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

Spatio-temporal evolution of the seismic phenomenon underway in Mayotte

Auteurs : D. Bertil, A. Lemoine, A. Roullé (BRGM) - 11.23.2018
This note reviews changes over space and time in the seismic sequence currently occurring in Mayotte and reports on advances in understanding its causes up to mid-November 2018.

Distribution of the seismic sequence over time

The seismic sequence that has affected Mayotte since 10 May 2018 is continuing. However, observed magnitudes have lessened in the last few months (the last earthquake of more than magnitude 5 occurred on 27 June 2018). This indicates that the seismic energy being released is on the wane, although some earthquakes have been felt by the population since July (26 August, 18 September, 16 October and 7 November 2018).

Distribution over time and magnitudes of the seismic events detected by the BRGM since the start of the seismic crisis in Mayotte.

Distribution over time and magnitudes of the seismic events detected by the BRGM since the start of the seismic crisis in Mayotte.

Cumulative seismic moment since the start of the seismic crisis in Mayotte.

Cumulative seismic moment since the start of the seismic crisis in Mayotte. The seismic moment is a measurement of the energy of an earthquake (expressed here in Newton metres). The graph shows the cumulative energy released by all the seismic events since May 2018. A seismic event releasing all this energy at once would equate to an earthquake of magnitude 6.4. We see here that the energy released by the 15 May earthquake (magnitude 5.8) represents less than one quarter of the energy released to date during the entire swarm period. In terms of energy, activity has weakened considerably since the end of June 2018, with a cumulative release of energy of only 0.5 E+18 N.m over 5 months as against 4 E+18 N.m during the first month of activity. While less energy has been released in the last few months, numerous earthquakes are still occurring daily but at low magnitudes.

Location of the seismic events

Since the end of August 2018, the locations of the earthquakes as determined by BRGM seismologists show that the active zone has moved, with a second patch of seismic activity appearing about 40 km to the north-east of Mamoudzou. This active zone, which did not appear at the start of the crisis, is where most of the seismic activity since the end of September has occurred, reaching a magnitude of 4.8 on 16 October and 7 November 2018, the highest observed since the end of June.

Changes over time of the distance from the YTMZ station (Mamoudzou, Mayotte) of the hypocentre of earthquakes of magnitude 3.5 or more.

Changes over time of the distance from the YTMZ station (Mamoudzou, Mayotte) of the hypocentre of earthquakes of magnitude 3.5 or more. Three distinct sequences can be seen: the main sequence with the strongest earthquakes extends from 10 May to end June. The seismic activity ranges across a distance varying from 45 to 60 km, the longest distance being observed around the end of June. The pattern changes in late June to early July, with seismic activity observed over 45-50 km. It then continues regularly within this range of distances until the end of September. From 23 August, the range of seismic activity changes again to around 37 km, a pattern not previously observed. Since early October, seismic activity has essentially been concentrated in this zone.

Location of earthquakes above magnitude 3.8 since the beginning of the episode up to 29 October (Day 0 = 10 May 2018; Day 180 = 5 November 2018).

Location of earthquakes above magnitude 3.8 since the beginning of the episode up to 29 October (Day 0 = 10 May 2018; Day 180 = 5 November 2018). The earlier seismic events are shown in mauve and blue; more recent events are in orange and red. The cloud of dots around 45.5°E began to appear in late August 2018. While the current seismic network accurately detects seismic events above magnitude 3.5, it cannot produce the precise location of these events. The degree of uncertainty is 10-15 km for an earthquake swarm barely 20 km in extent. Consequently, the location uncertainties are too high to indicate the main direction of the seismic activity. The depth of seismic activity is not determined by the observation network as currently configured. © BRGM / Bathymetry HOMONIM, SHOM et GEBCO (2014)

Advances in knowledge about the causes of the earthquake swarm

New information from GPS data available for the island (IGN) after processing by a team from the Geology Laboratory of the Ecole normale supérieure in Paris show that significant movements have occurred on  Mayotte since July 2018 (see the summary note drafted by Pierre Briole). These movements were not detectable at the start of the crisis and cannot be accounted for by a tectonic source alone. They show - significantly so for the first time - the presence of a volcanic component among the causes of the current seismic sequence.

On 11 November 2018, an atypical low frequency monochrome signal, with a period of about 17s (estimation by A. Lomax, Twitter), was detected by the international networks, including at very long range, that did not correspond to a high-magnitude earthquake (CTBTO, Twitter). The signal was also visible at the Chiconi station at a time when small seismic events appeared (magnitude of around 3). Signals of this type are characteristic of volcanic phenomena.

Seismic signals on 11/11/18, 09:30, at different stations used to locate earthquakes in Mayotte.

Seismic signals on 11/11/18, 09:30, at different stations used to locate earthquakes in Mayotte. Three small seismic events can be seen at the YTMZ accelerometric station and a superimposed low-frequency signal at the MCHI broadband station, also visible at more distant broadband stations (SBC, SBV, ABPO).

These observations 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.

In the last few months, the scientific community has joined forces to understand the phenomenon and provide answers to the questions it is raising. Possibilities are being investigated for deploying new instruments on land and at sea to improve earthquake detection and location. In parallel with this observation mission, an oceanographic campaign has been applied for to explore both the swarm zone between Mayotte and Madagascar and, more broadly, the Comoros region as a whole.

TO FIND OUT MORE

Read our news bulletin on the Mayotte earthquake swarm

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