Geology topics

Mark E. Reid

Publications and source records attributed to Mark E. Reid.

At least 19 recordsLinked to original sources

USGS2026

Map depicting locations susceptible to landslide hazards triggered by prolonged intense rainfall, Utuado Municipio, Puerto Rico

Introduction Landslides commonly occur in Puerto Rico during or soon after rainstorms with prolonged intense rainfall. Rainfall-triggered landslides present significant hazards to the built environment and human safety (for example, Jibson, 1986; Larsen and Torres Sanchez, 1992; Bessette-Kirton and others, 2019a). These hazards were illustrated during September 2017 (Hughes and others, 2019) when Hurricane María triggered more than 70,000 landslides across Puerto Rico (fig. 1), causing loss of life and widespread destruction (figs. 2–4). In Utuado Municipio, about 18,000 landslides were observed after Hurricane María (Hughes and others, 2019). Most intense rainfall-triggered landslides in Puerto Rico occur in soil overlying bedrock and travel downslope from where they initiate (for example, Jibson, 1986; Bessette-Kirton and others, 2019a, 2020). Hazards to human life and infrastructure can differ substantially between where landslides initiate and where they travel. At landslide initiation locations, the ground breaks up (fails) as it moves downslope, removing support for buildings, roadways, and other structures located upon the landslides (fig. 3A, B). This ground failure often causes structures to collapse and suffer severe damage or complete destruction, rendering building occupants at extreme risk. Roadways and other vital infrastructure, such as water, power, and communication lines, are often severed. During intense rainfall events, landslide initiation and movement downslope can occur in seconds to hours. Downslope from where they initiate, landslides in Puerto Rico often move very rapidly (faster than a person can walk or run) and can impact and destroy or fill structures with mud and debris (fig. 4) as well as bury roadways (fig. 3A, C). Moving landslides present extreme hazards to human safety; some fatalities during Hurricane María resulted from mud and debris that flowed through windows and doors of a home and buried occupants. Many landslides in Puerto Rico become fast-moving liquefied masses (debris flows), which are especially common in channels where they can quickly travel kilometers downstream. Multiple debris flows may occur in a channel during a single rainstorm (some channels experienced more than 50 debris flows during Hurricane María; Bessette-Kirton and others [2020] ). This hazard map separately depicts locations and the relative likelihood of where landslides may initiate, where they may move downslope, and where channelized debris flows may travel during or soon after intense rainfall like that during Hurricane María. An estimated average of 399 millimeters of rainfall fell across Utuado during September 19–21, 2017 (Ramos-Scharrón and Arima, 2019). Lower rainfall amounts, durations, and intensities also trigger landsliding in Puerto Rico, although landslides and debris flows become more numerous as rainfall intensities, durations, and cumulative amounts increase. The map does not show potential landslide locations in the karst terrain of northern Utuado (Bawiec, 1999) because Hurricane María-induced landslides were very sparse in this region (Hughes and others, 2019; Baxstrom and others, 2021a), even though rainfall amounts were similar to those in other parts of Utuado. Based on data from Hughes and others (2019), landslides were about 10 times more likely to occur outside of the karst terrane than within it, and our preliminary analyses indicated relatively low susceptibility to rainfall-induced landslide hazards within the karst. Landslide-prone soils appear to be poorly developed on the very steep slopes present in the karst region, where weathering by dissolution appears to dominate.

Puerto Rico, Utuado Municipio↗

USGS2026

Map depicting locations susceptible to landslide hazards triggered by prolonged intense rainfall, Naranjito Municipio, Puerto Rico

