landslides – Cabot Institute for the Environment blog

Developments in geophysical methods used to monitor surface and subsurface changes prior to landslides can lead to improved prediction and early warning.

Drone photo from the Hollin Hill Landslide Observatory in North Yorkshire, UK, taken in August 2018. This landslide exhibits progressive “slip-stick” failure, which is linked to increases in subsurface moisture-content caused by prolonged and extreme precipitation. This backscarp feature, located in Lias Group mudrocks (Whitby Mudstone Formation), has developed slowly since first appearing in April 2016. Prior to this, there were few clues at the ground surface indicating the impending failure. Credit: British Geological Survey

Every year, landslides cause fatalities and destruction in locations worldwide. Nevertheless, what triggers them and when they occur can often be difficult to predict. A recent article in Reviews of Geophysics examined developments in landslide monitoring using insights and methods from geophysics. Here, one of the authors of the paper answers some questions about landslide monitoring and prediction.

Why is the monitoring of landslides important, and what role can geophysics play?

Sometimes the most effective option for mitigating the risk from landslides is monitoring.

In an ideal world, we would have the geotechnical and financial resources to be able to remove all landslide hazards through slope stabilization and remediation. In reality, this just isn’t possible, and sometimes the most effective option for mitigating the risk from a landslide is to monitor it.

Historically, this has been done by monitoring deformation at the surface and by looking at changes from localised points in the subsurface; for example, by measuring fluctuations in the water table in a borehole. Variations in these data may provide clues about when slope failure is imminent.

The advantage of geophysical methods is that they can not only monitor subsurface properties and how they change over time but can also do so at much higher spatial resolution and over a wider area than point sources of information, such as boreholes.

What are the different types of landslides and why are geophysical methods particularly useful for monitoring “moisture-induced” landslides?

“Landslide” is one of those words that sounds simple enough to define but in reality is very complex.

One of the distinctions we can make between landslide types is their triggering mechanism; most landslides are caused by the direct consequences of increased rainfall and shaking by earthquakes, but they can also be a result of secondary factors such as deforestation.

Between 2007 and 2016, 83% of landslides globally were triggered by rainfall or other hydrological events. This is why we use the term “moisture-induced” in our review article, as it reflects the complicated nature of all sources of water present in landslide systems, including rainfall, snow-melt, and groundwater, amongst others.

Introducing increased amounts of water into a landslide changes the properties of the subsurface, which leads to destabilization and, when a critical threshold is exceeded, slope failure. These changes in material properties can be monitored by geophysical methods and, by comparing data collected over time, it is possible to make inferences about the destabilizing processes that are occurring in the subsurface of the landslide system.

Changes in subsurface ground moisture derived from a semi-permanent, 3D electrical resistivity (ER) array at the Hollin Hill Landslide Observatory, North Yorkshire, UK. The left image shows wet winter conditions, in which the western lobe of the landslide has significantly more subsurface moisture than the eastern lobe. The right image shows drier summer conditions, showing subsurface drainage from the failing Whitby Mudstone Formation to the underlying Staithes Sandstone Formation, despite dry ground at the surface of the landslide. Credit: Uhlemann et al. [2017], Figure 11

What different geophysical methods are used to gather information about moisture-induced landslides?

The majority of studies used passive seismic and active geoelectrical methods.

Our review article looks at published case studies from the past 12 years to see what kinds of methods are being applied to monitor moisture-induced landslides. What struck us was that the majority of studies used one of two methods: passive seismic and active geoelectrical methods.

Passive seismic monitoring has been used for many decades in global seismological studies, but really only started to be scaled down to look at smaller scale features, such as landslides, in the mid-1990s.

Although passive seismic monitoring has been around longer, monitoring landslides using active geoelectrical methods, primarily electrical resistivity (ER), has really taken off in the last decade or so. There have been several studies in which ER technologies have been developed specifically for landslide monitoring approaches. Consequently, ER monitoring is currently able to provide more information than passive seismic monitoring on the pre-failure conditions of landslides.

Lower equipment costs and power consumption, combined with better data management and equipment durability, means we can collect more geophysical data for longer from landslides. Each of the points in this plot shows information gathered from published case studies about the length of time and amount of data acquired during a single geophysical monitoring campaign. Multiannual campaigns are becoming increasingly common compared to nearly a decade ago. Credit: Whiteley et al. [2018], Figure 6

What do these methods tell us about the subsurface conditions of landslides?

The two approaches provide an opportunity to better understand the variable nature of the subsurface in time and space.

