At about 5:38am local time on 24 June 2017 (21:38 on 23 June 2017 UTC), a massive landslide struck Xinmo Village, Maoxian County, Sichuan Province in China. Sichuan province is prone to earthquakes, including the 2008 Mw 7.9 Wenchuan earthquake that killed over 70,000 people, as well as the 1933 Mw 7.3 Diexi earthquake with a death toll of up to 9300.
Authorities have confirmed that the Maoxian landslide was triggered by heavy rain. The Maoxian landslide swept 64 homes in Xinmo village, blocking a 2km section of river and burying 1,600 meters of road. The collapsed rubble was estimated to be about eight million cubic meters (Figure 1a). Three days later (on 27 June 2017), a second landslide hit Xinmo Village; almost in the same time, another landslide occurred in Shidaguan Town, 20km away from Xinmo Village (Figure 1b).
As part of the ESA’s Copernicus Program, the Sentinel-1 mission comprises a constellation of two polar-orbiting satellites, operating day and night performing C-band synthetic aperture radar imaging in all-weather conditions. Sentinel-1 images acquired before and after events such as landslides, earthquakes or volcanoes offer information on the extent and surface displacements of affected areas, which can be used for damage and future hazard assessment.
A joint team from Newcastle University (UK), Chengdu University of Technology, Tongji University, China Academy of Space Technology and Wuhan University (China) have been racing against time to respond these two events by combining ESA’s Sentinel-1, Chinese Gaofen-2/3 with field observations.
“Sentinel-1 acquired a post-event image thirteen and half hours after the Xinmo event, and provided us the first interferogram for the Xinmo landslide.” said Professor Zhenhong Li, Professor of Imaging Geodesy at Newcastle University, “This first Sentinel-1 interferogram, together with its corresponding coherence and amplitude maps, not only helped us identify the source area of this massive landslide, but also assisted with mapping the landslide boundary (Figure 2).”
“More importantly, through the analysis of the archived Sentinel-1 data, we found that pre-event movements exhibited in the source area during the period from 14 May to 19 June 2017 for the Xinmo event.”
Pre-event signals are even clearer for the Shidaguan landslide, suggesting it had been sliding for a while.
“It is well known that landslides are hard to predict” said Professor Qiang Xu, Geologist with Chengdu University of Technology, “This study convincingly demonstrates that InSAR can be used to detect and map active landslides, which is a great achievement.”
After presenting the joint research findings in the Dragon-4 symposium at Copenhagen on 27 June 2017, Professor Deren Li, Academician of Chinese Academy of Sciences (CAS) and Chinese Academy of Engineering (CAE) at Wuhan University concluded, “This joint effort suggests that landslide Early Warning System (EWS) might be encouraging. There are a range of factors we should consider for EWS, such as real time, automatic, spatial and temporal resolutions, and data uncertainty.”
Dramatic data from last year’s major earthquake in Kaikoura, New Zealand, will change the way scientists think about earthquake hazards in tectonic plate boundary zones.
The South Island earthquake was one of the most comprehensively recorded earthquakes in history. Satellite images of the earthquake and its aftermath have enabled scientists to analyse the quake in an unprecedented level of detail.
COMET Director Tim Wright, study co-author, said: “We’ve never seen anything like the Kaikoura quake before, it was one of the most complex ever recorded.
“An earthquake commonly ruptures across a single fault line or faults that are closely grouped; Kaikoura ruptured at least 12 major faults. This challenges many assumptions about how individual faults control earthquake ruptures.”
Immediately following the start of Kaikoura’s earthquake in November, the Sentinel-1 and ALOS-2 satellites were tasked with gathering images and data. COMET researchers at the University of Leeds used the data to provide a rapid analysis of what was happening during the quake.
They found that seismic readings of the earthquake were not giving accurate assessments of where the ruptures were occurring.
Professor Wright said: “Seismic readings are currently the fastest method of gathering earthquake data as they can quickly read shockwaves sent through the earth – but they paint a crude picture. The complexity of the Kaikoura earthquake caused seismologists to completely misinterpret the earthquake based on these seismic waves.
“Satellites such as the European Space Agency’s Sentinel-1 measure the way the ground deforms at very high resolution. The detail in the images showed us that ruptures took place across many separate faults.
