HR: 0800h
AN: G11A-0780    [Abstracts]
TI: Repeated Historic Surface Ruptures of the Denali Fault at Delta River, Alaska During Large Earthquakes in 1912 and 2002
AU: * Plafker, G
EM: george@plafker.com
AF: Plafker Geohazards Consultants, 235 Highland Terrace, Woodside, CA 94062 United States
AU: Carver, G
EM: cgeol@alaska.com
AF: Carver Geologic, 12021 Middle Bay Drive, Kodiak, AK 99615 United States
AU: Metz, M
EM: metzak@cascadeaccess.com
AF: Metz Geotechnical, 2541 Curlew Circle #A, Anchorage, AK 99502 United States
AU: Cluff, L
EM: lloydcluff@aol.com
AF: Pacific Gas and Electric Co., P.O. Box 770000 MC N4C, San Francisco, CA 94114 United States
AB: The Denali fault ruptured through stands of mature spruce trees on the Delta River valley floor during an Ms 7.2-7.4 earthquake on July 6, 1912 and again during the Mw 7.9 earthquake of November 3, 2002. In both events, most trees on the surface rupture along 2 km of the fault trace were damaged by splitting and tilting. Displacements in both events were dextral with a subordinate dip-slip component (south side down). Tree ring counts from older damaged trees on the fault trace closely date the age of the penultimate event at 1912. The only earthquake that fits the requirements for timing, location, and size to have caused the pre-2002 tree damage is an Ms 7.2-7.4 event on July 6, 1912, the epicenter of which had been located 40 km southwest of the Delta River fault crossing by Boyd and Lerner-Lam (1988). Intensity data for the widely felt 1912 earthquake are compatible with unilateral westward rupture on the Denali Fault. Empirical data for the estimated magnitude range of the 1912 earthquake suggest a surface rupture length of 60-84 km, average horizontal displacement of 140-210 cm, and maximum horizontal displacement of 240-390 cm (Wells and Coppersmith, 1994). The 2002 surface rupture was marked by large dextral surface slip (to 800+ cm) and variable dip slip along 240 km of the Denali Fault and 65 km of the Totschunda fault in the central Alaska Range. The Delta River valley, 90 km east of the 2002 epicenter, lies within a transition zone about 10 km wide in which dextral slip diminishes from 600 cm or more east of the valley to less than 450 cm west of the valley. Geodetic data for the 2002 rupture along the TAPS oil pipeline in the Delta River valley indicate a total of 580 cm dextral slip and 130 cm dip slip distributed over a zone 1,000 m wide. Surface trace of the fault is poorly developed on the valley floor because of distributive deformation in the thick underlying unconsolidated deposits; offsets on individual fissures are less than 130 cm dextral and 60 cm vertical. Paleoseismologic data from test pits at the west bank of the Delta River indicate two earlier surface faulting events on the Denali Fault probably comparable in size to the 2002 earthquake. Preliminary 14C dates of these paleo-events suggest a recurrence interval of about 350-400 years and an average long term slip rate of about 15 mm/yr. Rupture during the smaller 1912 earthquake contributes to reduce the size of any 2002 slip deficit that may exist along the Denali fault in the Delta River valley and probably also in the low-slip segment extending 70 km west of the valley to the end of the 2002 Denali fault surface trace.
DE: 7221 Paleoseismology
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
DE: 8107 Continental neotectonics
DE: 7209 Earthquake dynamics and mechanics
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting