UC Santa Barbara

In active mountain ranges, hillslopes steepen towards a threshold angle beyond which landslide failures tend to occur. These hillslopes steeper than the critical angle, termed “excess topography”, are prone to landslide failures, which in turn consume the over-steepened hillslopes. However, it remains less well understood the magnitude, pattern, and controls of the loss of excess topography by landslides. Here, we investigate how earthquake-induced landslides fail excess topography to better understand how seismoectonic activities destroy over-steepened hillslopes. We quantify a loss rate (RL) scale by calculating and comparing the ratio of coseismic landslide volume to the volume of excess topography in 28 earthquake-impacted mountain ranges spanning diverse seismotectonic and climatic settings at a global scale. We find that RL is generally low but spans 3 to 4 orders of magnitude (0.001% - 3%). We then examine how RL relates to diverse seismotectonic, lithological, topographic, and climatic factors. Our results show that reverse faults generate higher amounts of RL and that regional tectonic shortening rate exerts a nonlinear control over RL. Other seismotectonic factors, including earthquake magnitude and hypocenter depth, do not reveal clear controls. RL scales positively with topographic steepness and the extent of sedimentary rocks, but RL shows a nonlinear relation with mean annual precipitation where it peaks at moderate annual precipitation and dampens under both low and high precipitation magnitudes. RL does not exhibit a clear trend with the rate of precipitation during extreme events, implying that events with higher frequencies but lower magnitudes exert a stronger control over RL than low frequency, high magnitude events. The monotonic controls are explained by the distinct correlations between the controlling factors, the volume of excess topography, and the coseismic landslide volumes. The non-linear controls likely reflect a competition between processes that precondition versus overconsume hillslopes for landslide failures. We also examine how coseismic landslides sample the landscapes in the excess topography domain and find that coseismic landslides either oversample or undersample hillslopes with the highest excess topography. We interpret the variable sampling behavior to reflect a combination of stochastic processes (e.g. random survival of unstable slopes) and deterministic forces (e.g. high rock strength that sustains high excess topography) that regulate hillslope steepening. Overall, our results provide new insights into the failures of over-steepened topography by earthquake-induced landslides, holding broad implications for landscape evolution, landslide hazards, and sediment transport in mountain systems.