“One of the aims of COMET is to expand satellite capability to provide rapid response earthquake data to ensure residents and rescue workers have access to fast and accurate information.”
The satellites provided pre- and post-earthquake images to measure the extent of land movement. Kaikaoura’s earthquake caused sections of earth to move up to 25 metres and created surface ruptures measuring 12 metres. This caused large scale landslides and triggered a tsunami.
In a study published today in Science, the team of researchers reports the full range of data analysed from Kaikaoura’s earthquake, including satellite imagery, field observation, GPS data and coastal uplift data.
The research will prompt reassessment of how many different faults can be involved in a single earthquake and could potentially feed into revaluations of seismic hazard models.
Lead researcher Dr Ian Hamling, a natural hazards geodesist from New Zealand research institute GNS Science, said: “There was growing evidence internationally that conventional seismic hazard models are too simple and restrictive.
“The message from Kaikoura is that earthquake science should be more open to a wider range of possibilities when rupture models are being developed. It underlines the importance of re-evaluating how rupture scenarios are defined for seismic hazard models.”
New Zealand’s complex network of faults are similar to those found in western United States, Japan and central Asia.
Leeds co-author and COMET Associate Dr John Elliott said: “While earthquakes like Kaikoura’s do not commonly occur, the data we’ve gathered from this event will expand our understanding of similar boundary zones around the world.
“Not only could the data help inform us for the future but it may change how we’ve interpreted ancient earthquakes. If an earthquake like Kaikoura’s took place thousands of years ago, current methods of paleo-seismology would possibly see it as a series of earthquakes over a long period of time, rather than as one large single quake.”
The full paper, ‘Complex multi-fault rupture during the 2016 M7.8 Kaikoura earthquake, New Zealand’ was published on 23 March in Science First Release.
An international team of scientists, led by COMET’s John Elliott, has shed new light on the earthquake that devastated Nepal in April 2015, killing more than 8,000 people.
In a study published in the journal Nature Geoscience, the scientists show that a kink in the regional fault line below Nepal explains why the highest mountains in the Himalayas are seen to grow between earthquakes.
Dr Elliott explained: “We have shown that the fault beneath Nepal has a kink in it, creating a ramp 20km underground. Material is continually being pushed up this ramp, which explains why the mountains were seen to be growing in the decades before the earthquake. The earthquake itself then reversed this, dropping the mountains back down again when the pressure was released as the crust suddenly snapped in April 2015.
“Using the latest satellite technology, we have been able to precisely measure the land height changes across the entire eastern half of Nepal. The highest peaks dropped by up to 60cm in the first seconds of the earthquake.”
Mount Everest, at more than 50km east of the earthquake zone, was too far away to be affected by the subsidence in this event.
The team, which included academics from the USA and France, also demonstrate that the rupture on the fault stopped 11km below Kathmandu. This leaves an upper portion that remains unbroken and will build up more pressure over time as India continues to collide with Nepal, and indicates that another major earthquake could take place within a shorter timeframe than the centuries that might be expected for the area.
Dr Elliott added: “As this part of the fault is nearer the surface, the future rupture of this upper portion has the potential for a much greater impact on Kathmandu if it were to break in one go in a similar sized event to that of April 2015.
“Work on other earthquakes has suggested that when a rupture stops like this, it can be years or decades before it resumes, rather than the centuries that might usually be expected.”
Study co-author Dr Pablo González, from the School of Earth and Environment at the University of Leeds and also a COMET member, said: “We successfully mapped the earthquake motion using satellite technology on a very difficult mountainous terrain. We developed newly processing algorithms to obtain clearer displacement maps, which revealed the most likely fault geometry at depth to make sense of the puzzling geological observations.”
COMET researchers have unravelled a complex seismic sequence using a combination of techniques, explaining not only the earthquake sequence itself but also the formation of the mountain range where it occurred.
On August 18 2014, an Mw 6.2 earthquake struck Murmuri, near Dehloran in the Zagros Mountains of South West Iran (Figure 1), and was followed by five aftershocks of Mw of at least 5.4. The largest of these was a Mw 6.0 aftershock which took place 16 hours after the main event.
