Nisqually Earthquake 2002
Landslides triggered east of Puget Sound, Washington, by the Feb. 28, 2001 Nisqually Earthquake
WARNING: Provisional Report, Subject to Revision
From Rex Baum, Al Chleborad, Ed Harp, Randall Jibson, David Keefer, and Scott Miles, U.S. Geological Survey Geologic Hazards Team, Golden, CO. and U. S. Geological Survey, Menlo Park, CA, written. Report is incomplete and preliminary as of this time.
| Sites Visited 3/1/01 | Sites Visited 3/2/01 | Sites Visited 3/3/01 | Sites Visited 3/4/01 |
Sites visited 3/1/01
Cedar River
A earth slide and flow, approximately 200-300 m³, destroyed the shoulder and eastbound lane of a local road and partially blocked the Cedar River about 3-4 km east of the Elliott Bridge and the slides that blocked the Cedar River and destroyed the Van Dussen home. The earth slide appears to have formed in moderately sloping colluvial and alluvial deposits.
Sites visited 3/2/01
Snoqualmie
About 1 km northwest of Snoqualmie Falls, a 200-m³ pulse of landslide debris from a preexisting active landslide temporarily dammed Tokul Creek adjacent to a state fish hatchery. New cracks defining the head of an incipient slide formed east of the active slide and cracks in the eastbound lane of Snoqualmie-Fall City Road (State Highway 202) widened. Smaller cracks also opened along the shoulder of Highway 202 about 200 m southeast of the slide. The cracks extend for a distance of about 100 m along Highway 202. In Snoqualmie, an incipient slide formed in the south bank of the Snoqualmie River at the intersection of River St. and Park St., which was also the site of a water-main break.
Mount Si, near North Bend
USFS Rangers and local residents reported rock fall from the west face of Mount Si. The rocks reportedly broke down some trees and raised dust clouds, but evidently did not damage any property. We were able to confirm only approximate locations of the rock falls, which were near the north end of the west face of Mt. Si.
Enumclaw
A local resident reported closure of local road near Washington State Highway 410 after a slide had caused about 30 cm vertical offset in the road. We were unable to locate the site of the damaged road, which reportedly goes to a local factory or processing facility.
Highway 410, east of The Dalles Campground
About 15-25 m³ of rock fell 15-20m from a cliff onto the north shoulder of Highway 410 about 1 km east of the Dalles campground. The largest boulder measured about 3 m x 2 m x 1.5 m.
Sites visited 3/3/01
Gold Bar
Settlement and shallow sliding occurred at the east abutment of the U.S. Highway 2 bridge that crosses the Skykomish River 3 km east of Gold Bar. A pair of cracks that extend 10-15 m along the south edge of the pavement marked the head of a shallow slide in the south face of the highway embankment. The highway settled as much as 5 cm across its entire width, but settlement was greatest at the south side.
Index
About 1.5 km west of Index, cobbles showed evidence of recent disturbance in an active talus slope or rock-fall area on the south side of Highway 410. About 1 km east of Index, on the north side of Highway 410, a 5-m³ rock fall occurred in fractured rock. The fractured rock is an active rock fall area as evidenced by rock bolts and wire-mesh nets in place to mitigate rock-fall hazard.
Skykomish
Small (5-10 m³) deposits from recent rock falls were present on the north side of Highway 410 both east and west of Skykomish. The deposit to the west appears to be a few weeks old based on very slight weathering/discoloration of wood surfaces exposed in broken guardrail posts. We were unable to confirm that any of the rocks had fallen in the last few days. The deposit east of Skykomish similarly lacks any evidence that the rocks were deposited in the last few days.
Stevens Pass
Steep snow-covered slopes on the approach to Stevens Pass showed probable evidence of recent shaking in the form of miniature avalanche tracks. The avalanche tracks stopped within 0.5 km of Stevens Pass. Fresh snow (which may have fallen as recently as 3/2/01) covered most rocks at the base of the slopes along Highway 410, so it was difficult to determine how recently any of the rocks fell. A small soil fall covered snow directly downslope from it near the 3000-ft contour. Based on the avalanche tracks and small soil fall, we believe this location may be an eastern limit of earthquake induced landslides.
Marysville
A 15-m-long segment of bank failed on the east shore of Lake Nina, which is 0.5 km west of Interstate 5 and the north end of Marysville. A small dock was lost as a result of the slide. We were able to observe a scarp about 0.6 m high along the shore. Newspaper reports indicate that the slide also threatened a deck attached to a nearby home, but we were unable to enter the property to observe whether any ground cracks formed between the deck and the bank.
Woodway
The head of the 1997 Woodway landslide showed no evidence of significant cracking or movement. A small soil fall (<2 m³) occurred near the top of the scarp in the southern one third of the slide.
Sites visited 3/4/01
Ohop Valley
Several small, shallow soil slides, (<1-2 m³ each) and a 5-m³ rock fall occurred in road cuts of Washington State Highway 7 along the west wall of Ohop Valley. The rock fall source was an area of fractured rock on the valley wall.
LaGrande
An 8-m³ rock fall on a steep road cut about 1 km east of LaGrande on Highway 7. The source area was 5 m above road level in fractured rock. The deposit contained blocks up to 1 m across.
Glenoma
On State Highway 12, east of Morton Scars of two shallow soil slides appear expose bare soil on a steep bluff above Rainy Creek. The scars are 6-12 m high and about 5 m wide. We were able to view the scars only from the highway so we could not determine how recently material had slid at either site. Of the two scars, the westernmost one looked fresher. The one to the east, which we photographed, appeared to have a small amount of grass growing on it. The grass was brown and probably represents last summer's growth.
Randle
Deposits from a shallow landslide were present on a steep slope 0.5 km west of Randle. The 2-m wide scar was about 4 m above the road. The slide occurred in a thin layer of soil on a basalt slope. A large rock fall on the south side Cline Road, a few km southeast of Randle, broke the top out of a tree and skinned the bark off several others. The largest intact rock at the foot of the slope was a block of basalt about 2 m x 1.5 m x 1 m. Gouge marks in the soil, exposed wood from the broken tree and other signs of disturbance indicated that the rock fall was only a few days old and likely caused by the February 28 earthquake. The source was on a 60-m-high basalt cliff, but trees and shrubs obscured the exact location where the rock detached.