Introduction Landslides commonly occur in Puerto Rico during or soon after rainstorms with prolonged intense rainfall. Rainfall-triggered landslides present significant hazards to the built environment and human safety (for example, Jibson, 1986; Larsen and Torres Sanchez, 1992; Bessette-Kirton and others, 2019a). These hazards were illustrated during September 2017 (Hughes and others, 2019), when Hurricane María triggered more than 70,000 landslides across Puerto Rico (fig. 1), causing loss of life and widespread destruction (figs. 2–4). In Naranjito Municipio, about 2,000 landslides were observed after Hurricane María (Hughes and others, 2019). Most intense-rainfall-triggered landslides in Puerto Rico occur in soil overlying bedrock and travel downslope from where they initiated (for example, Jibson, 1986; Bessette-Kirton and others, 2019a; 2020). Hazards to human life and infrastructure can differ substantially between where landslides initiate and where they travel. At landslide initiation locations, the ground breaks up (fails) as it moves downslope, removing support for buildings, roadways, and other structures located upon the landslides (figs. 3B, 4A, B). This ground failure often causes structures to collapse and suffer severe damage or complete destruction, rendering building occupants at extreme risk. Roadways and other vital infrastructure, such as water, power, and communication lines are often severed. During intense rainfall events, landslide initiation and movement downslope can occur in seconds to hours. Downslope from where they initiate, landslides in Puerto Rico often move, or runout, very rapidly (faster than a person can walk or run) and can impact and destroy or fill structures with mud and debris (figs. 3A, 4A, C) as well as bury roadways (fig. 4A). Moving landslides present extreme hazards to human safety; some fatalities during Hurricane María resulted from mud and debris that flowed through windows and doors of a home and buried occupants. Many landslides in Puerto Rico become fast-moving debris flows, which are especially common in channels where they can quickly travel kilometers downstream. Multiple debris flows may occur in a channel during a single rainstorm (some channels experienced more than 50 debris flows during Hurricane María; Bessette-Kirton and others, 2020). This map separately depicts locations and the relative likelihood of where landslides may initiate, where they may move downslope, and where channelized debris flows may travel during or soon after intense rainfall like that during Hurricane María. An estimated average of 529 millimeters of rainfall fell across Naranjito during September 19–21, 2017 (Ramos-Scharrón and Arima, 2019). Lower rainfall amounts, durations, and intensities also trigger landsliding in Puerto Rico, although landslides and debris flows become more numerous as rainfall intensities, durations, and cumulative amounts increase.

Naranjito Municipio, Puerto Rico↗

USGS2026

Map depicting locations susceptible to landslide hazards triggered by prolonged intense rainfall, Lares Municipio, Puerto Rico

Introduction Landslides commonly occur in Puerto Rico during or soon after rainstorms with prolonged intense rainfall. Rainfall-triggered landslides present significant hazards to the built environment and human safety (for example, Jibson, 1986; Larsen and Torres Sanchez, 1992; Bessette-Kirton and others, 2019a). These hazards were illustrated during September 2017 (Hughes and others, 2019), when Hurricane María triggered more than 70,000 landslides across Puerto Rico (fig. 1), causing loss of life and widespread destruction (figs. 2 and 3). In Lares Municipio, about 3,500 landslides were observed after Hurricane María (Hughes and others, 2019). Most intense-rainfall-triggered landslides in Puerto Rico occur in soil overlying bedrock and travel downslope from where they initiated (for example, Jibson, 1986; Bessette-Kirton and others, 2019a; 2020). Hazards to human life and infrastructure can differ substantially between where landslides initiate and where they travel. At landslide initiation locations, the ground breaks up (fails) as it moves downslope, removing support for buildings, roadways, and other structures located upon the landslides (figs. 2, 3A, B). This ground failure often causes structures to collapse and suffer severe damage or complete destruction, rendering building occupants at extreme risk. Roadways and other vital infrastructure, such as water, power, and communication lines are often severed. During intense rainfall events, landslide initiation and movement downslope can occur in seconds to hours. Downslope from where they initiate, landslides in Puerto Rico often move, or runout, very rapidly (faster than a person can walk or run) and can impact and destroy or fill structures with mud and debris (figs. 3A, C) as well as bury roadways (fig. 3A). Moving landslides present extreme hazards to human safety; some fatalities during Hurricane María resulted from mud and debris that flowed through windows and doors of a home and buried occupants. Many landslides in Puerto Rico become fast-moving debris flows, which are especially common in channels where they can quickly travel kilometers downstream. Multiple debris flows may occur in a channel during a single rainstorm (some channels experienced more than 50 debris flows during Hurricane María; Bessette-Kirton and others, 2020). This map separately depicts locations and the relative likelihood of where landslides may initiate, where they may move downslope, and where channelized debris flows may travel during or soon after intense rainfall like that during Hurricane María. An estimated average of 256 millimeters of rainfall fell across Lares during September 19–21, 2017 (Ramos-Scharrón and Arima, 2019). Lower rainfall amounts, durations, and intensities also trigger landsliding in Puerto Rico, although landslides and debris flows become more numerous as rainfall intensities, durations, and cumulative amounts increase. The map does not show potential landslide locations in the karst terrain of northern Lares (Bawiec, 1999) because Hurricane María-induced landslides were very sparse in this region (Hughes and others, 2019; Baxstrom and others, 2021a), even though rainfall amounts were similar to those in other parts of Lares. Based on data from Hughes and others (2019), landslides were about ten times more likely to occur outside of the karst terrane than within it, and our preliminary analyses indicate relatively low susceptibility to rainfall-induced landslide hazards within the karst. Landslide-prone soils appear to be poorly developed on the very steep slopes present in the karst region, where weathering by dissolution appears to dominate.

