Observation of the long-period monotonic seismic waves of the November 11, 2018, Mayotte event by Iranian broadband seismic stations

Hossein Sadeghi,Sadaomi Suzuki

doi:10.1186/s40623-021-01408-1

Abstract

On November 11, 2018, an event generating long-lasting, monotonic long-period surface waves was observed by seismographs around the world. This event occurred at around 09:28 UTC east of the Mayotte Island, in the Indian Ocean off the coast of East Africa. This event is unusual due to the absence of body waves in the seismograms and no feeling of earth shaking by people locally. 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. The stations are located at epicentral distances ranging from 4542 to 5772 km north-northeast of the event’s epicenter. 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. Frequency analysis of the seismograms shows a clear peak at 0.064 Hz (15.6 s/cycle). The maximum amplitude of the transverse components is less than a half of the radial components. 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 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. A surface wave magnitude of Ms 5.07 ± 0.22 was estimated. Just before the monochromatic signal arrives, there is some dispersion in the surface waves. This observation may suggest a regular earthquake of Ms 4.3 ± 0.11 that triggered the November 11, 2018, event. 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.

Highlights

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
Because the interpretation of the Monotonic very-long-period (MVLP) triggering mechanism may differ depending on the accuracy of the delay time between the just before the monochromatic event” (JBE) and MVLP, we examined the record of the ABPO broadband station in Madagascar (Fig. 1)
We studied the Mayotte MVLP, which occurred on November 11, 2018, using the records of the Iranian National Broadband Seismic Network

Summary

Introduction

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. Mayotte Island is one of the four main islands in the volcanic Comoros archipelago. 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. Before May 2018, the seismic activity in this archipelago was dispersed and moderate, with only a few earthquakes having a magnitude greater than 4. In the year following the Mayotte volcanoseismic sequence on May 10, 2018, 32 earthquakes with a magnitude greater than 5 were recorded

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Conclusion