Tom R. Robinson

Regional coseismic landslide hazard assessment without historical landslide inventories: A new approach

Currently, regional coseismic landslide hazard analyses require comprehensive historical landslide inventories as well as detailed geotechnical data. Consequently, such analyses have not been possible where these data are not available. A new approach is proposed herein to assess coseismic landslide hazard at regional scale for specific earthquake scenarios in areas without historical landslide inventories. The proposed model employs fuzzy logic and geographic information systems to establish relationships between causative factors and coseismic slope failures in regions with well-documented and substantially complete coseismic landslide inventories. These relationships are then utilized to estimate the relative probability of landslide occurrence in regions with neither historical landslide inventories nor detailed geotechnical data. Statistical analyses of inventories from the 1994 Northridge and 2008 Wenchuan earthquakes reveal that shaking intensity, topography, and distance from active faults and streams are the main controls on the spatial distribution of coseismic landslides. Average fuzzy memberships for each factor are developed and aggregated to model the relative coseismic landslide hazard for both earthquakes. The predictive capabilities of the models are assessed and show good-to-excellent model performance for both events. These memberships are then applied to the 1999 Chi-Chi earthquake, using only a digital elevation model, active fault map, and isoseismal data, replicating prediction of a future event in a region lacking historic inventories and/or geotechnical data. This similarly results in excellent model performance, demonstrating the models predictive potential and confirming it can be meaningfully applied in regions where previous methods could not. For such regions, this method may enable a greater ability to analyze coseismic landslide hazard from specific earthquake scenarios, allowing for mitigation measures and emergency response plans to be better informed of earthquake-related hazards.

Evaluation of coseismic landslide hazard on the proposed Haast-Hollyford Highway, South Island, New Zealand.

ABSTRACT Coseismic landsliding presents a major hazard to infrastructure in mountains during large earthquakes. This is particularly true for road networks, as historically coseismic landsliding has resulted in road losses larger than those due to ground shaking. Assessing the exposure of current and planned highway links to coseismic landsliding for future earthquake scenarios is therefore vital for disaster risk reduction. This study presents a method to evaluate the exposure of critical infrastructure to landsliding from scenario earthquakes from an underlying quantitative landslide hazard assessment. The method is applied to a proposed new highway link in South Island, New Zealand, for a scenario Alpine Fault earthquake and compared to the current network. Exposure (the likelihood of a network being affected by one or more landslides) is evaluated from a regional-scale coseismic landslide hazard model and assessed on a relative basis from 0 to 1. The results show that the proposed Haast-Hollyford Highway (HHH) would be highly exposed to coseismic landsliding with at least 30–40 km likely to be badly affected (the Simonin Pass route being the worse affected of the two routes). In the current South Island State Highway network, the HHH would be the link most exposed to landsliding and would increase the total network exposure by 50–70% despite increasing the total road length by just 3%. The present work is intended to provide an effective method to assess coseismic landslide hazard of infrastructure in mountains with seismic hazard, and potentially identify mitigation options and critical network segments.

Near‐Real‐Time Modeling of Landslide Impacts to Inform Rapid Response: An Example from the 2016 Kaikōura, New Zealand, EarthquakeNear‐Real‐Time Modeling of Landslide Impacts to Inform Rapid Response

Reliable methods for the near‐real‐time modeling of landslide hazard and associated impacts that follow an earthquake are required to provide crucial information to guide emergency responses. After the 2016 Kaikoura earthquake in New Zealand, we undertook such a near‐real‐time modeling campaign in an attempt to pinpoint the location of landslides and identify the locations where roads and rivers had been blocked. The model combined an empirical analysis of landslide hazard (based on previously published global relationships) with a simple runout model (based on landslide reach angles). It was applied manually, with a first iteration completed within 24 hrs of the earthquake and a second iteration (based on updated shaking outputs) within ∼72  hrs . Both models highlighted the expectation that landsliding would be widespread and that impacts to roads likely meant that the township of Kaikoura was cut off from the surroundings. These results were used by responders at the time to formulate aerial reconnaissance flight paths and to identify the risk that landslide dams could cause further damage. Subsequent model verification based on available landslide inventories shows that although these models were able to capture a large percentage of landslides and landslide impacts, the outputs were overpredicted, limiting their use for pinpointing the precise locations of triggered landslides. To make future versions of the model more useful for informing emergency responses, continued work must be done on modification and adaptation of the approach so that this overprediction is minimized. Nevertheless, the results from this study show that the model is a promising initial attempt at near‐real‐time landslide modeling and that efforts to automate the approach would greatly increase the utility and speed of modeling in future earthquakes.