Lares Municipio, Puerto Rico↗

USGS2026

From start to stop: Simple methods for mapping susceptibility to landslide runout and debris-flow inundation

Landslide runout and debris-flow inundation can disrupt areas well beyond their initial sources, causing widespread damage and extensive fatalities. Understanding where they start and how far they might travel is essential in many locations worldwide. However, most landslide susceptibility maps focus on initiation areas and fail to incorporate runout areas. Furthermore, hazards vary greatly with the degree of landslide mobility, and debris flows can grow as they travel beyond their initial source. Here, we use the USGS Grfin Tools software suite to map susceptible areas from start to stop in the Federated States of Micronesia where landslide and debris-flows runout onto gentle ground is a deadly threat. The DEM-based models in Grfin Tools use simple, empirical, and well-documented approaches that require minimal parameters and, thus, can be used in areas that lack detailed physical properties. We illustrate methods to estimate the required parameters using direct landslide observations, and we suggest alternate methods if direct observations are absent. Our simple parameter estimations in the Federated States of Micronesia successfully predict areas that match past landslide runout and debris-flow inundation, as well as zones with no inundation. In addition to creating preliminary regional assessments, Grfin Tools can be used for comparing multiple scenarios and/or identifying areas for further investigations.

Chuuk, Kosrae, Pohnpei↗

USGS2025

Topographic controls on landslide mobility: Modeling hurricane-induced landslide runout and debris-flow inundation in Puerto Rico

In 2017, Hurricane Maria triggered more than 70 000 landslides in Puerto Rico. After initiation, these predominantly shallow landslides were mobilized to varying extents – some landslides only traveled partway downslope, whereas others reached drainage channels and were mobilized into long-traveled debris flows that could severely impact roads and infrastructure. Thus, forecasting potential landslide runout and inundation zones is critical for estimating landslide and debris-flow hazards. Here we conduct an in-depth topographic analysis of landslide-affected areas from nine study areas and apply a linked modeling technique to estimate locations susceptible to varying degrees of landslide runout in the Lares, Utuado, and Naranjito municipalities. We find that the longest runout lengths are observed on high-relief escarpments, although highly mobile long-runout debris flows also occurred in lower-relief dissected uplands. These topographic differences indicate that landslides that are initiated under similar conditions and possess equal potential to be mobilized as debris flows may not travel the same distances or affect the same areal extent. Our modeling approach allows the local topography to automatically control the implementation of two runout methods: (1) H/L runout zones are assigned directly downslope of landslide source zones, and (2) debris-flow inundation zones are estimated in the presence of a channel network. Debris-flow volumes are calculated as a function of area-integrated growth factors, estimated as a function of the upstream areas susceptible to shallow landslides. Applying our empirical modeling scheme over an area of 560 km 2 , our results highlight the efficacy of our methods for the assessment of the potential for landslide runout and debris-flow inundation over diverse terrains with varied susceptibility.

