Gregory J. Leonard

Geomorphic and geologic controls of geohazards induced by Nepal’s 2015 Gorkha earthquake

Nepals quake-driven landslide hazards Large earthquakes can trigger dangerous landslides across a wide geographic region. The 2015 Mw 7.8 Gorhka earthquake near Kathmandu, Nepal, was no exception. Kargal et al. used remote observations to compile a massive catalog of triggered debris flows. The satellite-based observations came from a rapid response team assisting the disaster relief effort. Schwanghart et al. show that Kathmandu escaped the historically catastrophic landslides associated with earthquakes in 1100, 1255, and 1344 C.E. near Nepals second largest city, Pokhara. These two studies underscore the importance of determining slope stability in mountainous, earthquake-prone regions. Science, this issue p. 10.1126/science.aac8353; see also p. 147 Satellite imaging isolated hazard potential for earthquake-triggered landslides after the 2015 Gorkha earthquake in Nepal. INTRODUCTION On 25 April 2015, the Gorkha earthquake [magnitude (M) 7.8] struck Nepal, followed by five aftershocks of ≥M 6.0 until 10 June 2015. The earthquakes killed ~9000 people and severely damaged a 550 by 200 km region in Nepal and neighboring countries. Some mountain villages were completely destroyed, and the remote locations, blocked roads, and landslide-dammed rivers prevented ground access to many areas. RATIONALE Our “Volunteer Group” of scientists from nine nations, motivated by humanitarian needs, focused on satellite-based systematic mapping and analysis of earthquake-induced geohazards. We provided information to relief and recovery officials as emergency operations were occurring, while supported by one of the largest-ever NASA-led campaigns of responsive satellite data acquisitions over a vast disaster zone. Our analysis of geohazards distribution allowed evaluation of geomorphic, tectonic, and lithologic controls on earthquake-induced landsliding, process mechanisms, and hazard process chains, particularly where they affected local populations. RESULTS We mapped 4312 coseismic and postseismic landslides. Their distribution shows positive associations with slope and shaking intensity. The highest areal densities of landslides are developed on the downdropped northern tectonic block, which is likely explained by momentary reduction of the normal stress along planes of weakness during downward acceleration. The two largest shocks bracket the high-density landslide distribution, the largest magnitudes of the surface displacement field, and highest peak ground accelerations (PGAs). Landslides are heavily concentrated where PGA was >0.6g and slope is >30°. Additional controls on landslide occurrence are indicated by their clustering near earthquake epicenters and within specific lithologic units. The product of PGA and the sine of surface slope (defined as the landslide susceptibility index) is a good indicator of where most landslides occurred. A tail of the statistical distributions of landslides extends to low values of the landslide susceptibility index. Slight earthquake shaking affected vulnerable materials hanging on steep slopes—such as ice, snow, and glacial debris—and moderate to strong shaking affected poorly consolidated sediments deposited in low-sloping river valleys, which were already poised near a failure threshold. In the remote Langtang Valley, some of the most concentrated destruction and losses of life outside the Kathmandu Valley were directly due to earthquake-induced landslides and air blasts. Complex seismic wave interactions and wave focusing may have caused ridgetop shattering and landslides near Langtang but reduced direct shaking damage on valley floors and at glacial lakes. CONCLUSION The Gorkha earthquake took a tremendous, tragic toll on human lives and culture. However, fortunately no damaging earthquake-caused glacier lake outburst floods were observed by our satellite analysis. The total number of landslides was far fewer than those generated by comparable earthquakes elsewhere, probably because of a lack of surface ruptures, the concentration of deformation along the subsurface thrust fault at 10 to 15 km depth, and the regional dominance of competent high-grade metamorphic and intrusive igneous rock types. Landslide distribution and effects of a huge landslide. (A) Landslides (purple dots) are concentrated mostly north of the tectonic hinge-line. Also shown are the epicenters of the main shock and largest aftershock. Displacements are from the JAXA ALOS-2 ScanSAR interferogram (21 Feb and 2 May 2015 acquisitions). (B and C) Before-and-after photographs obtained by D. Breashears in Langtang Valley showing complete destruction of a large part of Langtang village by a huge landslide. The Gorkha earthquake (magnitude 7.8) on 25 April 2015 and later aftershocks struck South Asia, killing ~9000 people and damaging a large region. Supported by a large campaign of responsive satellite data acquisitions over the earthquake disaster zone, our team undertook a satellite image survey of the earthquakes’ induced geohazards in Nepal and China and an assessment of the geomorphic, tectonic, and lithologic controls on quake-induced landslides. Timely analysis and communication aided response and recovery and informed decision-makers. We mapped 4312 coseismic and postseismic landslides. We also surveyed 491 glacier lakes for earthquake damage but found only nine landslide-impacted lakes and no visible satellite evidence of outbursts. Landslide densities correlate with slope, peak ground acceleration, surface downdrop, and specific metamorphic lithologies and large plutonic intrusions.