These were the first large seismic events in the region since important developments in satellite Earth Observation (EO) have allowed us to study earthquakes in unprecedented detail, providing the potential to combine a range of satellite-based and seismological approaches.
The study team, which included COMET scientists from Cambridge and Oxford along with colleagues from Iran, Colorado, and Canada, saw this as an opportunity to shed light not only on the Murmuri earthquake but also on how the Zagros Mountains themselves are evolving.
Figure 1 below shows the distribution of earthquakes in the region along with the 2014 Murmuri event.
The depth of the earthquake-generating thrust faults in the Zagros Mountains has been the subject of debate for some time. Previous studies have disagreed on whether the faults break a thick sequence of sedimentary layers, or are confined to the underlying crystalline rocks.
Importantly, rather than using a single technique, the team combined satellite-based EO techniques with studies of the seismic waves generated by the earthquake and aftershocks. The first step was to identify their locations, which was crucial to understanding the relationship between the ground motions detected by InSAR and the fault planes that caused the earthquakes. They then used the results to generate models of the faults.
Modelling the seismic waves showed that all except one of the events were caused by thrust faults. The smooth signals in the InSAR interferograms meanwhile showed that the faulting which led to the earthquake was buried deep under the surface.
The next question was whether the earthquakes had been caused by a single or multiple faults. The interferograms showed distinct lobes extending to the east and southeast of the main affected area, suggesting that the displacements on the surface were caused by two if not three separate faults.
COMET’s Alex Copley, from the Department of Earth Sciences at the University of Cambridge, explained: “ We found that a single-fault model couldn’t reproduce the deformation patterns shown by the interferograms, so we investigated by applying multiple-fault models instead.”
The team modelled the faults to establish characteristics including their direction, length and angle. When they used the interferograms alone there were a wide range of different fault parameters that could produce models that matched the data, but by including the seismic data the team could narrow down these characteristics.
Dr Copley added: “The only way we managed to work out what actually happened was by using seismological techniques, and then using these results to interpret the satellite measurements.”
The results showed that the 18 August 2014 event involved significant slip on two planes, which produced a complex displacement pattern in the InSAR, and that there were two separate events big enough to produce surface deformation signals, hence the two lobes on the interferograms.
It also became clear that most if not all of the faulting took place in the sedimentary layers rather than the igneous rocks below, at depths of 3-9km. The faults were also found to be longer than they were deep, which is relatively unusual – most faults tend to be more or less equal in length and depth. This could be because changes in the mechanical properties of the rocks below stop the faults from extending any deeper.
As well as explaining the events at Murmuri, the results throw light on the large scale tectonics of the Zagros Mountains, showing which combination of tectonic forces and material properties of the rocks can give rise to the shape and deformation pattern of the mountain range.
Dr Copley summarised: “If we had used seismology or satellite measurements alone we would have failed to learn much that was new about this earthquake sequence. Instead, our approach allowed us to shed light not only on the formation of the Zagros, but also how similar fold-thrust belts form across the globe.”
Active faults and the devastating earthquakes they can trigger do not respect political borders. Whilst the recent earthquakes in Nepal did most damage to the mountain kingdom itself, hundreds of people were also killed or injured in neighbouring China, India and Bangladesh.
The region’s history tells a similar story – the three countries’ earthquake records over the last 500 years are intertwined by shared proximity to the Himalayan mountain belt and its underlying megathrust fault.
Ways of improving resilience to earthquake hazard also need to transcend political boundaries, bringing together scientists and policymakers from the affected countries to share knowledge, experience and ideas.
This principle has led to the Earthquakes without Frontiers (EwF) partnership – a diverse group of natural and social scientists from around the UK. Led by James Jackson of both COMET and the University of Cambridge, EwF is a 5-year initiative funded by NERC and ESRC (the Natural Environment and Economic and Social Research Councils) under the Improving Resilience to Natural Hazards programme.
As well as COMET, the partnership includes researchers from Cambridge, Durham, Hull, Leeds, Northumbria and Oxford Universities, the British Geological Survey, the Overseas Development Institute and Durham’s Institute of Hazard, Risk and Resilience.
