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Earth tremors: is Rodrigues really at risk?

8 novembre 2015, 11:53

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Recent concerns of Rodriguan and Mauritian authorities regarding the latest earth tremors near Rodrigues and possible aftermath effects prompt me to make a scientific assessment of the phenomenon in order to clarify certain issues. 

 

One major planetary submarine geo-structural feature of the Indian Ocean floor is the Mid Indian Ocean Ridge (MIOR) and the Triple Junction where three major tectonic plates converge. They are the Indo-Australian Plate, the Antarctic Plate (Figure 2) and the African Plate which has a subset known as the Somali Plate which carries the Mascarene Islands. One important factor to bear in mind is that the plates are divergent plates i.e. they move away from each other leading to magmatic upwelling and higher temperature patterns and as such do not trigger destructive earthquakes like the ones which occur along the margins of convergent Plates. 

 

The Triple Junction meets at latitude 19º South, just around 300 km to the East of the island of Rodrigues. This is why that specific area is also known as the Rodrigues Triple Junction. 

 

Analysis of multi beam bathymetric data collected during several seafloor expeditions between the Triple Junction from 68° E to the Rodrigues Triple Junction reveal that the ridge has been in evolution since 8 May. It has also been deducted from scientific expeditions conducted by IFREMER Cruise in 1999 that horizontal component of strain increases dramatically between 69°30 E and the Rodrigues Triple Junction (RTJ). The magmatic upwelling and the resulting tectonic forces lead to deformation of the older crust of the Mid Indian Ocean Ridge. At times the forces lead to rupture generating minor tremors inside the sea floor at varying depth.

 

Figure 1 shows the Rodrigues Ridge as an E-W trending structure more than 600 km-long and 2500 m-high above the seafloor. Small volcanic ridges such as Three Magi and Gasitao ridges extend it further eastwards to the Mid Indian Ocean Ridge at latitude 19° S,  i.e. latitude of Rodrigues island.

 

Figure 1. Rodrigues Ridge and the Mid Indian Ocean Ridge (MIOR). Notice the two small ridges east of Rodrigues. 

 

Tremor events in the Rodrigues region.

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

 

a) with the upwelling of magma along the divergent plates, earth tremors do occur as the magma forces it way upwards. They are categorised as earth tremors due to the lower intensity as the great majority of the earth tremors are less than 5 on Richter scale; 

 

b) almost all the low magnitude tremors are located on either side of the divergent plates margin quite far from Rodrigues.

Figure 2 shows the exact location of the tremors recorded from January to December 2009 and table 1 the monthly frequency of earth tremors during the same year.

 

 

Therefore, earth tremors with category 5 on the Richter scale in terms of magnitude are a common feature on the ocean floor east of Rodrigues. There are no indications that any one of them has triggered destructive tidal waves that have affected Rodrigues. 

 

A magnitude of 5.0 is classified as a moderate earthquake, hence the term “tremors” which if occurred near land can cause major damage to poorly constructed buildings and slight damage to well-designed buildings. The risk of a major earthquake with an epicenter on Rodrigues, i.e. on land, is very low indeed.

 

During a recent meeting of the American Geophysical Union held in 2011, Geophysicists argued that molten rock or rising magma from underneath the ocean 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 is a valid argument as shock waves travel at lower velocity through molten rocks.

 

So, the scale of the tremors is smaller because the hot rising magma warms up the fault lines and makes them more difficult to break the rocks. This explains the lower magnitude of earth tremors in the region of Rodrigues.

 

It is good to point out that in order to monitor closely earth tremors in the region, a seismometer installed in the region of Citronelle is essential. The equipment will help detect and record tremors but will not be able to predict the occurrence of such a phenomenon. 

 

For the Mascarene Islands, the danger of a tsunami hitting the coastal areas would be more likely to come from the southern and south eastern segment of the Indonesian subduction arc (Central Sumatra through to the Java Trench) where the curvature faces the group of islands of the South West Indian Ocean. 

 

However, our islands have an advantage. They are located some 5,000 kms from the tectonically active subduction zone and the time of travel (approx. 5-7 hours) would provide enough time for the islands to put forward their evacuation plans and in the process considerably reduce loss of life and property. 

 

The mitigating role of ocean currents along the equator and the topography of the Mascarene Plateau with an isobaths as low as – 147 m which may help deflect and decrease the impact of the tsunami waves on the coastal areas of the islands of the South West Indian Ocean is not to be underestimated.