Spatio-temporal evolution of the seismic phenomenon underway in Mayotte

Auteurs : D. Bertil, A. Lemoine, A. Roullé, I. Thinon (BRGM) - 07.08.2019
This note reviews the spatial and temporal changes in the seismic sequence currently occurring in Mayotte and unfolding knowledge of its causes up to the end of June 2019.

Distribution of the seismic sequence over time

The seismic sequence that has affected Mayotte since 10 May 2018 is continuing. However, observations over several months have shown that the magnitudes have lessened. This indicates that the seismic energy released has weakened since the start of the crisis, although some tremors are still being felt by residents.

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.

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

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

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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 be the equivalent of an earthquake of magnitude 6.5. We see here that the energy released by the 15 May earthquake (magnitude 5.8) represents less than one tenth 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 1.7 E+18 N.m over 12 months as against 4.7 E+18 N.m during the first month of activity. While less energy has been released in the last few months, numerous tremors are still occurring daily but at low magnitudes.

Location of the seismic events

Since the end of August 2018, the locations of the tremors 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 on 7 November 2018, the highest observed since the end of June.

 S-P (S-P=4.0s -> Hypocentral distance ~35 km; S-P = 5.5 -> distance ~48 km)

Changes over time in the time difference between wave S and wave P measured at station YTMZ (Mamoudzou, Mayotte) for earthquakes of magnitude greater than 3.5. This S-P time difference can be directly related to the hypocentral distance: S-P (S-P=4.0s -> Hypocentral distance ~35 km; S-P = 5.5 -> distance ~48 km)

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Three distinct sequences can be seen: the main sequence with the strongest earthquakes extended from 10 May to the end of June. The hypocentral distance of the seismic activity ranged from 45 to 60 km, with the longest distances being observed around the end of June. The pattern changed in late June to early July, with seismic activity observed at a distance of 45 to 50 km. It then continued regularly within this range of distances until the end of September. From 23 August, the hypocentral distance of seismic activity changed again to around 35 km, a pattern not previously observed. Since early October, seismic activity has essentially been concentrated in this zone.

Location of earthquakes with magnitude greater than 3.8 since the beginning of the episode up to 29 October (Day 0 = 10 May 2018; Day 360 = 5 May 2019). The earlier seismic events are shown in mauve and blue; more recent events are in orange and red. The point cloud around 45.5°E appeared in late August 2018. While the seismic network up until May 2019 accurately detects seismic events with a magnitude of more than 3.5, it cannot determine the precise location of the events. The depth is arbitrarily set at 10 km. 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 installation of additional real-time acquisition stations in Mayotte (MTSB, PMZI and KNKL in the figure) from May 2019 and the acquisition of sea-floor seismometer data from the MAYOBS scientific programme should enable more precise locations and an estimate of earthquake depths. BRGM / HOMONIM, SHOM and GEBCO bathymetry (2014)

Location of earthquakes with magnitude greater than 3.8 since the beginning of the episode up to 29 October (Day 0 = 10 May 2018; Day 360 = 5 May 2019). The earlier seismic events are shown in mauve and blue; more recent events are in orange and red. The point cloud around 45.5°E appeared in late August 2018. While the seismic network up until May 2019 accurately detects seismic events with a magnitude of more than 3.5, it cannot determine the precise location of the events. The depth is arbitrarily set at 10 km. 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 installation of additional real-time acquisition stations in Mayotte (MTSB, PMZI and KNKL in the figure) from May 2019 and the acquisition of sea-floor seismometer data from the MAYOBS scientific programme should enable more precise locations and an estimate of earthquake depths. BRGM / HOMONIM, SHOM and GEBCO bathymetry (2014)

 

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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 de Paris show that  significant movements have occurred in the  Mayotte region 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 detectable at the Chiconi station at a time when small seismic events occurred (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. Three small seismic events could be seen at the YTMZ accelerometer station and a superimposed low-frequency signal at the MCHI broadband station, also detectable at more distant broadband stations (SBC, SBV, ABPO).

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

In the last few months, the scientific community has joined forces to investigate the phenomenon and provide answers to the questions raised. New instruments on land and at sea (part of the INSU-CNRS TELLUS and SISMAYOTTE project) were deployed to improve earthquake detection and location.

The acquisition of new data during the MAYOBS oceanographic campaigns (INSU-CNRS TELLUS project) confirmed the hypothesis of a combination of tectonic and volcanic effects to explain the seismic crisis in Mayotte. A recent active volcanic edifice was observed directly above the May 2018 swarm. The last MAYOBS campaign showed that effusions had occurred to the south of this new volcano from May to June 2019. Signs of seismic activity were recorded along the north-facing 110° ridge from Petite-Terre to the new volcano.

In parallel with these observation and acquisition missions, research projects and new, more regional data acquisitions are being developed.

TO FIND OUT MORE

Read our news bulletin on the Mayotte earthquake swarm