Detection, location and characterization of VLF events during the 2018-2020 seismovolcanic crisis in Mayotte

Abstract:

The seismovolcanic crisis of Mayotte (Comoros Archipelago) started in May 2018 with a sudden activation of seismicity to the east of the island. Along with thousands of volcano-tectonic earthquakes, an energetic very low frequency signal (VLF, 15 seconds of period) was recorded on November 11, 2018 at seismic stations worldwide. This exceptional monochromatic signal was not an isolated event: in this study we detected and analyze several hundredths of such signals, all along the duration of the Mayotte crisis.

We used a time-frequency detector based on the filtered signal spectrogram. The data comes from local and regional stations in Mayotte, Comoros and Madagascar, and from OBH (Ocean Bottom Hydrophones) installed during marine campaigns aboard the R/V Marion Dufresne II, between May and July 2019 [https://doi.org/10.18142/291], and shorter campaigns since July 2019.

Event location is performed though spatial 3D back-projection of station-pair cross-correlation functions [Poiata et al., 2016], assuming surface-wave speed. Further work is made to confirm the location of the VLFs compared to the seismic swarm and Mayotte island.

While the VLF epicentral location is reasonably well constrained (within 10 km) by the back-projection analysis, depth resolution is poor. For this reason, we explore VLF depth and focal mechanism from direct modelling of body and surface waves at regional and teleseismic stations.

To study the possible physical origin of such signals, we compare different types of source models, such as the resonant cavity and the fluid-filled crack model.