Toward Using Seismic Interferometry to Quantify Landscape Mechanical Variations after Earthquakes

Abstract

In mountainous terrain, large earthquakes often cause widespread coseismic landsliding as well as hydrological and hydrogeological disturbances. A subsequent transient phase with high landslide rates has also been reported for several earthquakes. Separately, subsurface seismic velocities are frequently observed to drop coseismically and subsequently recover. Consistent with various laboratory work, we hypothesize that the seismic-velocity changes track coseismic damage and progressive recovery of landscape substrate, which modulate landslide hazard and hydrogeological processes, on timescales of months to years. To test this, we analyze the near-surface seismic-velocity variations, obtained with single-station high-frequency (0.5–4 Hz) passive image interferometry, in the epicentral zones of four shallow earthquakes, for which constraints on landslide susceptibility through time exist. In the case of the 1999 Chi-Chi earthquake, detailed landslide mapping allows us to accurately constrain an exponential recovery of landslide susceptibility with a relaxation timescale of about 1 yr, similar to the pattern of recovery of seismic velocities. The 2004 Niigata, 2008 Iwate, and 2015 Gorkha earthquakes have less-resolved constraints on landsliding, but, assuming an exponential recovery, we also find matching relaxation timescales, from ∼0.1 to ∼0.6 yr, for the landslide and seismic recoveries. These observations support our hypothesis and suggest that systematic monitoring of seismic velocities after large earthquakes may help constrain and manage the evolution of landslide hazard in epicentral areas. To achieve this goal, we end by discussing several ways to improve the link between seismic velocity and landscape mechanical properties, specifically by better constraining time-dependent near-surface strength and hydrogeological changes. Hillslopes displaying coseismic surface fissuring and displacement may be an important target for future geotechnical analysis and coupled to passive geophysical investigations.

Seismic Velocity Recovery in the Subsurface: Transient Damage and Groundwater Drainage Following the 2015 Gorkha Earthquake, Nepal

“…We did not correct for this event or any aftershocks occurring between the 25 April 2015 and the 6 June 2015. Nevertheless, we predict that most of the NLME effects are contained within the first approximate year ( R 155 and R 250 , Figure 8), a value consistent with the inferred recovery of landslide rates in the Bhote Koshi (∼1 yr; Marc et al., 2021). If we assume that our inferred δv estimated at rather high frequency (4–8 Hz) is a good proxy for shallow subsurface damage, this comparison with landsliding shows that our model is realistic and does not support a longer effect for NLME, such as inferred on model R 846 (Figure 8).…”

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Following the passage of seismic waves, a wide range of transient effects have been observed near the Earth's surface, including increased landslide rates (Marc et al, 2015), enhanced permeability (Manga et al, 2012;Xue et al, 2013), and perturbations of frictional properties in fault zones (Pei et al, 2019). These observations suggest that earthquakes induce a lingering effect in the properties of near-surface rocks that may be linked to non-linear mesoscopic elasticity (NLME, e.g., Gassenmeier et al, 2016;Marc et al, 2021). This phenomenon is generally expressed by a drop in elastic moduli after a dynamic or static strain perturbation that is followed by a non-instantaneous recovery of these moduli.

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Near‐Surface Geomechanical Properties and Weathering Characteristics Across a Tectonic and Climatic Gradient in the Central Nepal Himalaya

“…Although these mechanisms are not mutually exclusive, we suggest that topographic stress‐induced fractures are likely to be particularly relevant in the Himalaya, not only due to the dramatic scale of the topography but also because of the strong regional compressive stress which has been previously shown to determine the extent to which ridges are more‐weathered than channels (Moon et al., 2017; Slim et al., 2015; St. Clair et al., 2015). Another factor that might contribute to topographic‐dependent bedrock fractures is topographic‐amplification of seismic rock damage induced during the recent 2015 Gorkha Earthquake (e.g., Huang et al., 2019), although we note that previously documented reductions in shallow seismic velocity following large earthquakes are generally less than 5%, which is far less than the factor of ∼2 difference we observe in the median V s profiles for ridge versus channel sites (Hobiger et al., 2016; Marc et al., 2021; Takagi et al., 2012). The specific mechanism notwithstanding, the strength of the correlation between the degree of weathering and proximity to ridges, channels, and hillslopes is an important result because it suggests that the geomorphic control on chemical and physical bedrock weathering dominates over other environmental variables.…”

“…Clair et al, 2015). Another factor that might contribute to topographic-dependent bedrock fractures is topographic-amplification of seismic rock damage induced during the recent 2015 Gorkha Earthquake (e.g., Huang et al, 2019), although we note that previously documented reductions in shallow seismic velocity following large earthquakes are generally less than 5%, which is far less than the factor of ∼2 difference we observe in the median V s profiles for ridge versus channel sites (Hobiger et al, 2016;Marc et al, 2021;Takagi et al, 2012). The specific mechanism notwithstanding, the strength of the correlation between the degree of weathering and proximity to ridges, channels, and hillslopes is an important result because it suggests that the geomorphic control on chemical and physical bedrock weathering dominates over other environmental variables.…”

Controls on Post‐Seismic Landslide Behavior in Brittle Rocks

“…The mechanisms responsible for this temporal pattern are poorly constrained (Rosser et al., 2021). Transient, elevated (i.e., above rainfall‐normalized baseline conditions) rates of post‐seismic landsliding are not ostensibly controlled by external seismic or meteorological forcing (Marc et al., 2015) and have instead been attributed to a combination of erosion of regolith weakened by earthquake ground shaking (Fan et al., 2018; Kincey et al., 2021; Lin et al., 2008; Wang et al., 2015), and/or recovery of hillslope strength in the post‐seismic phase following initial disturbance and weakening during ground shaking (Leshchinsky et al., 2020; Marc et al., 2015, 2021). The mechanisms responsible for the latter are poorly constrained but have been postulated to result from a range of “healing” processes that include the re‐establishment of plant‐root cohesion (e.g., Jacoby, 1997; Yunus et al., 2020) and the reversal of dilation experienced during an earthquake as rock and soil masses settle and re‐establish frictional contacts (e.g., Lawrence et al., 2009).…”