T24 Earthquake Geology in Reverse-Faulting Terrains

Sunday, 2 November 2003
8:00 AM - 12:00 PM


PALEOSEISMICITY AND STRAIN PARTITIONING IN THE FOREARC FOLD AND THRUST BELT OF THE KODIAK SEGMENT, EASTERN ALEUTIAN SUBDUCTION ZONE

Paper number: T24-10
Program: Technical Sessions
Time: 10:05 AM – 10:25 AM


Authors


Abstract

The Kodiak segment of the eastern Aleutian subduction zone is characterized by a high convergence rate (~57 mm/yr), wide (~ 100 km) low angle (7-10 degree) locked zone and small left oblique component of relative plate motion (~8 mm/yr). The architecture of the upper plate includes a well developed fold and thrust belt in the outer part of the forearc above the locked zone (Albatross Banks - Kodiak Shelf fault zones), a mid-forearc high above the transition zone (Kodiak Archipelago), and a large forearc basin adjacent to the arc (Shelikof Strait). Late Quaternary uplift of the Kodiak Islands has resulted in the formation of a widespread raised coastal terrace interpreted to have been cut during oxygen isotope stage 5e (133-120 ka). The elevation of the terrace increases gradually from about 15 m on the northwest side to about 40 m on the southeast side of the islands, reflecting long term uplift rates of about 0.15 mm/yr (west side) to 0.3 mm/yr (east side). However, at Narrow Cape, located near the zero isobase of the 1964 earthquake, the elevation of the terrace increases abruptly to ~280 m. Narrow Cape is traversed by at least four active faults (post-glacial, ~14 ka) including the Narrow Cape fault, one of principal structures in the arc-side margin of the fold and thrust belt. At the Cape the late Pleistocene coastal terrace is displaced by the Narrow Cape fault vertically ~180 m (vertical slip rate ~ 1.5 mm/yr) and post-glacial fluvial channels crossing the fault are offset left laterally as much as 30 m (horizontal slip rate ~2 mm/yr). Trenches across post-glacial scarps on the Claudia, Bison, and Bear Paw faults at the Cape show at least four post-glacial episodes of high angle left lateral and left normal oblique slip, the most recent less than about 3 ka. Holocene slip rates for these faults derived from offset tephras and soils exposed in the trenches range from ~0.03 to ~ 0.1 mm/yr. We interpret the faulting at the Cape to reflect deformation at a large releasing stepover on the Narrow Cape fault. The Narrow Cape fault and nearby Kodiak Island fault are major active predominately strike slip structures in the arc-side margin of the fold and thrust belt. These faults appear to accommodate much of the lateral component of subduction and constitute sources of large strike-slip earthquakes in the forearc.