Passive seismic and active geoelectrical approaches complement each other very well. First, they tell us about different aspects of the subsurface conditions beneath a landslide. Seismic methods are able to tell us about the strength of the ground, while ER methods provide information about subsurface moisture dynamics. Both of these aspects are very important when trying to predict landslide movements.

Second, passive approaches tend to have great temporal resolution, but their spatial coverage can be limited by the number of seismic sensors deployed on a slope, usually due to cost or power requirements. On the other hand, ER methods can provide very high spatial resolution, but as they are dependent on collecting a set of data from many measurements, their temporal resolution can be limited. Together, the two approaches provide an opportunity to better understand the variable nature of the subsurface in time and space.

What advances in equipment and data analysis have improved understanding of landslide processes?

The financial, computational, and energy cost of equipment is continually reducing, which means we can collect more data for longer periods, and send data from the field to the lab for near real-time analysis.

Also, data telemetry means we can send data from the field to the lab for near real-time analysis. Both of these are crucial when using geophysical methods for early-warning of landslide failure.

Recently, there has been an increase in the use of 3D surveys and petrophysical relationships linking geophysical The financial, computational and energy cost of equipment is continually reducing, which means we can collect more data for longer periods. Also, data telemetry means we can send data from the field to the lab for near real-time analysis. Both of these are crucial when using geophysical methods for early-warning of landslide failure.

In ER monitoring, movements in the electrode array would have historically produced errors in the resistivity model, but developments in ER data inversion can now use this source of “error” to track movements in the landslide. Similarly seismic “ambient noise” is being used in innovative ways to monitor landslides, even though these background signals would have traditionally been undesirable in seismological surveys.

Left: The “Automated time-Lapse Electrical Resistivity” (ALERT) geoelectrical monitoring system installed at Hollin Hill, North Yorkshire, UK. Right: Inside the cabinet, the system acquires geoelectrical, geotechnical and weather data. Collecting geophysical measurements alongside local displacement and environmental data allows for more robust interpretations of the changes in subsurface geoelectrical data over time. Credit: British Geological Survey

Where is the field of geophysical monitoring of moisture-induced landslide heading?

The challenge now is to start looking for clues to identify precursory conditions to slope failure and to develop geophysical thresholds to inform early-warning approaches.

The great news is that this is a very active area of research! There is a lot of work being done in environmental seismology to increase the number of low-cost, low-power seismic sensors that can be deployed in landslide settings. This is important, as it will allow us to monitor landslides at very high-resolution in both the spatial and temporal domain.

Looking to the future, one can envision “smart sensor” sites that provide power, data storage, and telemetry, accommodating a wide range of integrated geophysical, geotechnical, and environmental monitoring methods. These could include seismic and electrical arrays, wireless sensor networks, and weather stations, with data relayed back to central processing sites for near-real time assessment, and early-warnings of impending failure based on calibrated geophysical thresholds.

———————————
This blog was written by Cabot Institute for the Environment member James Whitely, postgraduate researcher at University of Bristol’s School of Earth Science and the British Geological Society, with contributions from the articles co-authors. The blog was originally published by Editors’ Vox.

Original blog Citation: Whiteley, J. (2019), Downhill all the way: monitoring landslides using geophysics, Eos, 100 https://doi.org/10.1029/2019EO111065

Earlier this year, I wrote about my experiences of attending an interdisciplinary workshop in Mexico, and how these approaches foster a rounded approach to addressing the challenges in communicating risk in earth sciences research. In the field of geohazards, this approach is increasingly becoming adopted due to the concept of “cascading hazards”, or in other words, recognising that when a natural hazard causes a human disaster it often does so as part of a chain of events, rather than as a standalone incident. This is especially true in my field of research; landslides. Landslides are, after all, geological phenomena studied by a wide range of “geoscientists” (read: geologists, geomorphologists, remote sensors, geophysicists, meteorologists, environmental scientists, risk assessors, geotechnical and civil engineers, disaster risk-reduction agencies, the list goes on). Sadly, these natural hazards affect many people across the globe, and we have had several shocking reminders in recent months of how landslides are an inextricable hazard in areas prone to earthquakes and extremes of precipitation.

