Geologic Hazards Science Center U.S. Geological Survey Golden, Colorado Landslides triggered by the 2018 Anchorage Earthquake 2019 Symposium on the 2018 M7.1 Anchorage Earthquake Randall W. Jibson ([email protected]) Rock falls and rock slides Slump/earth flows 1964 translational landslides Abstract

Investigation of landslides triggered by the 2018 Mw 7.1 Anchorage earthquake showed that landslides and ground cracking caused significant damage. Shallow rock falls and slides were the most abun- dant types of landslides, but they occurred in smaller numbers than global models that are based on earthquake magnitude predict; this might result from the 2018 earthquake being an intraslab event. Ground cracking, possibly related to incipient landsliding, was pervasive near the edges of slopes Ÿ The 1964 Alaska earthquake (Mw 9.2) in hilly areas and caused perhaps the most signifi- triggered several deep, translational cant property damage of all types of ground failure. A complex of slump/earth flows was triggered along block slides in Anchorage. Ÿ coastal bluffs in southern Anchorage where slides Slumps that transitioned into earth flows Ÿ Small cracks were observed after the also occurred in 1964; the 2018 slides involved both were small in number but were very dam- 2018 earthquake along pre-existing fail- mobilization of new landside material and reactiva- aging. tion of parts of the 1964 landslide deposits. Large, ure planes from the 1964 movement at translational slides that formed during the 1964 Ÿ Road-fill failures at Vine Road in Wasilla Government Hill (photograph at upper Alaska earthquake showed evidence of deformation Ÿ Shallow, disrupted falls and slides in rock (photograph at upper right) and at the In- left) and Turnagain Heights (photograph along preexisting failure surfaces but did not reacti- vate with new net downslope displacement. Model- and debris were the most numerous types ternational Airport Road disrupted traffic. at upper right). of landsides triggered, as is common in ing suggests that ground motion in 2018 was of in- Ÿ Failures along the railroad grade northeast Ÿ No net downslope movement of the sufficient duration and too high frequency to trigger most earthquakes. reactivation of the deep landslides. of Anchorage (photograph at upper left) dis- slides occurred: they responded to the Ÿ These landslides were sparsely scattered rupted rail service in several areas. shaking but did not fully reactivate. throughout the western Chugach Mountains Conclusions Ÿ Near Potter Hill in south Anchorage, land- and along coastal bluffs rimming Turnagain Ÿ The 2018 earthquake produced less ground failure slides along the bluff occurred in the same Ground cracks than would be predicted by present models that and Knik Arms of Cook Inlet. location where large landslides occurred in depend on magnitude alone. Other factors such as Ÿ the deep focus and normal mechanism probably Slides were perhaps an order of magnitude the 1964 earthquake (photographs at bot- contributed to the relative paucity of ground failure. less numerous than global magnitude- tom left and right). based models would predict. This could be Ÿ Landslides triggered by the 1964 Alaska earth- Ÿ The Potter Hill landslides involved reacti- quake responded to the 2018 shaking but did not the result of this being an intraslab earth- vation of parts of the 1964 deposits as well reactivate. The duration and period of ground mo- quake. tion in 1964 were much longer and are the primary as failure of fresh bluff material. drivers of movement of these large landslides. Ÿ The largest known rock slide detached from Ÿ A few complex slump/earth flows in highway fill Rainbow Peak above the Seward Highway and glacial-marine sediment produced long-runout along Turnagain Arm (upper left photo- landslides and significant damage to roads and rail graph). The slide covered an area of about Ÿ Ground cracks that suggest incipient lines. Near Rabbit Creek, slides occurred in the 50,000 m2 and traveled 700 m from its same location as the Potter Hill landslides that landsliding and topographic amplifica- were triggered by shaking in the 1964 earthquake. source. tion near slope edges were widespread. Ÿ Ÿ Much of the damage and continuing hazard re- Rock falls temporarily closed the Seward Ÿ Cracks on the outboard edges of build- lates to the extensional cracks and possible incipi- Highway along Turnagain Arm in a few ing sites caused extensive damage and ent landslides on and near steep slopes in Peters places but otherwise did little damage. Creek, Eagle River, and southern Anchorage. pose continuing hazards.

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E E O V L R earthquake.usgs.gov O U M 9 U.S. Geological Survey G S A Earthquake Hazards Program RCH 9, 184 I C A L earthquake.usgs.gov