Puerto Rico↗

USGS2025

Grfin Tools—User guide and methods for modeling landslide runout and debris-flow growth and inundation

The software package, Grfin Tools, can estimate potential runout from landslides or inundation from geophysical mass flows such as debris flows, lahars from volcanoes, and rock avalanches within a digital elevation model (DEM). Grfin is an acronym of gr owth + f low + in undation. The tools within this package apply simple, well-tested, empirical models of runout that are computationally efficient and require minimal parameters. These tools can be used individually (for example, to estimate debris-flow inundation) or in combination to represent a more complete series of linked processes, from landslide source areas, to unchannelized transport, to channelized flows. Grfin Tools can rapidly assess potential runout and inundation over large areas and the results are readily visualized in a geographic information system. Tools for assessing areas affected by runout and flow inundation include a height-to-length ( H / L ) ratio, angle-of-reach approach for estimating open-slope, unchannelized landslide runout, and volume-area scaling relations for assessing flow inundation in channels. Potential landslide areas that constitute the sources of runout or inundation can be delineated with topographic features, such as slope and (or) curvature, derived by the software package, or by employing potential sources derived from other landslide susceptibility models. Grfin Tools also has the capability to assess inundation from flows that grow volumetrically downstream. This is a vital feature, as larger flows commonly result in longer runout and larger inundation. The software uses empirically derived growth factors applied over upslope contributing source areas or upstream channel lengths to integrate the effects of various growth processes, such as channel entrainment, streambank failures, adjacent landslides, and hillslope erosion. Inundation follows a drainage network defined with a separate tool that uses topographic curvature to identify channel initiation locations. This document includes information on using Grfin Tools, the basis and methods underlying the tools and models, detailed descriptions of the software input and output files, and tips for handling special conditions such as roads and large water bodies. Multiple detailed examples illustrating different applications are also presented. Grfin Tools relies on the freely available TauDEM software package (Tarboton, 2005). The Grfin Tools software release is available from Cronkite-Ratcliff and others (2025).

Techniques and Methods↗

USGS2025

Shear surface undulations modulate clayey gouge strength and contribute to divergent landslide acceleration

Landslides display a spectrum of speeds for incompletely known reasons. Sliding occurs along slickensided undulatory shear surfaces within boundary shear gouge. Laboratory tests reveal that gouge shear strength generally decreases with finite cumulative displacement during relatively rapid failure and may increase or decrease with increasing shear rate; these behaviors can result in accelerating or decelerating landslide motion, which significantly affects consequent hazards. However, mechanisms responsible for such behaviors are poorly understood. We performed advanced ring shear strength testing that revealed such variable strength of a landslide near Oso, Washington, USA. We hypothesized that millimeter-scale undulations along shear surfaces caused the strength variability by imparting shear strength but while also modifying stresses that locally increase and decrease the typically considered particle-scale shear strength. We tested our hypotheses in the laboratory and with finite element soil deformation modeling. Lab results suggest that undulations contribute strength that decays with finite cumulative displacement. Modeling similarly reveals this, and that rapid shearing across undulations locally reduces effective normal stress by persistently elevating pore-water pressure and causing dilation. Consequent effects on strength differ by material with granular-rich, high-friction gouge losing strength and clay-rich, low-friction gouge losing little or gaining strength as shear rates increase. Ample testing by others reveals similar patterns. Hence, the propensity for gouge-controlled accelerating or decelerating failure may be estimated from simple index tests. Our findings on the effects of undulations reveal previously unknown mechanisms that may help to explain why some landslides reactivate catastrophically while others do not.

Washington↗

USGS2024

Assessing locations susceptible to shallow landslide initiation during prolonged intense rainfall in the Lares, Utuado, and Naranjito municipalities of Puerto Rico

Hurricane Maria induced about 70 000 landslides throughout Puerto Rico, USA, including thousands each in three municipalities situated in Puerto Rico's rugged Cordillera Central range. By combining a nonlinear soil-depth model, presumed wettest-case pore pressures, and quasi-three-dimensional (3D) slope-stability analysis, we developed a landslide susceptibility map that has very good performance and continuous susceptibility zones having smooth, buffered boundaries. Our landslide susceptibility map enables assessment of potential ground-failure locations and their use as landslide sources in a companion assessment of inundation and debris-flow runout. The quasi-3D factor of safety, F 3 , showed strong inverse correlation to landslide density (high density at low F 3 ). Area under the curve (AUC) of true positive rate (TPR) versus false positive rate (FPR) indicated success of F 3 in identifying head-scarp points (AUC = 0.84) and source-area polygons (0.85 ≤ AUC ≤ 0.88). The susceptibility zones enclose specific percentages of observed landslides. Thus, zone boundaries use successive F 3 levels for increasing TPR of landslide head-scarp points, with zones bounded by F 3 at TPR = 0.75, very high; F 3 at TPR = 0.90, high; and the remainder moderate to low. The very high susceptibility zone, with 118 landslides km −2 , covered 23 % of the three municipalities. The high zone (51 landslides km −2 ) covered another 10 %.

