ML20151P748

Seismic Potential of Gorda Segment of Cascadia Subduction Zone, Presented at 880506-08 Meeting in Seattle,Wa

ML20151P748
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Site:	Trojan File:Portland General Electric icon.png
Issue date:	05/06/1988
From:	Carver G HUMBOLDT STATE UNIV., ARCATA, CA
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References
NUDOCS 8808100175
Download: ML20151P748 (1)

Text

6 Seismic Potential of the Gorda Segment of the Cascadia Subduction Zone Gary A. Carver, Humboldt State University, Arcata, CA 95521

., Assessing the seismic potential of the Cascadia subduction zone is especially difficult because the plate boundary lies well offshore along most of its length. Acoustic profiles of the sea floor across the subduction zone reveal a complex system of east dipping thrust faults and associated folds comprise a prominent fold and thrust belt parallel landward of the shallow dipping mega thrust. At its southern end the subduction zone converges with the northern California coast and the mega-thrust approaches within 25 km of Cape Mendocino. North of the Cape the fold and thru .t belt extends onshore where it can be studied directly.

Jeformation of Pleistocene and Holocene sediments and marine te: races in northern California records northeast-southwest contraction ,

across the 75 km wide portion of the fold and thrust belt exposed on (and.

l The contraction is represented by the growth of 15 major thrust faults and l

folds, and has averaged at least 20 25 mm/yr during the late Quaternary.

The contraction accommodates a large part of the Gorda North American l otate convergence and is interpreted to reflect strong coupling across the upper 100-120 km wide portion of the plate boundary.

Field relations indicating sudden displacement on the thrust faults and jerky growth of the associated folds demonstrate that the deformation has been coseismic. Paleoseismic investigations of the Little Salmon thrust show a late Holocene history characterized by individual slip events of at least 5 meters, and possibly 10 meters, at 400 to 600 year intervals; the most recent event occurred less than 415 C14 yrs B P. Carbon-14 dates from offset horizons in trenches across traces of the McKinleyville and Mad River faults show offsets of 3 to 4 meters recurring at intervals of a few thousand years. Raised marine terraces and associated terrace cover sediments at McKinleyville indicate two jerk uplift events have effected this portion of the coast in the last 1170 C1 yrs, and buried saltmarsh peats record repeated episodes of late Holocene jerky subsidence where the axis of the Freshwater syncline intersects the margins of Humboldt bay. Growth ring pattems reflecting damage to trees on landslides along the Little Salmon thrust and buried trees on the raised Holocene terrace at Mckinleyville support an age estimate of about 300 yrs for the most recent subduction event.

In tctal, the geologic evidence s~trongly indicates that the Gorda segment of the Cascadia subduction zone generates very large earthquakes.

These earthquakes are accompanied by thrusting and folding in the accretionary prism, and may involve simultaneous coseismic grcwth of multiple faults and folds. Preliminary assessment of the recurrence interval and time since the last event suggest the Gorda segment of the subduction zone may be approaching the end of the present seismic cycle.

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