In 2017, Hurricane Maria devastated islands throughout the Caribbean, in particular, much of Puerto Rico and Dominica, causing thousands of landslides that damaged infrastructure and blocked key road networks needed to provide aid and supplies to more remote communities. Also in 2017, the Mocoa landslide in Colombia, triggered by local heavy rains, resulted in 329 fatalities. The 2016 Mw7.8 Kaikoura, New Zealand earthquake triggered widespread landsliding and resulted in key roads being blocked for over a year with significant impact to the local economy from loss of tourism (Figure 1). The key decision making needs articulated directly from local stakeholders to response agencies and the scientific community were essentially the same: Where are the landslides? What are the impacts (exposure and risk)? What might we expect after the triggering event (e.g. cascading hazards and risk)? How can we build resilience to these hazards in support of disaster risk reduction?

This project supports key decision-making and resilience-building capabilities at a range of spatial and temporal scales that are directly relevant to our stakeholder decision making and response needs. Specifically, the proposed work supports both real-time situational awareness of potential landslide activity and impacts at a global scale and scenario-based regional assessment of cascading landslide hazards resulting from extreme weather and earthquakes. This work aligns with key knowledge gaps within the decision support structures of our stakeholder communities.

The collisional boundaries between Earth’s tectonic plates produce dramatic mountain ranges that generate destructive natural hazards as well as chemical and physical exchanges that control the surface

environment. Major mountain belts alter local and regional climate, act as “water towers” supplying freshwater to much of the world’s population, and are hotspots of erosion and CO2-consuming

weathering reactions. Major questions remain to be answered about these processes and the links between them, and thus about the role of active orogens in the Earth’s surface system. This proposal is for a multi-

disciplinary investigation framed around understanding four outstanding problems: (1) how rock is transformed physically and chemically during exhumation in tectonically active mountains, (2) how the steep mountainous topography of active orogens develops, and particularly what the role is for landslides, (3) how water makes its way from precipitation, through the subsurface, and into streams and rivers

draining active orogens, and (4) how chemical weathering depends on tectonic and climatic conditions in these settings.

Landslides triggered by large earthquakes pose both immediate and prolonged hazards at the same time that present, however, opportunities to understand how seismicity contributes to erosion and associated landscape evolution. The Mw 7.8 earthquake that occurred in April 2015 in central Nepal offers a particular opportunity to understand seismically-triggered landslides and their effects over a range of timescales. Gradients in landslide occurrence across a well-characterized portion of central Nepal have allowed us to frame questions and hypotheses of wide interest to the geosciences. One major goal of the project is better constrain strong ground motions from this poorly-instrumented event. Here we aim to integrate landslide distributions with a novel method to estimate seismic strong ground motions from specific types of failures in alluvial terraces. A second key goal of the project is to evaluate variability in rock strength in a tectonically active orogen, and specifically, to understand the interplay between physical and chemical weathering processes in the evolution of rock strength at hillslope scales. We know that rock strength decreases over orders of magnitude from intact, unweathered rock to transportable soil. However, we lack generalized models of rock weakening on which regionally-based landscape evolution processes can be based. In this project, we aim to capitalize on the large forcing event provided by the earthquake, which instantaneously produced tens of thousands of slope failure examples that illuminate rock properties. Targeted field studies complement our regional interpretations of these failures

The broad research questions posed by this project address when, why and how landslides occur; the nature of seismic energy release during major earthquakes on continental thrust faults; and how chemical weathering and physical denudation are related. This project also includes scientific results that will be of societal significance as outcomes will improve both our understanding of seismic ground shaking in poorly instrumented, high relief terrain, and our capacity to predict landsliding caused by earthquakes, as well as “follow-on” hazards such as debris flows. This research supports, fully or partially, several PhD students, several undergraduate student researchers, and three faculty members, and it fosters domestic and international collaboration with partners at the USGS, in Nepal and in Europe. In addition, novel curriculum development will include a virtual reality landslide field trip developed with video footage collected during fieldwork.

The geomechanical properties of geologically young, clastic sedimentary rocks are highly sensitive to burial histories up to 5 km because of the physical changes to porosity and cementation that accompany compaction and diagenetic changes to the rock mass. Because young sedimentary rocks are a common rock type in active tectonic environments, a generalized model that incorporates burial history into estimates for rock strength will significantly advance co-seismic landslide hazard models. We propose a multi-scale investigation of a stratigraphic sequence in southern California on which a generalized model can be proposed. These results will be applied to scenario predictions for cosesimic landsliding based on strong ground motion for active faults in southern California, thereby addressing the NEHRP priorities by reducing co-seismic landslide losses.