The i-FSC proxy for predicting near-source topographic site effects and studying earthquake-induced landslide distributions

During earthquakes, a high degree of spatial variation in damage distribution, encompassing both structural damage to buildings and co-seismic landslides, is commonly observed in mountainous regions near the seismic source. Among other factors, this spatial variability can be partly attributed to the amplification of seismic waves caused by surface topography. Our study focuses on predicting ground-motion amplification due to topography in close proximity to earthquakes and examining its potential influence on co-seismic landslide distribution patterns.

To achieve this goal, we employ neural network analysis on previously available synthetic data from 3D finite-differences simulations of seismic wave propagation. The analysis aims at developing a physics-based estimator of topographic site effects in close distances to the source, referred to as the i-FSC proxy (Illuminated Frequency Scaled Curvature). This proxy depends on the S-wavelength, the curvature of the topographic surface, and a new parameter called the "normalized seismic illumination angle", which quantifies the slope's exposure to the incoming wavefield. The inclusion of the illumination parameter substantially decreases the uncertainties of the proxy by a factor of 2 compared to estimators that rely solely on curvature as a key parameter. The i-FSC proxy is a user-friendly tool that does not require high computational resources; it utilizes only a digital elevation map and the position of the seismic source to predict amplification factors at any point on the surface topography. This estimator allows exploring the spatial variations in topographic amplification caused by nearby seismic sources, representing a significant breakthrough as areas closest to the fault typically sustain the most damage during earthquakes.

Subsequently, the i-FSC proxy is used to investigate the correlation between ground-motion amplification and the spatial distribution of earthquake-induced landslides triggered by large events such as the 2015 Gorkha earthquake (M_W 7.8). The results indicate that more than 71% of co-seismic landslides tend to be localized in amplified areas. Different physical controls on the landslide triggering at different frequencies have been identified. The results also highlight the crucial importance of considering the effect of topographic amplification, together with other classical factors such as slope steepness, for a better understanding of the complex mechanisms governing the spatial distribution of earthquake-induced landslides at local and regional scales. The obtained results could provide valuable insights for future researches, guiding efforts towards more effective risk assessment and mitigation strategies in mountainous regions.