View over the Maars de Moya on Petite-Terre (Moya, Mayotte, 2012). © BRGM - Dominique Tardy
 

Earthquake swarm in Mayotte: Scientific FAQ

11.27.2018
In these FAQ, the BRGM presents the knowledge it has collected on the earthquake swarm that began in Mayotte on 10 May 2018. They correspond to a preliminary analysis and supplement the FAQ published by the Mayotte Prefecture on its website.

What is an earthquake swarm?

An "earthquake swarm" is an episode of multiple earthquakes that occur in a particular zone over several days or weeks, as opposed to the usual pattern of one main shock followed by several smaller aftershocks. Because of their nature as a "swarm", these episodes cannot be characterised in terms of magnitude and duration.

Have there been other earthquake swarms in the world?

These phenomena are quite common and have been recorded in many regions of the world. Seismic swarms also occur in mainland France, for example in late 2017 in the Maurienne area. Earthquake swarms are not the same as the seismic sequences associated with several major and successive earthquakes (of comparable magnitude), each of which is followed by aftershocks. Those closest to Mayotte were observed in 1985 to the north of the Davie Ridge (offshore from Mozambique), then in 1989 and 2005 in Ethiopia (Afar region), in tectonically active zones associated with different rift systems.

Have these swarms produced any major earthquakes?

The earthquake swarms observed so far do not always reach their maximum at the same time. For example:

  • Sequence to the north of the Davie Ridge (1985): a severe magnitude 6.3 earthquake occurring at the start of the sequence was followed many smaller quakes.
  • Dôbi seismic crisis (Afar, 1989): a magnitude 6.2 earthquake occurred at the start of the crisis, followed over 48 hours by a succession of magnitude ≥6 earthquakes (6 altogether), each producing aftershocks. This is referred to as a seismic sequence.
  • The Manda-Hararo Dabbahu crisis (Afar region, volcano-tectonic episode in 2005-2010): a magnitude 5.6 earthquake occurred 6 after the start of the sequence. Other magnitude 4.9 to 5.4 earthquakes occurred as the sequence continued.

Could Mayotte experience an earthquake of greater magnitude than those already recorded? 

The record for Mayotte until now was the magnitude 5.2 earthquake in 1993. This has been substantially exceeded during the current sequence. The 1993 earthquake has been used as a reference for hazard studies and to define the seismicity zone officially. The entire département of Mayotte is within the "moderate" seismicity zone (3rd in rank in the French seismic zoning plan). This means, as in other "moderately" seismic zones in France (Oléron, Vendée, Massif central), that only "moderate" seismic observations are provided for, a fact that also accounts for the limited extent of current knowledge on local seismicity.

Today, we no longer have a local reference with which to define a possible maximum magnitude. It is therefore not possible to exclude the occurrence, during this swarm or any subsequent episode, of an earthquake of greater magnitude than those already observed.

Read the report on "Predicting the next destructive earthquake" ("Prédire où et quand aura lieu le prochain séisme destructeur") on the Plan Séisme website (in French)

Could these earthquakes form a new volcano?

Volcanic eruptions or the formation of dykes (when lava or magmatic rock rises through a fissure) have been observed following earthquake swarms, but they do not occur systematically.

At this stage, volcanic-tectonic activity is one of several working assumptions considered by the scientists studying this seismic crisis.

A tectonic origin is certain because hypocentre mechanisms have been described, showing mechanisms at the source that are typical of ruptures along faults.

The volcanic component is under study thanks to additions to the observation network and new measurements.  New information from the processing of GPS data available for the island (IGN data) by a team from the Geology Laboratory of the Ecole normale supérieure de Paris show that  significant movements have occurred on  Mayotte since July 2018. These movements were not detectable at the start of the crisis and cannot be accounted for by a tectonic source alone (in other words by the consequences of the seismic events in the Mayotte region only since May 2018). They show, for the first time since the start of the crisis, the presence of a volcanic component among the causes of the current seismic sequence (see the summary note by Pierre Briole).

In parallel, on 11 November, an atypical very low frequency signal was detected by the international networks. The signal was also visible at the Chiconi seismic station. Signals of this type are characteristic of volcanic phenomena.

These observations therefore back up the hypothesis of a combination of tectonic and volcanic effects accounting for a geological phenomenon involving a seismic sequence and a volcanic phenomenon. This hypothesis will need to be confirmed by future scientific studies and different types of observations.

If an eruption should occur in the swarm zone, 50 km offshore and at a depth of 3500 m, no significant impacts for Mayotte are considered likely at present, although these aspects will require additional study once the phenomenon now under way has been better characterised.

Could a tsunami occur?

