In mountainous environments, quantifying the drivers of mass-wasting is fundamental for understanding landscape evolution and improving hazard management. Here, we quantify the magnitudes of mass-wasting caused by the Asia Summer Monsoon, extreme rainfall, and earthquakes in the Nepal Himalaya. Using a newly compiled 30-year mass-wasting inventory, we establish empirical relationships between monsoon-triggered mass-wasting and monsoon precipitation, before quantifying how other mass-wasting drivers perturb this relationship. We find that perturbations up to 5 times greater than that expected from the monsoon alone are caused by rainfall events with 5-to-30-year return periods and short-term (< 2 year) earthquake-induced landscape preconditioning. In 2015, the landscape preconditioning is strongly controlled by the topographic signature of the Gorkha earthquake, whereby high Peak Ground Accelerations coincident with high excess topography (rock volume above a landscape threshold angle) amplifies landscape damage. Furthermore, earlier earthquakes in 1934, 1988 and 2011 are not found to influence 2015 mass-wasting.

The Eastern Mediterranean is one of the most seismically active regions in Europe. Crete, located in the centre of the Eastern Mediterranean, should experience tsunamis resulting from large magnitude earthquakes or volcanic eruptions. At three locations, boulders were observed that may relate to tsunami or storm events. At Lakki, the size of the boulders slightly favours a tsunami origin for deposition. By contrast, at Kommos, boulder size and geomorphology are consistent with storm parameters in the Mediterranean. The most compelling evidence for tsunami transport is found at Diplomo Petris, where a lithologically varied grouping of large boulders (≤690 t) is exposed at sea level. The calculated storm wave heights (15 m) required to transport the observed boulders significantly exceeds winter averages: therefore, these accumulations are interpreted as tsunami deposits. Radiocarbon dating of encrusting biological material was undertaken to constrain periods of boulder motion. Encrustations from Diplomo Petris and Lakki predate the 365 CE earthquake, suggesting that this event transported the largest boulders; the first time that boulder deposits have been identified on Crete from this tsunami. Therefore, these data are important for developing local and regional hazard assessments but also to inform numerical models of tsunami propagation in the Mediterranean.

The 25th April 2015 M7.6 Gorkha earthquake caused significant damage to buildings and infrastructure in both Kathmandu and surrounding areas as well as triggering numerous, large landslides. This resulted in the loss of approximately 8600 lives. In order to learn how the impact of such events can be reduced on communities both in Nepal and elsewhere, the Earthquake Engineering Field Investigation Team (EEFIT) reconnaissance mission was undertaken, aiming to look at damage patterns within the country. Passive, microtremor recordings in severely damaged areas of the Kathmandu Valley, as well as at the main seismic recording station in Kathmandu (USGS station KATNP) are used to determined preliminary shear wave velocity (Vs) profiles for each site. These profiles are converted into spectral acceleration using the input motion of the Gorkha earthquake. The results are limited, but show clear site amplification within the Siddhitol Region. The resulting ground motions exceed the design levels from the Nepalese Building Codes, indicating the need for site-specific hazard analysis and for revision of the building code to address the effect of site amplification

Thousands of landslides occurred during the April 2015 Gorkha earthquake in Nepal. Previous work using satellite imagery mapped nearly 25,000 coseismic landslides. In this study, the satellite-based mapping was analyzed in three areas where field deployment was also conducted—the Budhi Gandaki, Trishuli, and Indrawati river valleys—to better characterize the landslides. Unmanned aerial vehicles (UAVs) were deployed to map the three-dimensional (3-D) geometry of failed slopes using photogrammetry, as well as to characterize rock structure and strength. The majority of landslides were rock slides along the ridges and the steeper portions of the basins primarily involving the weathered rock zone. Additional landslides included rock falls and soil failures. Satellite imagery analysis indicated that landsliding was concentrated north of the physiographic transition, in steep areas, and in close proximity to the major rivers. The Trishuli area experienced the lowest landslide density in terms of number of landslides compared to the Budhi Gandaki and Indrawati areas, although all three areas had similar density in terms of total landslide area and other landslide statistics.

Abstract. Boulder movement can be observed not only in rockfall activity, but also in association with other landslide types such as rockslides, soil slides in colluvium originating from previous rockslides, and debris flows. Large boulders pose a direct threat to life and key infrastructure in terms of amplifying landslide and flood hazards as they move from the slopes to the river network. Despite the hazard they pose, boulders have not been directly targeted as a mean to detect landslide movement or used in dedicated early warning systems. We use an innovative monitoring system to observe boulder movement occurring in different geomorphological settings before reaching the river system. Our study focuses on an area in the upper Bhote Koshi catchment northeast of Kathmandu, where the Araniko highway is subjected to periodic landsliding and floods during the monsoons and was heavily affected by coseismic landslides during the 2015 Gorkha earthquake. In the area, damage by boulders to properties, roads, and other key infrastructure, such as hydropower plants, is observed every year. We embedded trackers in 23 boulders spread between a landslide body and two debris flow channels before the monsoon season of 2019. The trackers, equipped with accelerometers, can detect small angular changes in the orientation of boulders and large forces acting on them. The data can be transmitted in real time via a long-range wide-area network (LoRaWAN®) gateway to a server. Nine of the tagged boulders registered patterns in the accelerometer data compatible with downslope movements. Of these, six lying within the landslide body show small angular changes, indicating a reactivation during the rainfall period and a movement of the landslide mass. Three boulders located in a debris flow channel show sharp changes in orientation, likely corresponding to larger free movements and sudden rotations. This study highlights the fact that this innovative, cost-effective technology can be used to monitor boulders in hazard-prone sites by identifying the onset of potentially hazardous movement in real time and may thus establish the basis for early warning systems, particularly in developing countries where expensive hazard mitigation strategies may be unfeasible.