An earthquake of Magnitude 6.7 (M6.7) which struck in the struck in the Indian Ocean was not among the world’s most significant events of the year, but nevertheless is worthy of comment given its geological and tectonic context. The earthquake, which, according to USGS data, took place on July 25th, 2012 around 250 miles north east of Mauritius occurred at a mid-ocean ridge – a setting where most earthquakes are of relatively small magnitude compared with elsewhere.
The Tectonic Setting of the Mauritius/Reunion Earthquake
The Earth is composed of many large slabs of crust (tectonic plates) which move very slowly relative to one another. New crust is generated where hot molten rock moves upwards from the earth’s interior along rift zones thousands of miles long, almost all in the deep oceans. Earthquakes are associated with these zones as rocks are pushed apart.
The July 2012 Indian Ocean earthquake occurred in just such a setting. Here, new crust is generated as the African and Australian plates move apart: the movement of molten rock to the surface causes movement and earthquakes are typically shallow. USGS data show that the July earthquake, which occurred at a depth of around 10km, was typical of mid ocean ridge earthquakes in this respect.
A glance at a map of seismicity in the area shows that all of the recorded earthquakes occurring along the relevant section of the ridge since 1990 have had a depth of less than 35km and that the majority of them have a magnitude of less than M6.0, making the July tremor significant.
Mid-Ocean Ridge Earthquakes
The magnitude of earthquakes on subduction zones is controlled by various factors. At ocean ridges, the rock is hot and more malleable, making it less likely to accumulate strain. Furthermore, ocean ridges are broken into segments and earthquake magnitude is known to be dependent upon the length of these segments. Work by Clark et al, however, indicates that at ocean ridges, this applies only up to a maximum magnitude of around M7.0, a size beyond which segment length has no influence.
Against this background, it is evident that the July 2012 earthquake is a very large one for its location. It is not, however, the largest ocean ridge tremor on record: this took place in 1942 in the Pacific Ocean. Although there’s some doubt about its actual magnitude, this quake was recorded as having a magnitude of M7.7. Though this might be considered a significant event, it is by no means a major earthquake in world terms – the largest earthquakes at subduction zones, for example, can exceed M9.0.
Clark, A., Wetzel, L. R., Frohlich, C. Earthquake Magnitude and the Length of Mid-Ocean Ridge Segments. (2005). American Geophysical Union, Fall Meeting 2005. Accessed July 26, 2012.
Okal, E.A. and Stein, S. The 1942 Southwest Indian Ocean Ridge Earthquake: largest ever recorded on an oceanic transform. (1987). Published in Geophysical Research Letters. Accessed July 26, 2012.
United States Geological Survey. Magnitude 6.7 – MAURITIUS – REUNION REGION. (2012). Accessed July 26, 2012.