Residual shear strength variability as a primary control on movement of landslides reactivated by earthquake‐induced ground motion: Implications for coastal Oregon, U.S.

Abstract

Most large seismogenic landslides are reactivations of preexisting landslides with basal shear zones in the residual strength condition. Residual shear strength often varies during rapid displacement, but the response of residual shear zones to seismic loading is largely unknown. We used a ring shear apparatus to perform simulated seismic loading tests, constant displacement rate tests, and tests during which shear stress was gradually varied on specimens from two landslides to improve understanding of coseismic landslide reactivation and to identify shear strength models valid for slow gravitational failure through rapid coseismic failure. The landslides we studied represent many along the Oregon, U.S., coast. Seismic loading tests resulted in (1) catastrophic failure involving unbounded displacement when stresses represented those for the existing landslides and (2) limited to unbounded displacement when stresses represented those for hypothetical dormant landslides, suggesting that coseismic landslide reactivation may be significant during future great earthquakes occurring near the Oregon Coast. Constant displacement rate tests indicated that shear strength decreased exponentially during the first few decimeters of displacement but increased logarithmically with increasing displacement rate when sheared at 0.001 cm s À1 or greater. Dynamic shear resistance estimated from shear strength models correlated well with stresses observed during seismic loading tests, indicating that displacement rate and amount primarily controlled failure characteristics. We developed a stress-based approach to estimate coseismic landslide displacement that utilizes the variable shear strength model. The approach produced results that compared favorably to observations made during seismic loading tests, indicating its utility for application to landslides.

Earthquake-driven acceleration of slow-moving landslides in the Colca valley, Peru, detected from Pléiades images

“…Understanding the mechanics of landsliding under seismic forcing is thus a major issue. Several processes have been pointed out which could explain the mechanics of co-seismic landslides, including variations in the frictional properties during and after the shaking (Lacroix, Perfettini, Taipe, & Guillier, 2014;Newmark, 1965;Schulz & Wang, 2014;Togo, Shimamoto, Dong, Lee, & Yang, 2014), damage to the landslide body (Dadson et al, 2004;Petley, Dunning, Rosser, & Kausar, 2006), and transient pore water pressure changes due to the shaking (Jibson, Prentice, Borissoff, Rogozhin, & Langer, 1994;Wang, Cheng, Chin, & Yu, 2001;Wasowski, Pierri, Pierri, & Capolongo, 2002). The quantification of all these mechanisms remains difficult due to the paucity of in-situ measurements of landslide triggering by earthquakes.…”

Quantifying the Sensitivity of Progressive Landslide Movements to Failure Geometry, Undercutting Processes and Hydrological Changes

“…The former two landslides are located on the central Oregon coast, while the Arizona Inn slide is on the south coast. Much of the movement observations for the first two slides (Johnson Creek and Carmel Knoll) leveraged data from inclinometer and borehole extensometer readings documented in Priest et al (), Schulz et al (), and Schulz and Wang (). According to these studies, movements and piezometric data were primarily recorded until December 2014, recorded at approximately 15‐min intervals (Schulz & Wang, ).…”

Landslides in the western Columbia Gorge, Skamania County, Washington

“…In particular, the potential for earthquake-induced ground accelerations to destabilize the terrain behind the 100-to 350-m-high headscarps of the Bonneville and Red Bluffs landslides, or to reactivate certain seasonally active or dormant landslides (Schulz and Wang, 2014), has not been investigated. With the probability of a magnitude 9 Cascadia Subduction Zone (CSZ) earthquake occurring within the next 50 years calculated at about 1 chance in 10 ( Petersen and others, 2002;Goldfinger and others, 2012), such investigations are warranted.…”