Geology and landscape evolution of the Hellas region of Mars

Hellas basin on Mars has been the site of volcanism, tectonism, and modification by fluvial, mass-wasting, and eolian processes over its more than 4-b.y. existence. Our detailed geologic mapping and related studies have resulted in the following new interpretations. The asymmetric distribution of highland massifs and other structures that define the uplifted basin rim suggest a formation of the basin by the impact of a low-angle bolide having a trajectory heading S60°E. During the Late Noachian, the basin was infilled, perhaps by lava flows, that were sufficiently thick (>1 km) to produce wrinkle ridges on the fill material and extensional faulting along the west rim of the basin. At about the same time, deposits buried northern Malea Planum, which are interpreted to be pyroclastic flows from Amphitrites and Peneus Paterae on the basis of their degraded morphology, topography, and the application of a previous model for pyroclastic volcanism on Mars. Peneus forms a distinctive caldera structure that indicates eruption of massive volumes of magma, whereas Amphitrites is a less distinct circular feature surrounded by a broad, low, dissected shield that suggests generally smaller volume eruptions. During the Early Hesperian, a ∼1- to 2-km-thick sequence of primarily fined-grained, eolian material was deposited on the floor of Hellas basin. Subsequently, the deposit was deeply eroded, except where armored by crater ejecta, and it retreated as much as 200–300 km along its western margin, leaving behind pedestal craters and knobby outliers of the deposit. Local debris flows within the deposit attest to concentrations of groundwater, perhaps in part brought in by outflow floods along the east rim of the basin. These floods may have deposited ∼100–200 m of sediment, subduing wrinkle ridges in the eastern part of the basin floor. During the Late Hesperian and Amazonian, eolian mantles were emplaced on the basin rim and floor and surrounding highlands. Their subsequent erosion resulted in pitted and etched plains and crater fill, irregular mesas, and pedestal craters. Local evidence occurs for the possible former presence of ground ice or ice sheets ∼100 km across; however, we disagree with a hypothesis that suggests that the entire south rim and much of the floor of Hellas have been glaciated. Orientations of dune fields and yardangs in lower parts of Hellas basin follow directions of the strongest winds predicted by a recently published general circulation model (GCM). Transient frost and dust splotches in the region are, by contrast, related to the GCM prediction for the season in which the images they appear in were taken.

Himalayan glaciers: The big picture is a montage

Unusual miscarriages of science (1, 2) recently rocked climate change science and glaciology. An infamous paragraph, uncharacteristic of the rest of the contribution of Working Group II to the Intergovernmental Panel on Climate Change Fourth Assessment, claimed that Himalayan glaciers would disappear by 2035 (1). In such a monumental report, errors can be expected. However, this error, explicated in ref. 3, shredded the reputation of a large and usually rigorous international virtual institution. The gaffe by the Intergovernmental Panel on Climate Change helped to trigger a global political retreat from climate change negotiations, and it may prove to have been one of the more consequential scientific missteps in human history. An equally incorrect claim, on a different timescale, was that large Himalayan glaciers may be responding today to climate shifts 6,000–15,000 y ago (2). However, both mistakes (1, 2) and some solid scientific reporting on Himalayan glacier dynamics (4–10) highlight large gaps in the observational record. In PNAS, Fujita and Nuimura (11) competently reduced the knowledge gap.

Satellite Monitoring of Pakistan's Rockslide‐Dammed Lake Gojal

On 4 January 2010, a rockslide 1200 meters long, 350 meters wide, and 125 meters high dammed the Hunza River in Attabad, northern Pakistan, and formed Lake Gojal. The initial mass movement of rock killed 20 people and submerged several villages and 22 kilometers of the strategic Karakoram Highway linking Pakistan and China. Tens of thousands of people were displaced or cut off from overland connection with the rest of the country. On 29 May, the lake overflow began to pour through a spillway excavated by Pakistani authorities. On approximately 20 July, the lake attained a maximum depth of 119 meters and a torrent at least 9 meters deep issued over the spillway, according to Pakistans National Disaster Management Authority (NDMA). To date, the natural dam is holding and eroding slowly. However, the threat of a catastrophic outburst flood remains.