Puerto Rice↗

USGS2023

Lateral edifice collapse and volcanic debris avalanches: A post-1980 Mount St. Helens perspective

The 1980 eruption of Mount St. Helens was instrumental in advancing understanding of how volcanoes work. Lateral edifice collapses and the generation of volcanic debris avalanches were not widely recognized prior to that eruption, making assessment of their hazards and risks challenging. The proliferation of studies since 1980 on resulting deposits and evaluation of processes leading to their generation has built on the insights from the 1980 eruption. Volcano-related destabilizing phenomena, such as strength reduction by hydrothermal alteration, deformation and structural modifications from shallow magma intrusion, and thermal pressurization of pore fluids supplement those factors also affecting nonvolcanic slopes and can lead to larger failures. Remote and ground-based monitoring techniques can aid in detecting potentially destabilizing dynamic processes and in forecasting the size and location of future large lateral collapses, although forecasting remains a topic of investigation. More than a thousand large lateral collapse events likely ≥ 0.01 km 3 in volume have now been identified from deposits or inferred from source area morphology, leading to a recognition of their importance in the evolution of volcanoes and the hazards they pose. Criteria for recognition of debris-avalanche deposits include morphological factors and textural characteristics from outcrop to microscopic scale, allowing discrimination from other volcaniclastic deposits. Lateral edifice failure impacts a broad spectrum of volcanic structures in diverse tectonic settings and can occur multiple times during the evolution of individual volcanoes. Globally, collapses ≥ 0.1 km 3 in volume have been documented 5–6 times per century since 1500 CE, with about one per century having a volume ≥ 1 km 3 . Smaller events < 0.1 km 3 are underrepresented in the earlier record but also have high hazard impact.

Wasgington↗

USGS2023

Using integrated growth to delineate debris-flow inundation

Debris-flow volume is fundamental to mobility, yet many debris flows change volume as they travel. Growth can occur through diverse processes such as channel-bed entrainment, bank failures, aggregation of landslides, and coalescence of multiple flows. Integrating growth, either over upslope area or stream length, combines the effects of these growth processes and requires specification of only the growth zone extent and a growth factor. To delineate potential debris-flow inundation, we implement integrated growth factors and simple volume-area relations in a new USGS software package, Grfin Tools. We present two examples of forecasting debris-flow inundation – one using an area growth factor in Puerto Rico and another using a channel-length growth factor in Oregon, USA. The use of growth zones and growth factors enables scenario-based hazard assessments for geomorphic settings with debris-flow growth.

Conference Paper↗

USGS2021

When hazard avoidance is not an option: Lessons learned from monitoring the postdisaster Oso landslide, USA

On 22 March 2014, a massive, catastrophic landslide occurred near Oso, Washington, USA, sweeping more than 1 km across the adjacent valley flats and killing 43 people. For the following 5 weeks, hundreds of workers engaged in an exhaustive search, rescue, and recovery effort directly in the landslide runout path. These workers could not avoid the risks posed by additional large-scale slope collapses. In an effort to ensure worker safety, multiple agencies cooperated to swiftly deploy a monitoring and alerting system consisting of sensors, automated data processing and web-based display, along with defined communication protocols and clear calls to action for emergency management and search personnel. Guided by the principle that an accelerating landslide poses a greater threat than a steadily moving or stationary mass, the system was designed to detect ground motion and vibration using complementary monitoring techniques. Near real-time information was provided by continuous GPS, seismometers/geophones, and extensometers. This information was augmented by repeat-assessment techniques such as terrestrial and aerial laser scanning and time-lapse photography. Fortunately, no major additional landsliding occurred. However, we did detect small headscarp failures as well as slow movement of the remaining landslide mass with the monitoring system. This was an exceptional response situation and the lessons learned are applicable to other landslide disaster crises. They underscore the need for cogent landslide expertise and ready-to-deploy monitoring equipment, the value of using redundant monitoring techniques with distinct goals, the benefit of clearly defined communication protocols, and the importance of continued research into forecasting landslide behavior to allow timely warning.

