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 31В s, and most likely ofвЂ‰~вЂ‰7В s only. This observation may suggest a regular earthquake of Ms 4.3вЂ‰В±вЂ‰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 5.07вЂ‰В±вЂ‰0.22 was estimated. The average apparent phase velocities were calculated as 3.31 and 2.97В km/s, in the transverse and radial directions, corresponding, respectively, to Love and Rayleigh waves in the frequency range of 0.05вЂ“0.07В Hz. This is in agreement with the theoretical radiation pattern of Rayleigh and Love waves at a frequency of 0.06В 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 0.064В Hz (15.6В 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 30В min. The stations are located at epicentral distances ranging from 4542 to 5772В km north-northeast of the eventвЂ™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. Long-term studies of active volcanoes could reveal how changes in the size and shape of those peaksвЂ™ magma reservoirs correlate with eruptions.On November 11, 2018, an event generating long-lasting, monotonic long-period surface waves was observed by seismographs around the world. The tubes that occasionally channel molten rock to the surface during eruptions are too small to be discerned by the analyses, the scientists report, but the volcanoвЂ™s magma chamber, which lies at least 500 meters below the surface, shows up clearly. The seismic data from those blasts revealed that a large blob of magma-which in some places slowed down seismic waves by as much as 1 kilometer per second-lies beneath the northwestern slope of the volcano. During the 2008вЂ“2009 field season, the team set off a dozen small blasts on or near the peak, which was surrounded by a roughly 4-kilometer-by-4-kilometer network of 23 seismic instruments that had been deployed the previous summer.

On an even smaller scale and using a small network of a few dozen seismometers, researchers mapped out parts of the plumbing system beneath Mount Erebus, a volcano in Antarctica ( 6).

The seismac waves of an earthwuake