An earthquake can cause a tsunami when strong earthquakes, typically more than 6.5 in magnitude, combine with vertical movement of the ocean floor caused by rupturing to form waves. These conditions do not appear to be present together in Mayotte as the movements triggered by the earthquakes observed seem to be essentially horizontal ("strike-slip" faulting). 

Tsunamis can also be triggered when submarine landslips are destabilised by earth earthquakes. Some seismic events such as those currently observed have triggered moderate tsunamis in other parts of the world with a steeply sloping ocean floor, but these were associated with stronger earthquakes.

Furthermore, the configuration of the ocean floor around Mayotte (which plunges very steeply beyond the coral barrier) is not conducive to the amplification of tsunami waves.

How long is this earthquake swarm likely to last?

The duration of a swarm is impossible to predict. The measurements now being taken give the number and characteristics of the earthquakes occurring, but there is no law that can predict when the phenomenon will end.

In other cases of earthquake swarms, seismic activity has gradually lessened over several consecutive days. The fact that fewer earthquakes are observed over one or two days cannot be interpreted as indicating the end of the phenomenon.

Earthquake swarms described as examples in the literature (bibliographic study under way) and located in volcanic-tectonic contexts similar to that in Mayotte last from a few hours to several days. We have not yet identified any swarms of similar magnitudes or lasting for such a long time. Earthquake swarms in other contexts (in the Alps, for example) can last for a long time but with smaller magnitudes.

Where are the earthquakes located, and are they moving closer to Mayotte?

The location of the these seismic events is determined by analysing the signals measured by the monitoring stations. The BRGM's measurements locate them to within 10km and show that the epicentres are in a zone about 20 km in diameter some 50 km to the east of Mamoudzou: earthquakes have been located at distances ranging from 26 to 65 km from the YTMZ station in Mamoudzou.

Why do different institutes give different magnitudes and locations?

The BRGM estimates earthquake locations and magnitudes from 9 stations within 1400km from the epicentres, including three in Mayotte (40 to 50km distant from the epicentres) and two in Grande Comore (about 270 km from the epicentres). The international networks use more distant seismic monitoring stations, the nearest being 480km from Mayotte. The uncertainties are therefore greater.

In practice, the reliability of estimations depends on the magnitude of the earthquakes:

  • Earthquakes below magnitude 4.5 are not detected by the international networks, whereas the BRGM networks can detect earthquakes from magnitude 3.5.
  • Because of their configuration, the international network stations cannot produce reliable estimations of the location and magnitude of earthquakes of less than magnitude 4.9.
  • The differences between estimations become less and less significant above magnitude 5.0.

Why are earthquakes strongly felt even when they are moderate?

How a earthquake is felt depends at once on its characteristics, its effects on the spot and individual perceptions.  The following factors come into play:

  • the rupture mechanism at source: waves produced by earthquakes of the same magnitude can be amplified more strongly in the direction of the rupture (comparable to the Doppler effect). In a swarm, ruptures can be of different types. Further studies on this point are being considered;
  • the effects of wave propagation can differ geographically: North-South differences in perception are known in the Caribbean, but not documented in Mayotte;
  • local site effects can amplify ground movements depending on the topography or the nature of the soil.  These effects are known and documented in Mayotte;
  • effects due to the building where the earthquake is felt: amplification at higher levels, responses of the building and furnishings;
  • individual factors: it is recognised that perceptions differ with individuals and with the same individual depending on conditions: day/night, emotional state, activity, etc.

What are the causes of the earthquakes now occurring?

Seismic events occur regularly and quite frequently along the 500 km of the Comoros archipelago, including earthquakes of magnitude 5 or more across the zone as a whole. The current episode is therefore in line with the fairly moderate seismic activity known to occur in the Mozambique Channel.

In the current state of knowledge, the seismic events now occurring can be attributed to a combination of dynamic geological phenomena:

  • Tectonic causes: due to the relative movements of tectonic plates, some sectors in the region experience active seismicity along major structures, especially the East African Rift and the Davie Ridge to the west of the Comoros. Active tectonic structures associated with the Comoros Archipelago are still poorly documented, but the seismic events recorded testify to ruptures along their faults.
  • A volcanic component: this hypothesis is backed up in particular by studies of movements on the surface.

Observations therefore back up the hypothesis of a combination of tectonic and volcanic effects accounting for a geological phenomenon involving a seismic sequence and a volcanic phenomenon. This hypothesis will need to be confirmed and refined by further data acquisitions and future scientific work.

TO FIND OUT MORE

Read the FAQ and safety advice distribute by the Mayotte Prefecture (in French)

BRGM - 3 avenue Claude-Guillemin - BP 36009 45060 Orléans Cedex 2 - France Tel.: +33 (0)2 38 64 34 34