Introduction: Global glacier monitoring— a long-term task integrating in situ observations and remote sensing

This book focuses on the complexities of glaciers as documented via satellite observations. The complexities drive much scientific interest in the subject. The essence—that the world’s glaciers and ice caps exhibit overwhelming retreat—is also developed by this book. In this introductory chapter, we aim at providing the reader with background information to better understand the integration of the glacier-mapping initiative known as Global Land Ice Measurements from Space (GLIMS, http://www.glims.org ) within the framework of internationally coordinated glacier-monitoring activities. The chapter begins with general definitions of perennial ice on land and its global coverage, followed by a section on the relation between glaciers and climate. Brief overviews on the specific history of internationally coordinated glacier monitoring and the global monitoring strategy for glaciers and ice caps are followed by a summary of available data. We introduce the potential and challenges of satellite remote sensing for glacier monitoring in the 21st century and emphasize the importance of integrative change assessments. Lastly, we provide a synopsis of the book structure as well as some concluding remarks on worldwide glacier monitoring.

New Zealand’s Glaciers

New Zealand’s mountains support 3,153 inventoried glaciers, 99.4 % of this number (_99.9 % by volume) on South Island, and the remaining few on Mt. Ruapehu, a North Island volcano. Here we (1) provide a historical, geological, and climatic context for New Zealand’s glaciers; (2) review published knowledge of their current state and recent dynamics; (3) present a synoptic overview from ASTER imaging of the glaciers of Mt. Ruapehu (North Island), including relations to volcanic activity; use ASTER to examine changes affecting glaciers of Mt. Aoraki (Mt. Cook, South Island) and selected areas southward to Milford Sound; and (4) review limnological, climatic, and debris load controls on New Zealand’s glacier fluctuations. Half or more of New Zealand’s ice mass has disappeared since the Little Ice Age (LIA). New Zealand has some of the world’s highest ice mass accumulation rates, shortest glacier response times, and greatest concentrations of glacier debris discharge. For the smaller glaciers on steep slopes, especially those in high-precipitation zones and descending into warm climatic zones where ablation is rapid and response times are short, these small glaciers are not responding to the end of the LIA, but rather their observed fluctuations are a response to decadal climate oscillations and centennial-scale trends (including atmospheric warming). Decadalscale climate changes driving short-term glacier fluctuations of fast-response glaciers in New Zealand correlate, foremost, to the Antarctic Oscillation (AAO) and the Southern Oscillation Index (SOI), and, second, to the El Nino Southern Oscillation (ENSO). In contrast, the largest low-sloping valley glaciers have long response times due to their great thicknesses and insulating debris loads, and their lengths exhibit no discernible influence from decadal climate oscillations; consequently they are far out of equilibrium with the long-term warming and short-term fluctuating climate. Many glacier attributes are interrelated in a web of positive and negative dynamical feedbacks. For example, high ice discharge (which by itself is associated with short glacier response times) can remove surficial debris and allow rapid ablation, thereby further shortening response times. Large glacial lakes are characterized by a separate range of dynamic behavior. Lake formation and growth are promoted on slowresponse low-gradient glaciers with thick debris cover, and overdeepened valleys, as well as by climatic warming. Once the lakes enlarge, coalesce, and expand past a critical point, rapid calving and a host of other ablation processes accelerate, commonly beyond control by further climate change. New Zealand’s Southern Alps climate has a strong east–west gradient affecting all its climate parameters. However, thus far the dynamical responses of glaciers of comparable geomorphic types are very similar on the east and west sides of the Main Divide of the Southern Alps. Although we observe substantial climatic and climate change differences across the Alps, thus far glacier responses appear to be uniform across the entire mountain range.

Glacier-dammed ice-marginal lakes of Alaska

The climate across Alaska is changing, as are the melting and other dynamics of glaciers and their lengths, widths, thicknesses, and masses. One may reasonably expect that terminus and surface ablation of Alaskan glaciers would impact the occurrence and sizes of glacier-dammed lakes (GDLs) impounded by many of these glaciers. An individual lake and its damming glacier may have unique dynamics, exhibiting chaotic variations not directly attributable to climate change. Statistical analysis of a large GDL population, however, indicates that changes may be linked directly to shifting climatic conditions, thus providing an empirical basis for quantitative numerical models of future behavior. In this chapter we show that the rapidly changing distribution of glacier-dammed lakes is neither simple nor homogeneous, but that lake changes may be amenable to modeling. These lakes, their impounding glaciers, and their changes through time can be detected in satellite imagery. Presented here is research enabled by the GLIMS initiative, and the Terra/ASTER and Landsat programs. Archived imagery facilitated review and change analysis of 538 previously mapped GDLs across Alaska, U.S.A. All glacier perimeters within the study area were also reviewed for additional lakes forming since the prior survey in 1971. Change analyses of the combined lake population over time found nonuniform distributions of lake losses and gains from 1971 to 2000 across Alaska. Detected changes appear less related to elevation, latitude, or maritime influence than to the complexity, origin and terminus types of damming glaciers, the aspects of their ice dams, possibly the topographic gradient below and near the lake, and possibly the rate of recent temperature increases. The Copper River Basin (CRB), for example, has recorded low rates of atmospheric warming relative to adjoining areas over the past 50 years, and it has retained and developed the greatest proportion of GDLs. A concurrent detailed remote-sensing and field observation study of the CRB highlighted the dynamics of individual GDLs and their damming glaciers. Whereas there is no single typical or representative lake, Iceberg Lake, in the far eastern Chugach Mountains, provides an example showing how a climatic shift to warmer conditions may result in diminishment and even disappearance of these lakes. Iceberg Lake was comparatively stable for at least 1,500 years, but responded to > 100 years of thinning of its damming glacier with the initiation of episodic glacier lake outburst flood (GLOF) drainages every year or two, starting in 1999. The thinning of the damming glacier, in turn, is partly a response to a moraine-dammed lake that has formed since the 1970s at the terminus of the trunk (Tana) glacier, catalyzing disarticulation of that terminus and causing a furtherance of thinning of Iceberg Lake’s damming glacier. The entire system including the Bagley Icefield, outlet glaciers, multiple GDLs (Iceberg Lake among them), and oraine-dammed lakes, is dynamically complex; some parts appear to be responding to climate shifts, and other parts may be displaying intrinsic unsteadiness of flow (including a surge/waste cycle of Bering Glacier). At the same time as known lakes have been diminishing or disappearing, in recent decades at lower elevations, GDLs tended to form and persist at higher elevations.