The iRALL, or the ‘International Research Association on Large Landslides’, is a consortium of researchers from across the world trying to adopt this approach to understanding cascading hazards, with a particular focus on landslides. I was lucky enough to attend the ‘iRALL School 2018: Field data collection, monitoring and modelling of large landslides’ in October this year, hosted by the State Key Laboratory of Geohazard Prevention and Geoenvironment Protection (SKLGP) at Chengdu University of Technology (CDUT), Chengdu, China. The school was attended by over 30 postgraduate and postdoctoral researchers working in fields related to landslide and earthquake research. The diversity of students, both in terms of subjects and origins, was staggering: geotechnical and civil engineers from the UK, landslide specialists from China, soil scientists from Japan, geologists from the Himalaya region, remote sensing researchers from Italy, earthquake engineers from South America, geophysicists from Belgium; and that’s just some of the students! In the two weeks we spent in China, we received presentations from a plethora of global experts, delivering lectures in all aspects of landslide studies, including landslide failure mechanisms, hydrology, geophysics, modelling, earthquake responses, remote sensing, and runout analysis amongst others. Having such a well-structured program of distilled knowledge delivered by these world-class researchers would have been enough, but one of the highlights of the school was the fieldwork attached to the lectures.

The scale of landslides affecting Beichuan County is difficult to grasp: in this photo of the Tangjiwan landslide, the red arrow points to a one story building. This landslide was triggered by the 2008 Wenchuan earthquake, and reactivated by heavy rainfall in 2016.

The first four days of the school were spent at SKLGP at CDUT, learning about the cascading hazard chain caused by the 2008 Wenchuan earthquake, another poignant event which demonstrates the interconnectivity of natural hazards. On 12th May 2008, a magnitude 7.9 earthquake occurred in Beichuan County, China’s largest seismic event for over 50 years. The earthquake triggered the immediate destabilisation of more than 60,000 landslides, and affected an area of over 35,000 km2; the largest of these, the Daguangbao landslide, had an estimated volume of 1.2 billion m3 (Huang and Fan, 2013). It is difficult to comprehend numbers on these scales, but here’s an attempt: 35,000 km2 is an area bigger than the Netherlands, and 1.2 billion m3 is the amount of material you would need to fill the O2 Arena in London 430 times over. These comparisons still don’t manage to convey the scale of the devastation of the 2008 Wenchuan earthquake, and so after the first four days in Chengdu, it was time to move three hours north to Beichuan County, to see first-hand the impacts of the earthquake from a decade ago. We would spend the next ten days here, continuing a series of excellent lectures punctuated with visits to the field to see and study the landscape features that we were learning about in the classroom.

The most sobering memorial of the 2008 Wenchuan earthquake is the ‘Beichuan Earthquake Historic Site’, comprising the stabilised remains of collapsed and partially-collapsed buildings of the town of Old Beichuan. This town was situated close to the epicentre of the Wenchuan earthquake, and consequently suffered huge damage during the shaking, as well as being impacted by two large landslides which buried buildings in the town; one of these landslides buried a school with over 600 students and teachers inside. Today, a single basketball hoop in the corner of a buried playground is all that identifies it as once being a school. In total, around 20,000 people died in a town with a population of 30,000. Earth science is an applied field of study, and as such, researchers are often more aware of the impact of their research on the public than in some other areas of science. Despite this, we don’t always come this close to the devastation that justifies the importance of our research in the first place.

River erosion damaging check-dams designed to stop debris flows is still a problem in Beichuan County, a decade after the 2008 Wenchuan earthquake.

It may be a cliché, but seeing is believing, and the iRALL School provided many opportunities to see the lasting impacts of large slope failures, both to society and the landscape. The risk of debris flows resulting from the blocking of rivers by landslides (a further step in the cascading hazard chain surrounding earthquakes and landslides) continues to be a hazard threatening people in Beichuan County today. Debris flow check-dams installed after the 2008 Wenchuan earthquake are still being constantly maintained or replaced to provide protection to vulnerable river valleys, and the risk of reactivation of landslides in a seismically active area is always present. But this is why organisations such as the iRALL, and their activities such as the iRALL School are so important; it is near impossible to gain a true understanding of the impact of cascading hazards without bringing the classroom and the field together. The same is true when trying to work on solutions to lessen the impact of these cascading hazard chains. It is only by collaborating with people from a broad range of backgrounds, skills and experiences can we expect to come up with effective solutions that are more than the sum of their parts.

—————

This blog has been reposted with kind permission from James Whiteley. View the original blog on BGS Geoblogy. This blog was written by James Whiteley, a geophysicist and geologist at University of Bristol, hosted by British Geological Survey. Jim is funded through the BGS University Funding Initiative (BUFI). The aim of BUFI is to encourage and fund science at the PhD level. At present there are around 130 PhD students who are based at about 35 UK universities and research institutes. BUFI do not fund applications from individuals.

Tag: landslides

Downhill all the way: Monitoring landslides using geophysics