Controls on Post‐Seismic Landslide Behavior in Brittle Rocks. Issue 9 (16th September 2021)

• Record Type: Journal Article
• Authors: Brain, Matthew J.; Moya, Sebastian; Kincey, Mark E.; Tunstall, Neil; Petley, David N.; Sepúlveda, Sergio A.
• Is Part Of: Journal of geophysical research. Volume 126:Issue 9(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-09-16
• Subjects: landslide -- earthquake -- post‐seismic -- geotechnical testing -- path dependence -- dynamic loading; Geomorphology -- Periodicals; 551.3
• DOI: 10.1029/2021JF006242
• Languages: English
• ISSNs: 2169-9003

Abstract

Abstract: Earthquakes trigger widespread landsliding in tectonically active landscapes. The effects of strong ground shaking on hillslope stability persist into the post‐seismic stage; rates of landsliding remain elevated in the years following an earthquake. The mechanisms that control the spatial pattern and rate of ongoing landsliding are poorly constrained, hindering our ability to reliably forecast how landscapes and landslide hazard evolve. To address this, we undertook a detailed geotechnical investigation in which we subjected representative rock samples to dynamic loading, simulating the effects of earthquake ground shaking on hillslopes of different configuration. Our results indicate that post‐seismic hillslope strength is not an intrinsic rock property; rather, it responds to the amplitude of imposed dynamic loads and the degree of pre‐existing shear surface formation within the rock. This path‐dependent behavior results from differences in the character of fractures generated by dynamic loads of different amplitude, and the ways in which apertures are mobilized or degraded in subsequent (post‐seismic) shearing. Sensitivity to dynamic loading amplitude is greater in shallow landslides in which shear surfaces are yet to fully form; such hillslopes can be strengthened or weakened by earthquake events, depending on their characteristics. In contrast, deeper landslides on steeper hillslopes in which shear surfaces have largely developed are less likely to display differences in behavior in response to dynamic loading because strain accumulation along pre‐existing fractures is dominant. Our results demonstrate the need to consider path‐dependent hillslope stability in numerical models used to forecast how landscapes respond to earthquakes and how post‐seismic hazard evolves. Plain Language Summary: Landsliding is more common in the months and years after an earthquake. Our understanding of why this happens is limited but likely results from earthquake weakening of hillslopes. The mechanisms causing this weakening are difficult to assess at the landscape scale using satellite imagery. An alternative approach to improve our understanding of the controls on rock strength after an earthquake is to subject rock samples to seismic shaking in laboratory conditions. We used a custom‐built apparatus to subject rock samples to pressure conditions typical of those experienced in shallow and deep‐seated landslides. We simulated ground shaking of differing intensity to assess whether the rock was weaker or stronger after the shaking stopped. In simulated shallow landslides, ground shaking intensity affected how the sample cracked. Less intense shaking weakened the rock, implying it is more likely to fail following an earthquake. Conversely, more intense ground shaking strengthened the rock by creating a very uneven sliding surface, suggesting it is less likely to subsequently fail. Simulated deeper landslides on steeper hillslopes did not weaken, displaying less sensitivity to ground‐shaking intensity. Our findings improve understanding of how hillslopes respond to seismic shaking, assisting forecasts of how landscapes and hazard develop after an earthquake. Key Points: We used novel geotechnical testing to explore mechanisms that control post‐seismic hillslope strength and rheology in brittle rocks The amplitude of dynamic loading can govern the nature of fractures and asperities formed, affecting subsequent shear strength and rheology The shear deformation mechanism mobilized determines whether post‐seismic shear strength increases, decreases or remains unchanged