HR: 11:35h
AN: U42A-06    [Abstracts]
TI: Unraveling the Earthquake History of the Denali Fault System, Alaska: Filling a Blank Canvas With Paleoearthquakes
AU: * Schwartz, D P
EM: dschwartz@usgs.gov
AF: USGS, 325 Middlefield Road, Menlo Park, CA 94025 United States
AU: Haeussler, P J
U42A-06 AF: USGS, 4200 University Drive, Anchorage, AK 99508 United States
AU: Seitz, G G
U42A-06 AF: San Diego State University, 5500 Campanile Drive, San Diego, CA 92182 United States
AU: Dawson, T E
U42A-06 AF: USGS, 325 Middlefield Road, Menlo Park, CA 94025 United States
AU: Stenner, H D
U42A-06 AF: USGS, 325 Middlefield Road, Menlo Park, CA 94025 United States
AU: Matmon, A
U42A-06 AF: USGS, 325 Middlefield Road, Menlo Park, CA 94025 United States
AU: Crone, A J
U42A-06 AF: USGS, P.O. Box 25046, Denver, CO 80225 United States
AU: Personius, S
U42A-06 AF: USGS, P.O. Box 25046, Denver, CO 80225 United States
AU: Burns, P B
U42A-06 AF: Alaska DGGS, 3354 College Road, Fairbanks, AK 99709 United States
AU: Cadena, A
U42A-06 AF: Central Washington University, 400 East University Way, Ellensburg, WA 98926 United States
AU: Thoms, E
U42A-06 AF: USGS, 4200 University Drive, Anchorage, AK 99508 United States
AB: Developing accurate rupture histories of long, high-slip-rate strike-slip faults is is especially challenging where recurrence is relatively short (hundreds of years), adjacent segments may fail within decades of each other, and uncertainties in dating can be as large as, or larger than, the time between events. The Denali Fault system (DFS) is the major active structure of interior Alaska, but received little study since pioneering fault investigations in the early 1970s. Until the summer of 2003 essentially no data existed on the timing or spatial distribution of past ruptures on the DFS. This changed with the occurrence of the M7.9 2002 Denali fault earthquake, which has been a catalyst for present paleoseismic investigations. It provided a well-constrained rupture length and slip distribution. Strike-slip faulting occurred along 290 km of the Denali and Totschunda faults, leaving unruptured ?140km of the eastern Denali fault, ?180 km of the western Denali fault, and ?70 km of the eastern Totschunda fault. The DFS presents us with a blank canvas on which to fill a chronology of past earthquakes using modern paleoseismic techniques. Aware of correlation issues with potentially closely-timed earthquakes we have a) investigated 11 paleoseismic sites that allow a variety of dating techniques, b) measured paleo offsets, which provide insight into magnitude and rupture length of past events, at 18 locations, and c) developed late Pleistocene and Holocene slip rates using exposure age dating to constrain long-term fault behavior models. We are in the process of: 1) radiocarbon-dating peats involved in faulting and liquefaction, and especially short-lived forest floor vegetation that includes outer rings of trees, spruce needles, and blueberry leaves killed and buried during paleoearthquakes; 2) supporting development of a 700-900 year tree-ring time-series for precise dating of trees used in event timing; 3) employing Pb 210 for constraining the youngest ruptures in sag ponds on the eastern and western Denali fault; and 4) using volcanic ashes in trenches for dating and correlation. Initial results are: 1) Large earthquakes occurred along the 2002 rupture section 350-700 yrb02 (2-sigma, calendar-corrected, years before 2002) with offsets about the same as 2002. The Denali penultimate rupture appears younger (350-570 yrb02) than the Totschunda (580-700 yrb02); 2) The western Denali fault is geomorphically fresh, its MRE likely occurred within the past 250 years, the penultimate event occurred 570-680 yrb02, and slip in each event was 4m; 3) The eastern Denali MRE post-dates peat dated at 550-680 yrb02, is younger than the penultimate Totschunda event, and could be part of the penultimate Denali fault rupture or a separate earthquake; 4) A 120-km section of the Denali fault between tNenana glacier and the Delta River may be a zone of overlap for large events and/or capable of producing smaller earthquakes; its western part has fresh scarps with small (1m) offsets. 2004/2005 field observations show there are longer datable records, with 4-5 events recorded in trenches on the eastern Denali fault and the west end of the 2002 rupture, 2-3 events on the western part of the fault in Denali National Park, and 3-4 events on the Totschunda fault. These and extensive datable material provide the basis to define the paleoseismic history of DFS earthquake ruptures through multiple and complete earthquake cycles.
DE: 1105 Quaternary geochronology
DE: 1600 GLOBAL CHANGE
DE: 7221 Paleoseismology (8036)
DE: 8177 Tectonics and climatic interactions
DE: 8408 Volcano/climate interactions (1605, 3309)
SC: Union [U]
MN: Fall Meeting 2005