(Left) Map of area of the November 11 Mayotte event (called MVLP), and locations of Iranian broadband seismic stations (triangles). The objective of our study is to characterize the seismic signal and its source by analyzing seismic data recorded by the Iranian National Broadband Seismic Network: in particular, we aim at (1) evaluating its spectral properties, (2) estimating the magnitude, and (3) assessing the occurrence of an earthquake just before the event for possible interaction that may have triggered the event.

In this study, we investigate this event using the waveforms recorded by the Iranian National Broadband Seismic Network.

The event has been described in the framework of underwater volcanic activity, as the result of the resonance of a deep magma reservoir, in response to the sagging of the reservoir roof (Cesca et al. We call the November 11 event the “monotonic very-long-period” signal and abbreviate it as MVLP. Thus, there is still much to learn about these very-long-period (VLP) signals occurring during the Mayotte event. Moreover, several smaller tremors with similar characteristics were recorded during the 2018–2019 Mayotte seismo-volcanic crisis (Cesca et al. The long-period waves appeared for more than 30 min at both the regional and teleseismic distances. The most obvious feature of the Mayotte event is that while large long-period monotonic surface waves are visible, body waves are absent. The waveforms show clear differences in the nature of the propagated waves between the regular earthquake and the Mayotte event. The epicenters of these two events were located very close to each other, and the epicentral distances were 7 km from the ABPO and ASAO stations, respectively. Figure 1 shows the recorded waveforms for the May 15, 2018, earthquake, and the Mayotte event at a regional station, ABPO, Ambohimpanompo, Madagascar (part of the global IRIS/IDA seismic network), and at a teleseismically distant station, ASAO, Ashtian, Iran (part of the Iranian National Broadband Seismic Network).

The biggest earthquake ever recorded in the area was a Mw 5.9 event that happened on (USGS). But in the year following the Mayotte volcano-seismic sequence on May 10, 2018, 32 earthquakes with a magnitude greater than 5 were recorded (Lemoine et al. Before May 2018, the seismic activity in this archipelago was dispersed and moderate, with only a few earthquakes having a magnitude greater than 4. The November 11 Mayotte event, in the absence of body waves, caused large, long-lasting, monotonic long-period surface waves that traveled around the globe. Mayotte Island is one of the four main islands in the volcanic Comoros archipelago. The event occurred offshore around 9:28 UTC near Mayotte Island in the Mozambique Channel between the northern tip of Madagascar and the eastern coast of Africa. The November 11, 2018, Mayotte event was first introduced in the media by Wei-Haas ( 2018) in National Geographic as a strange earthquake in which seismic waves were recorded by instruments around the world, but, unusually, nobody felt shaking. The difference between the arrival times of the recorded surface waves of the two events is estimated to be less than 31 s, and most likely of ~ 7 s only. This observation may suggest a regular earthquake of Ms 4.3 ± 0.11 that triggered the November 11, 2018, event. Just before the monochromatic signal arrives, there is some dispersion in the surface waves.

A surface wave magnitude of Ms 5.07 ± 0.22 was estimated. The average apparent phase velocities were calculated as 3.31 and 2.97 km/s, in the transverse and radial directions, corresponding, respectively, to Love and Rayleigh waves in the frequency range of 0.05–0.07 Hz. This is in agreement with the theoretical radiation pattern of Rayleigh and Love waves at a frequency of 0.06 Hz for a vertical compensated linear vector dipole source mechanism. The maximum amplitude of the transverse components is less than a half of the radial components. Frequency analysis of the seismograms shows a clear peak at 0.064 Hz (15.6 s/cycle). The arrival of monochromatic long-period signals is visible around 10 UTC in the recordings of all the stations and the signals lasted for more than 30 min. The stations are located at epicentral distances ranging from 4542 to 5772 km north-northeast of the event’s epicenter. The purpose of this study is to investigate this unusual event using the waveforms recorded by 26 stations of the Iranian National Broadband Seismic Network. This event is unusual due to the absence of body waves in the seismograms and no feeling of earth shaking by people locally. This event occurred at around 09:28 UTC east of the Mayotte Island, in the Indian Ocean off the coast of East Africa. On November 11, 2018, an event generating long-lasting, monotonic long-period surface waves was observed by seismographs around the world.

Seismac waves