By 2050, 68% of people are projected to live in urban areas. As cities grow into steeper terrain, residents are increasingly exposed to hazards like rainfall-triggered landslides. However, despite thousands of yearly fatalities, few cities have established early warning systems to reduce weather-related landslide risk. Rainfall thresholds that identify landslide triggering conditions are a key component of early warning systems, but limited landslide inventory data has hindered efforts to determine thresholds for many cities. Furthermore, the magnitude and variability of thresholds in and between urban areas worldwide has yet to be quantified, leaving cities without inventories few options to learn from others. We compiled 1216 urban landslides to estimate rainfall intensity-duration thresholds for 26 cities worldwide and a global threshold for urban landslides with a multi-level regression model. We find that landslides were triggered under surprisingly similar rainfall conditions in most cities despite widely varying climates, topographies, and income classes. In 77% of cities, the median threshold is indistinguishable from the global average. We show that urban landslides occurred at lower threshold intensities than previously reported for multiple land-use types, while 31% of landslides were triggered below annual rainfall maxima. Our results suggest that anthropogenic hillslope modification and malfunctioning infrastructure facilitate slope failures in cities. We argue that urbanization harmonizes rainfall thresholds between cities, overprinting natural variability, and offer a baseline for warning in cities with sparse landslide records.

Landslides are among the most recognizable evidence of hillslope erosion in tectonically active mountains. Yet, how much of the distribution of landslides of different ages relates to, or inherits from, the pattern of topographic metrics of landscape evolution remains partially unresolved, and especially so in tropical areas. We derive such metrics for 650 catchments, including their mean hypsometric integral, local relief, geological lineament, density, and stepness variations and knickpoint density of river channels as proxies of tectonic activity; we test how these proxies match with, if not explain, the distribution of some 14,000 prehistoric and modern landslides in the Colombian Andes. A $K$-means cluster analysis of catchment hypsometry reveals four distinct groups of catchments. We interpret these groups to reflect different states of transience with clear contrasts in mean local relief, average hillslope inclination, channel steepness, and landslide density. We propose that tectonic uplift, base-level changes, and passing waves of incision control these different states of transience. Yet, we find that landslides occur widely without much spatial association to, or amassing near, major channel knickpoints. This observation reflects what we would expect from a threshold landscape in which landslides abound irrespective of contrasts in local river incision rates. Still, we notice a pronounced attraction of landslides to transient divides, where especially prehistoric landslides are preferentially preserved. We infer that, in our study area at least, differences in catchment hypsometry might be more useful to track potential tectonic controls on landslide patterns than comparing these to knickpoint distributions or channel metrics.

Moderate to large earthquakes can increase the amount of water feeding stream flows, raise groundwater levels, and thus grant plant roots more access to water in water-limited environments. We examine tree growth and photosynthetic responses to the Maule Mw 8.8 Earthquake in small headwater catchments of Chile’s Mediterranean Coastal Range. We combine high-resolution wood anatomic (lumen area) and biogeochemical ( of wood cellulose) proxies of daily to weekly tree growth on cores sampled from trees on floodplains and close to ridge lines. We find that, immediately after the earthquake, at least two out of six tree cores show changes in these proxies: lumen area increased and decreased in the valley trees, whereas the sign of change was reversed in trees on the hillslope. Our results indicate a control of soil water on this response, largely consistent with models that predict how enhanced post-seismic vertical soil permeability causes groundwater levels to rise on the valley floor, but fall along the ridges. Statistical analysis with boosted regression trees indicates that streamflow discharge gained predictive importance for photosynthetic activity on the ridges but lost importance on the valley floor after the earthquake. We infer that earthquakes may stimulate ecohydrological conditions favoring tree growth over days to weeks by triggering stomatal opening. The weak and short-lived signals that we identified, however, show that such responses are only valid under water-limited instead of energy-limited tree growth. Hence, dendrochronological studies targeted at annual resolution may overlook some earthquake effects on tree vitality.

Dense tree stands and high wind speeds characterize the dense temperate rainforests of southern Chilean Patagonia, where landslides frequently strip hillslopes of soils, rock, and biomass. Assuming that wind loads on trees promote slope instability, we explore the role of forest cover and wind speed in predicting mapped landslides with a robust Bayesian logistic regression. We find that more crown openness and higher wind speeds credibly predict higher probabilities of detecting landslides moderately well regardless of topographic location, though much better in low-order channels and on midslope locations than on open slopes. Wind speed has less predictive power in areas that were smothered by tephra fall from recent volcanic eruptions, while the influence of forest cover remains.