Washington↗

USGS2020

Progress and lessons learned from responses to landslide disasters

Landslides have the incredible power to transform landscapes and also, tragically, to cause disastrous societal impacts. Whereas the mechanics and effects of many landslide disasters have been analyzed in detail, the means by which landslide experts respond to these events has garnered much less attention. Herein, we evaluate nine landslide response case histories conducted by the U.S. Geological Survey over the past two decades and summarize the event history, the response conducted, and the lessons learned from each event. We group the responses into three categories—providing event context from past events, addressing ongoing hazards, and acquiring data for the future—and present the nine case studies accordingly. We also summarize the progress in landslide response that has been made over the past two decades, including insights and advancements on the preparation for such events, the use of new technologies, and the importance of clear communication between all parties during disasters. We believe that exchanging and sharing experiences such as these will promote more clear and successful approaches for responses to landslide disasters in the future.

Book chapter↗

USGS2020

Debris-flow growth in Puerto Rico during Hurricane Maria: Preliminary results from analyses of pre- and post-event lidar data

On September 20, 2017, Hurricane Maria triggered widespread debris flows in Puerto Rico. We used field observations and pre- and post-Maria lidar to study the volumetric growth of long-travelled (>400 m) debris flows in four basins. We found overall growth rates that ranged from 0.7 to 30.4 m 3 per meter of channel length. We partitioned the rates into two growth mechanisms, aggregation of multiple landslides, or erosion and entrainment of channel sediment. In three basins, landslides accounted for more than 80% of the total debris-flow volumes. In one basin, entrainment accounted for 96% of the volume. These results indicate that forecasting volumes for regional debris-flow inundation modeling is more complicated than estimating the number and volume of contributing landslide source areas, although this task is difficult by itself. In this preliminary analysis, we did not find geologic, topographic, or morphometric variables that correlated with the growth observations. We suspect that the observed growth rates were heavily influenced by local variations in environmental conditions, including antecedent soil moisture conditions, duration and intensity of rainfall, and availability of channel material. Given these considerations, regional debris-flow inundation modeling may be best achieved by using a suite of scenarios that capture possible mechanisms of debris-flow growth.

Puerto Rico↗

USGS2019

Enhanced landslide mobility by basal liquefaction: the 2014 SR530 (Oso), Washington landslide

Landslide mobility can vastly amplify the consequences of slope failure. As a compelling example, the March 22, 2014 landslide near Oso, Washington (USA) was particularly devastating, traveling across a 1-km+ wide river valley, killing 43 people, destroying dozens of homes, and temporarily closing a well-traveled highway. To resolve causes for the landslide’s behavior and mobility, we conducted detailed post-event field investigations and material testing. Geologic and structure mapping revealed a progression of geomorphological structures ranging from debris flow lobes at the distal end, through hummock fields, laterally continuous landslide blocks, back-rotated blocks, and finally colluvial slides and falls at the landslide headscarp. Primary structures, as well as stratigraphic and vegetation patterns, in the landslide deposit indicated rapid extensional motion of the approximately nine-million-m3 source volume in a closely timed sequence of events. We identified hundreds of transient sand boils in the landslide runout zone – evidence of widespread elevated pore-water pressures with consequent shear-strength reduction at the base of the slide. During the event, underlying wet alluvium liquefied and allowed quasi-intact slide hummocks to extend and translate long distances across the flat valley. Most of the slide material itself did not liquefy. Using geotechnical testing and numerical modeling, we examined rapid undrained loading, shear and collapse of loose saturated alluvium, and strong ground shaking as potential liquefaction mechanisms. Our analyses show that some layers in the alluvium can liquefy when sheared, as could occur with rapid undrained loading. Simultaneous ground shaking could have contributed to pore-pressure generation as well. Two key elements, a large and rapid failure overriding wet liquefiable sediments, enabled the landslide’s high mobility. Basal liquefaction may enhance mobility of other landslides in similar settings.

Washington↗

USGS2019

When volcanoes fall down—Catastrophic collapse and debris avalanches

Despite their seeming permanence, volcanoes are prone to catastrophic collapse that can affect vast areas in a matter of minutes. Large collapses begin as gigantic landslides that quickly transform to debris avalanches—chaotically tumbling masses of rock debris that can sweep downslope at extremely high velocities, inundating areas far beyond the volcano. Rapid burial by the debris avalanches themselves, associated eruptions and lahars (volcanic mudflows), and inundation by tsunamis triggered when avalanches impact bodies of water can all cause widespread devastation to people and property.