The project focuses on three broad regions – China, the Himalayan mountain front (Nepal and Northern India) and Central Asia (Kazakhstan and Kyrgyzstan) – with the key objective of furthering knowledge on earthquakes and landslides in the continental interiors.
Much of this involves using remotely sensed data which complements the project’s cross-border approach. EwF scientists use digital topography and multispectral and optical imagery to research landslide hazard and map active faults, alongside satellite radar to measure the warping of the Earth’s crust and steady interseismic motions as faults build up stress before the next seismic event.
All of this contributes to a wider programme of social and natural scientific research with scientists in the partner countries, as well as being used to run workshops and training events for young international scientists.
Crucially, this knowledge exchange extends to countries dealing with similar hazards – the same types of fault that threaten vast regions in China also cause earthquakes in Italy, and lessons learnt about Iranian faults can inform work on hazard in Kazakhstan and vice versa. As such, EwF brings together scientists from many countries to share knowledge and experience across an even wider network, culminating in the annual EwF partnership meetings, the most recent of which was held in Kathmandu in April 2015.
When a huge Mw7.8 earthquake struck Nepal on the 25th April, it came as a double blow to all within EwF. Nepal is not only one of our focus areas, but many of the UK team also had been in Kathmandu just one week before the earthquake, working and living alongside Nepali colleagues and friends.
The International Charter for Space and Major Disasters was invoked just 3 hours after the earthquake, and over the following few days, space agencies hurriedly tasked their satellites to acquire new imagery over Nepal. We all felt strongly that we should put our combined experience to good use in the immediate aftermath of the Nepal earthquake, and dashed to obtain satellite imagery of the area. In the weeks since we have been working to analyse this imagery in order to aid both disaster relief efforts and hazard re-evaluation.
EwF researchers, along with colleagues at the University of Leeds, used data from the European Space Agency’s Sentinel-1A satellite to measure how the ground was permanently warped by the earthquake. This was greatly assisted by COMET’s new automated processing facility, designed to cope with the vast amount of data from the Sentinel-1 satellites, which helped to produce some of the first radar interferograms of the Nepal earthquake. These mapped how the ground was warped along a 170 km stretch of the fault, moving by up to ~1.4 m near Kathmandu.
We are now modelling the data to understand how the fault slipped at depth, establish the relationship with the large Mw 7.3 aftershock on the 12th May, and gauge how these events may have stressed the surrounding regions, making them more likely to fail in future.
At the same time, EwF scientists at Durham University and the British Geological Survey have been using high-resolution optical and multispectral imagery to map landslides in the region. We have identified around 3,600 landslides that were either triggered or reactivated by the earthquake, using the maps to show where rivers are likely to be dammed and roads blocked. This has also highlighted the need to plan for the monsoon season which may reactivate or trigger even more deadly landslides.
Over the coming months, EwF researchers will continue to work on these topics as well as the many more questions raised by the Nepal earthquake. We hope that the lessons learned from this terrible event will bring us one step closer to improving resilience to future earthquakes, not just for Nepal and the countries across its borders, but for all earthquake-prone countries.
In 2013, a MW7.7 earthquake struck Balochistan, caused a huge surface offset and triggered a small tsunami in the Arabian Sea.
The apparently strange fault behaviour attracted the attention of scientists worldwide and discussion is still ongoing.
This an interesting case for paleoseismologists, not only because of the cascading earthquake effects, but also because of the surface rupture distribution, from which we might learn some important lessons.
COMET student Yu Zhou and his colleagues from Oxford University have published a new paper on this event, arguing that it might be not as unusual as it seems. Their research is based on the analysis of Pleiades stereo satellite imagery, which has proven to be a very useful data source.
On 25 April, a 7.8-magnitude earthquake struck Nepal, claiming over 8,000 lives and affecting millions of people.
Images from ESA’s Sentinel-1A satellite clearly showed the effects of the earthquake, including the maximum land deformation only 17km from Nepal’s capital, Kathmandu. This explains the extremely high damage to the area.
By combining Sentinel-1A imagery from before and after the quake, COMET scientists have been able to interpret the rainbow-coloured interference patterns in the image (known as an interferogram), and interpret them as changes on the ground. COMET scientists have also been analysing the 12 May aftershock. You can read more here.