Road impacts from the 2016 Kaikōura earthquake: an analogue for a future Alpine Fault earthquake?

ABSTRACT The 2016 MW 7.8 Kaikōura earthquake involved complex rupture of multiple faults for > 170 km, generating strong ground shaking throughout the upper South Island leading to widespread landsliding. As a result of surface fault rupture and landslides, State Highway (SH) 1 and SH70 were blocked, isolating Kaikōura and the surrounding communities, and necessitating evacuations by air and sea. In all these respects, the Kaikōura earthquake can be considered an analogue for a future Alpine Fault earthquake, providing lessons for the necessary emergency response. Landslide blockages primarily occurred where surrounding slopes averaged > 17° and where peak ground acceleration was > 0.43 g, peak ground velocity was > 41 cm/s, or the modified Mercalli intensity was > 7.9. Using a potential future Alpine Fault scenario earthquake, this study identifies locations on other key state highway routes that have similar predictive variables that may, therefore, become blocked in a future earthquake. This suggests that SH6 between Hokitika and Haast, SH73 near Arthur’s Pass, and SH94 south of Milford Sound are all likely to be affected. This will necessitate the evacuation of large numbers of spatially distributed tourists as well as the resupply of isolated local populations. The possibility of bad weather along with a lack of sea ports south of Hokitika will likely make such activities challenging. Contingency planning based on experiences from the Kaikōura earthquake is therefore necessary and likely to prove invaluable following an Alpine Fault earthquake.

Use of scenario ensembles for deriving seismic risk

Significance High death tolls from recent earthquakes have highlighted the need to better identify ways to effectively reduce seismic risk. We address this need by developing a new earthquake scenario ensemble approach. We model impacts from multiple different earthquake scenarios, identifying impacts that are common to multiple scenarios. This method allows us to estimate whether particular impacts are specific to certain earthquakes or occur irrespective of the location or magnitude of the next earthquake. Our method provides contingency planners with critical information on the likelihood, and probable scale, of impacts in future earthquakes, especially in situations where robust information on the likelihood of future earthquakes is incomplete, allowing disaster risk-reduction efforts to focus on minimizing such effects and reducing seismic risk. High death tolls from recent earthquakes show that seismic risk remains high globally. While there has been much focus on seismic hazard, large uncertainties associated with exposure and vulnerability have led to more limited analyses of the potential impacts of future earthquakes. We argue that as both exposure and vulnerability are reducible factors of risk, assessing their importance and variability allows for prioritization of the most effective disaster risk-reduction (DRR) actions. We address this through earthquake ensemble modeling, using the example of Nepal. We model fatalities from 90 different scenario earthquakes and establish whether impacts are specific to certain scenario earthquakes or occur irrespective of the scenario. Our results show that for most districts in Nepal impacts are not specific to the particular characteristics of a single earthquake, and that total modeled impacts are skewed toward the minimum estimate. These results suggest that planning for the worst-case scenario in Nepal may place an unnecessarily large burden on the limited resources available for DRR. We also show that the most at-risk districts are predominantly in rural western Nepal, with ∼9.5 million Nepalis inhabiting districts with higher seismic risk than Kathmandu. Our proposed approach provides a holistic consideration of seismic risk for informing contingency planning and allows the relative importance of the reducible components of risk (exposure and vulnerability) to be estimated, highlighting factors that can be targeted most effectively. We propose this approach for informing contingency planning, especially in locations where information on the likelihood of future earthquakes is inadequate.

Looking and Moving Forward

This chapter, the conclusion of the book, presents emerging themes based on the disciplinary perspectives of the book contributors.

Introduction to the Gorkha Earthquake

This chapter is the introduction to the book and briefly describes the Gorkha earthquake and its impact on Nepal. Progress since the 1950s and the political changes since the 1990s, including the Maoist-led insurgency, or the People’s War, is presented. The economic profile of the country and its dependence on international remittances is considered. The hazard profile of the country, with an emphasis on earthquakes, is briefly discussed. Last, the scope of the book is presented and a brief description of the contribution of each chapter is detailed.