Earthquakes and volcanoes : Are Rodrigues and Mauritius really at risk?

Recently, the inhabitants of Rodrigues have felt very much concerned as they have heard of repeated earth tremours near their island. This concern took another dimension following the January 2010 Haiti deadly earthquake. In the wake, Mauritians have also started to ask questions. This article attempts to give a clear scientific analysis of the geophysical set up  in our region with focus on Rodrigues concerning earth tremours and on Mauritius concerning any imminent volcanic upsurge.

The Mascarene islands of Réunion, Mauritius and Rodrigues are geologically young volcanic islands. They have the same  type of volcanism i.e. the hawaiian type which means that eruptions have been and will always tend to be effusive and hence quiet. This is in contrast with explosive type of volcanicity.The three islands are located on their respective submarine platform (see image) and are therefore not geophysically connected. Each has its own pedestal, its own volcanic history and specificity.

The Mascarene Islands and earthquake hazard

In order to understand clearly the risk of an earthquake affecting our islands, it is important to bear in mind the following key features:

Firstly, the three islands are not linked. Secondly, La Fournaise, on Réunion Island is still active and hence minor earth tremours associated with upwelling of magma from the magma reservoir is a common feature and are good pre-eruption signs for volcanologists. Thirdly, the case of Rodrigues is a special one as it is located on a submarine ridge, the Rodrigues Ridge. The ridge extends eastwards towards the Mid Indian Ocean Ridge. Fourthly, Rodrigues is the closest island to the Mid Indian Ocean Ridge along which three major tectonic plates diverge at a speed of 4.5 cm/year. What makes Rodrigues more at risk is that the island is very near the triple junction known as the Rodrigues Triple Junction.

Therefore, the case of Rodrigues is worth considering very closely.

The Rodrigues Triple Junction is a geologic submarine junction in the southern Indian Ocean where three major tectonic plates meet: the African Plate in the west, the Indo-Australian Plate in the east and the Antarctic Plate in the south. The triple point is so named as the three plates meet at latitude 19 º South just to the East of the island of Rodrigues.

One important factor to bear in mind is that the plates are divergent plates as they constitute oceanic spreading centers. Hence magmatic upwelling and temperature patterns around the triple junction are influenced by the relative divergence of three lithospheric plates.

The ridge has been in evolution since 8 million years. Therefore, since more than 8 million years, the region east of Rodrigues has been affected by earth tremours and there are no indications that any one of them has triggered destructive tidal waves that have affected Rodrigues.

Earthquake events in the Rodrigues region

A set of maps issued by the American Geophysical Union which trace seismicity along the Mid Indian Ocean Ridge between latitude 15°S and 22°S clearly reveal the following:

a) almost all the low magnitude earthquakes are located on either side of the divergent plates margin quite far from Rodrigues and
b) the great majority of the earth tremours are less than magnitude 5 on Richter scale.

A magnitude of 5.0 is classified as a moderate earthquake which if occured near land can cause major damage to poorly constructed buildings and slight damage to well-designed buildings. Given the geophysical set up and the relative movements of the plates near the Rodrigues Triple Junction, the risk of a major earthquake with an epicenter on Rodrigues i.e. on land is very low indeed. However, given the present situation, it is highly advisable to take precautions and devise hard engineering coastal defense against any tidal waves that may affect the strip of coast from Pointe aux Cornes to Pointe Mapou .
Figure 2 shows the exact location of the earthquakes recorded from January to December 2009. Note the number of shallow tremours along the Mid Indian Ocean Ridge and near the Rodrigues Triple Junction. The shallow quakes  are associated with magmatic upwelling along divergent plates. Contrary to dangerous earthquakes which happen along convergent plate margins only two events have reached level 6 on the Richter scale.

Geophysicists and oceanographers studying movements and earth tremours along the Mid Indian Ocean Ridge argue that molten rock or rising magma from underneath the sea along divergent plates margins lubricates the faults and cracks which are common in these areas, reducing the amount of friction that could cause strong earthquakes to occur.

This explains the lower magnitude earth tremours in the region of Rodrigues.

To conclude on Rodrigues, it is good to point out that in order to monitor closely earth tremours in the region, a seismometer will be installed soon at Citronelle. The equipment will help detect and record tremours but will  not predict any forthcoming tremours.

