. VL - 113 UR - http://www.sciencedirect.com/science/article/pii/S0277379115000220 JO - Beach ridges as paleoseismic indicators of abrupt coastal subsidence during subduction zone earthquakes, and implications for Alaska-Aleutian subduction zone paleoseismology, southeast coast of the Kenai Peninsula, Alaska ER - TY - BOOK T1 - Handbook of Sea-Level ResearchPre-fieldwork surveys Y1 - 2015 A1 - Witter, Robert C. ED - Shennan, Ian ED - Long, Antony J. ED - Horton, Benjamin P. KW - aerial photography KW - barrier island KW - bathymetric data KW - beach ridge KW - coastal geomorphology KW - geomorphological map KW - LiDAR KW - nautical chart KW - remote sensing KW - satellite imagery AB - Using maps, imagery, tidal measurements, and other historical data to assess the geomorphology of a study area prior to fieldwork helps select optimal sites, identify the most effective field methods to test research hypotheses, highlight the primary geomorphic processes, and may provide preliminary estimates of sea-level change. Here, I review the common types of data useful for pre-fieldwork surveys in sea-level research and discuss the many benefits of assessing the geomorphology of a site in advance of fieldwork. I also review briefly some case studies where historical data led to new directions in research and a scientific breakthrough. PB - John Wiley & Sons, Ltd CY - Chichester, UK SN - 9781118452585 UR - http://doi.wiley.com/10.1002/9781118452547.ch3 ER - TY - JOUR T1 - Heterogeneous rupture in the great Cascadia earthquake of 1700 inferred from coastal subsidence estimates JF - Journal of Geophysical Research: Solid Earth Y1 - 2013 A1 - Wang, Pei-Ling A1 - Engelhart, Simon E. A1 - Wang, Kelin A1 - Hawkes, Andrea D. A1 - Horton, Benjamin P. A1 - Nelson, Alan R. A1 - Witter, Robert C. AB - Past earthquake rupture models used to explain paleoseismic estimates of coastal subsidence during the great A.D. 1700 Cascadia earthquake have assumed a uniform slip distribution along the megathrust. Here we infer heterogeneous slip for the Cascadia margin in A.D. 1700 that is analogous to slip distributions during instrumentally recorded great subduction earthquakes worldwide. The assumption of uniform distribution in previous rupture models was due partly to the large uncertainties of then available paleoseismic data used to constrain the models. In this work, we use more precise estimates of subsidence in 1700 from detailed tidal microfossil studies. We develop a 3-D elastic dislocation model that allows the slip to vary both along strike and in the dip direction. Despite uncertainties in the updip and downdip slip extensions, the more precise subsidence estimates are best explained by a model with along-strike slip heterogeneity, with multiple patches of high-moment release separated by areas of low-moment release. For example, in A.D. 1700, there was very little slip near Alsea Bay, Oregon (~44.4°N), an area that coincides with a segment boundary previously suggested on the basis of gravity anomalies. A probable subducting seamount in this area may be responsible for impeding rupture during great earthquakes. Our results highlight the need for more precise, high-quality estimates of subsidence or uplift during prehistoric earthquakes from the coasts of southern British Columbia, northern Washington (north of 47°N), southernmost Oregon, and northern California (south of 43°N), where slip distributions of prehistoric earthquakes are poorly constrained. VL - 118 UR - http://doi.wiley.com/10.1002/jgrb.50101 IS - 5 ER -