Glacier Mapping and Monitoring Using Multispectral Data

Multispectral satellite data represent the primary data source for spaceborne glacier mapping and monitoring, and remote-sensing studies have generated significant results regarding global glacier observations and understandings. In this chapter we provide an overview of the use of multispectral data and the methods typically applied in glacier studies. Besides multispectral techniques based on the visible and near-infrared section and the shortwave infrared section of the spectrum, we also briefly discuss methods for analyzing thermal and radar data, with special emphasis on the mapping of debris-covered glacier ice. A further focus is on spectral change detection techniques applied to multitemporal data, with special attention to a novel image-differencing technique. Then we provide an overview of satellite image-based measurement of glacier flow. Finally, we offer a suggestion for a new combination of glacier observations to be made by both multispectral and radar/microwave remote-sensing sensors.

Massive collapse of two glaciers in western Tibet in 2016 after surge-like instability

Surges and glacier avalanches are expressions of glacier instability, and among the most dramatic phenomena in the mountain cryosphere. Until now, the catastrophic collapse of a glacier, combining the large volume of surges and mobility of ice avalanches, has been reported only for the 2002 130 × 106 m3 detachment of Kolka Glacier (Caucasus Mountains), which has been considered a globally singular event. Here, we report on the similar detachment of the entire lower parts of two adjacent glaciers in western Tibet in July and September 2016, leading to an unprecedented pair of giant low-angle ice avalanches with volumes of 68 ± 2 × 106 m3 and 83 ± 2 × 106 m3. On the basis of satellite remote sensing, numerical modelling and field investigations, we find that the twin collapses were caused by climate- and weather-driven external forcing, acting on specific polythermal and soft-bed glacier properties. These factors converged to produce surge-like enhancement of driving stresses and massively reduced basal friction connected to subglacial water and fine-grained bed lithology, to eventually exceed collapse thresholds in resisting forces of the tongues frozen to their bed. Our findings show that large catastrophic instabilities of low-angle glaciers can happen under rare circumstances without historical precedent.Two catastrophic glacier collapse events in western Tibet in 2016 were caused by a convergence of weather and glacier-bed conditions, according to an analysis of observations and modelling.

Himalayan Glaciers (India, Bhutan, Nepal): Satellite Observations of Thinning and Retreat

This chapter summarizes the current state of remote sensing of glaciers in the India, Nepal, and Bhutan regions of the Himalaya, and focuses on new methods for assessing glacier change. Glaciers in these Himalaya regions exhibit complex patterns of changes due to the unique and variable climatic, topographic, and glaciological parameters present in this region. The theoretical understanding of glaciers in the Himalaya is limited by lack of sufficient observations due to terrain breadth and complexity, severe weather conditions, logistic difficulties, and geopolitics. Mapping and assessing these glaciers with satellite imagery is also challenging due to inherent sensor limitations and information extraction issues. Thus, we still lack a complete understanding of the magnitude of feedbacks, and in some places even their sign, between climate changes and glacier response in this region. In this chapter we present the current status of glaciers in various climatic regimes of the Himalaya, ranging from the monsoon-influenced regions of the central-eastern Himalaya (Nepal, Garhwal, Sikkim, and Bhutan) through the monsoon transition zone of Himachal Pradesh (India), to the dry areas of Ladakh (western Himalaya). The case studies presented here illustrate the use of remote sensing and elevation data coupled with glaciermapping techniques for glacier area and elevation change detection and ice flow modeling in the context of the Himalaya.