What about Mauritius?

Earthquakes and volcanoes are natural  events that are very much linked. There has been talk these days of possible regain of volcanic activities in Mauritius. As a geomorphologist, I have carried intensive rersearch in the evolution of island volcanoes and more particularly in the geostructural evolution of Mauritius.

As such, I have always argued in national and international forum that our volcanoes are dormant and that a regain in volcanic activity is possible given the geophysical activities occuring beneath our island. Mauritius has been geologically constructed in five main phases and between each phase, there has been a period of calm ranging from half to one million years. Since our last lava emissions issuing from the L'Escalier volcano are dated only 20,000 years, a regain of volcanic activity is very much possible.

However, since our volcanicity is of the effusive type, there will be clear and visible pre eruption signs.

Various indicators are used to help predict forthcoming volcanic eruptions. We must be careful not to make hasty deductions based on only one observed parameter. There is a set of interrelated observed phenomena which should be used before we can say for sure that a volcanic eruption is imminent. In addition, the matter gets more complicated as pre eruption indicators tend to be highly individual to a volcano's particular internal structure.

For an alert geomorphologist, volcanic eruptions can be predicted by catching early symptoms. Therefore, continuous monitoring of dormant volcanoes, as is the case for Mauritius, is the only way to enable eruptive behavior forecasts. I recommend the following methods which are usually adopted in monitoring volcanoes and the symptomatic evolution of their activity:

1. The most reliable sign of a volcanic eruption is the number of small earth tremors around the vent of the dormant volcano.
2. Another sign is the emissions of a set of gases.
3. As magma rises slowly, it causes deformation of the cone and causes cracks on the sides. The cracks become visible.

Magma contains dissolved gases that are released into the atmosphere during eruptions. Gases are also released from magma that either remains below ground or is rising towards the surface. In such cases, gases may escape continuously into the sub soil and contaminate underground water by lowering its pH.

Taking the example of the Hawaiian active volcanoes of Kilauea, Mauna Loa and Mauna Kea of main Island and Haleokala of Maui, upwelling magma  release small amounts of gases, including sulphur dioxide (SO2), hydrogen sulphide (H2S), hydrogen (H2), carbon monoxide (CO), hydrogen chloride (HCL), hydrogen fluoride (HF), and helium (He).

In case these gases, even in low concentrations, are released into the atmosphere, they can cause immediate irritation of the nose, throat, eyes and moist skin within minutes. These irritations are therefore signals of upwelling gases. To verify the imminence of volcanic eruption, we must also look for and consider other parameters.

When gases associated with upwelling of magma from reservoirs below our volcanoes reach upper levels, they are expected to contaminate our underground water. The result is an increase in the water acidity reflected in lower pH values. This brought me to have a look at the chemistry of samples of our underground as scientifically analysed by the C.W.A. The water samples studied are from the N 20 ° orientation i.e along the central uplands where all our 23 volcanoes are located.and water tests were carried out repeatedly between January 2008 and March 2009.

The overall results show that pH ranges from 6.24 to 7.44 (Hollyrood), 5.77 to 6.94 (Moka), 6.28 - 7.01 (Mare -aux- Vacoas), 6.2 - 7.4 (La Marie) to quote just a few examples. The only factor worth mentioning is the relatively moderate amount of free carbon dioxide in the water samples from Moka. This is but an isolated case and no respected scientist will come forward with a hasty conclusion based on one single isolated case.

In conclusion

Based on the above geophysical consideration, on magmatic evolution on Réunion Island and on the past history of recorded earth tremours, especially in the Rodrigues region, the following deductions may be safely forwarded :

· Magmatic upwelling along the vent of La Fournaise active effusive volcano of Réunion Island will go on triggering earth tremours of low magnitude. At times, cauldron subsidence may cause slightly higher magnitude magmatic tremours.The risk that these will affect any one of the islands is low.
· Mauritius will every now and then feel the pulses vibrating from Réunion Island and since the island is located far from the Mid Indian Ocean Ridge, we are safe as far as earth tremours are concerned.
· Rodrigues needs to be given special attention. Although the epicenters of earth tremours  will be deep (10 km or more) in the sea towards the east, it is advisable to protect the eastern segment of the already fragile coastal environment from any land bound abnormal waves.

Prem Saddul - Geomorphologist