Installation of seismic stations to monitor collapses along the Maïdo rampart on La Réunion

Teams from the BRGM have installed three broadband seismic stations along the Maïdo rampart on La Réunion, which will be used to characterise the mechanisms causing the rampart formation to collapse.


24 July 2019
Installing seismic stations to monitor collapses along the Maïdo rampart

Installing seismic stations to monitor collapses along the Maïdo rampart in the Mafate cirque on La Réunion.

© BRGM

The Maïdo "rampart" is a towering rock wall nearly 1000 metres in height  with an incline varying from 50 to 60°, along the western edge of the Mafate cirque on La Réunion. The site is characterised by potentially unstable compartments where rock "flakes" become detached as cracks open up in the rock wall. These cracks can be over a metre in width and over ten metres in height, and have been observed more than 100 metres away from the edge of the Maïdo rampart.  

Assessing the risks of collapse  

Past collapses of La Réunion's mountain ramparts have been spectacular, causing over one million m3 of rock to fall at the Bras des Roches Noires in 2006 and more than 30 million m3 at Mahavel in 1965. The dangers arise both downstream and at the tops of the ramparts, which could suddenly break away several hundred metres from the edge.

To assess the risks of potentially major collapses along these ramparts, the BRGM has installed three seismic stations at the Maïdo site.

 Installing seismic stations to monitor collapses along the Maïdo rampart

Installing seismic stations to monitor collapses along the Maïdo rampart in the Mafate cirque on La Réunion.

© BRGM

Using seismic methods to characterise potentially unstable flaking rock  

Like musical instruments, all objects have their own vibration frequencies. Unstable flakes of rock tend to vibrate at certain frequencies, which are characteristic of their geometry, weight and rigidity, but also of how firmly they are attached to the stable rock mass. Seismometers can be used to record these frequencies and how they change over time. 

Measuring these vibrations over the long term will produce information on each rock flake (dimensions, attachment to the solid rock mass) but also on changes according to weather conditions (ice forming in the fissures, extreme heat, etc.). Understanding the thermomechanics of an unstable rock flake brings a better understanding of how it will eventually break away.

Installing seismic stations to monitor collapses along the Maïdo rampart

Installing seismic stations to monitor collapses along the Maïdo rampart in the Mafate cirque on La Réunion.

© BRGM

The RenovRisk and DEAL projects  

In the same context, the RenovRisk Erosion research programme is investigating gravity-related and erosion phenomena that could trigger substantial  disturbances on La Réunion, especially in  hurricane conditions. These projects aim to:  

  • characterise and acquire fundamental knowledge on landslips and rockfalls in mountain cirques;  
  • understand landslip mechanisms when movements are triggered by water;  
  • model the dynamics of landslips as affected by climate change;  
  • detect and monitor the transport of materials when destabilised by river spates so that the data can be incorporated into plans for hurricane risk prevention.  

Characterising potential major instabilities in the Maïdo rampart is one of the tasks allotted to the RenovRisk Erosion research programme.  

To complement this task from an operational point of view by specifying the characterisation methodology for hazards in contexts of this kind, the DEAL and BRGM are funding a study on the collapse of large potentially unstable rock "flakes" in the mountain ramparts of La Réunion. Results from the instruments installed along the Maïdo rampart will help to understand the long-term behaviour of these unstable "flakes" and, if possible, to specify the methodology for characterising landslip hazards when drawing up risk prevention plans.  

The partners  

With financial support from the European Union, the State and the Reunion Island Region.