Fact Sheet↗

USGS2019

Characterizing the catastrophic 2017 Mud Creek Landslide, California, using repeat Structure-from-Motion (SfM) photogrammetry

Along the rugged coast of Big Sur, California, the Mud Creek landslide failed catastrophically on May 20, 2017 and destroyed over 400 m of scenic California State Highway 1. We collected structure-from-motion (SfM) photogrammetry data using airborne platforms that, when combined with existing airborne lidar data, revealed that the area exhibited significant topographic change and displacement before, during and after the catastrophic failure. Before the catastrophic failure we document two areas of elevated change in the zone of depletion, which aligned with the double-peaked head scarp produced by the catastrophic failure. The catastrophic failure extended from 337 m elevation to at least 8 m below sea level, was 490 m wide, displaced ~3 million m3 of earth and rock, and deposited landslide debris at least 175 m seaward of the original shoreline. The failure was not a complete slope-clearing event, however, and several upslope and lateral regions that did not slip into the ocean exhibited significant displacement and topographic change during the days and months after catastrophic failure. Additionally, we use the post-slide data to quantify several other processes, including the time-varying rates of talus accumulation and coastal erosion of the landslide toe. We conclude that repeat SfM surveys from aerial imagery can provide valuable information about landslide evolution and the potential for deep-seated landslide hazards – especially in the lead up to catastrophic failure – if photos are collected and processed regularly.

California↗

USGS2019

Debris-flow initiation promoted by extension within a slow-moving landslide

The dynamics of slow landslide motion can predispose oversteepened and extended slide regions to debris-flow initiation. For more than 20 years, our real-time monitoring, combined with repeat high-precision GPS surveys, of the Cleveland Corral landslide complex, California, USA, reveals that debris flows initiate from slow-moving kinematic elements of this complex. Different slide elements move in different wet years, and all remain dormant in dry years. To explore controls on landslide-element kinematics, we use triaxial testing to define the critical state behavior of the landslide material, and use a large-diameter sampling ring to determine in situ material porosities in both extensional and compressional regions of the slide. Regions undergoing extension contain materials looser than their critical state, potentially aiding liquefaction and debris-flow mobilization from shallow, secondary slides. Although intense rainfall serves as a trigger for debris-flow initiation, slow deformation of the larger landslide promotes debris-flow formation by oversteeepening toe and lateral margins and by preferentially extending, and effectively loosening, material in these steep regions.

California↗

USGS2018

Combining multiphase groundwater flow and slope stability models to assess stratovolcano flank collapse in the Cascade Range

Hydrothermal alteration can create low‐permeability zones, potentially resulting in elevated pore‐fluid pressures, within a volcanic edifice. Strength reduction by rock alteration and high pore‐fluid pressures have been suggested as a mechanism for edifice flank instability. Here we combine numerical models of multiphase heat transport and groundwater flow with a slope‐stability code that incorporates three‐dimensional distributions of strength and pore‐water pressure to address the following questions: (1) What permeability distributions and contrasts produce elevated pore‐fluid pressures in a stratovolcano? (2) What are the effects of these elevated pressures on flank stability? (3) Finally, what are the effects of magma intrusion on potential flank failure in an edifice? Simulation results show that under a range of plausible parameters, water tables in a stratovolcano can be elevated or perched. These elevated water tables result in universally lower stability (lower factor of safety) compared with equivalent dry edifices, indicating a higher likelihood of flank collapse. Low‐permeability (<1 × 10 −17 m 2 ) layers such as altered pyroclastic deposits or breccias can result in locally saturated regions (perched water) and lower factors of safety near the ground surface but may actually reduce liquid water saturation and pore pressures in the core of the edifice and thus may favor small, shallow collapses over larger, deeper collapses. Magma intrusion into the base of the edifice increases pore‐fluid pressures and decreases the factor of safety. However, the shear strength of edifice rocks also exerts a significant control on stability, so both mechanical properties and pore‐fluid pressures are important for stability assessments.

Washington↗