Evolution of shallow volcanic seismicity in the hydrothermal system of La Soufrière de Guadeloupe following the April 2018 Mlv 4.1 earthquake

La Soufrière volcano in Guadeloupe presents a seismo-volcanic (VT) activity associated with an active hydrothermal system. This microseismicity is principally shallow, produced by repeating earthquakes and triggered in swarms. Four recurrent families of VT repeaters are detected, including a main family accounting for more than 80% of the catalog since at least 2014.

By stacking seismic waveforms of repeaters and using a temporary dense seismic network, we build a MASTER event with a high Signal-to-Noise Ratio (SNR) and we better constrain the absolute location of the main MASTER event.

We report positive residuals between observed and predicted P-wave arrival times at nearly all stations, suggesting that the velocity model of the shallow part of the dome could be improved. We significantly lower these residuals by raising the P-wave velocity from 2 to 2.7 km/s and reducing the Vp/Vs ratio from 1.8 to 1.69, leading to an improved local velocity model.

We then locate each VT event relatively to is own MASTER hypocenter and image the hydrothermal seismic activity along a sub-vertical conduit, beneath the Tarissan crater acid lake found at the summit of Soufrière.

We also define a linear relationship between the peak amplitude of seismic events and their duration to obtain a pseudo local magnitude. The approach allows us to automatically and accurately estimate the magnitude of each event at the detection stage.

The April 2018 earthquake (MLv 4.1), the largest since the last phreatic eruption in 1976-77, occurred 2 km northwest of the summit and generated an increase in the number of events and seismic energy released. This event also resulted in the emergence of a secondary significant VT family during the summer 2018, located above the first one. We show that the increase of shallow microseismicity, following the April 2018, is likely explained by dynamic damage of the hostrock below the dome, thanks to the analysis of the associated distribution of Coulomb stress variation and relative velocity variations.

Finally, using a statistical approach, we detect periodicities in the number of events and the released seismic moment at La Soufrière. A dominant peak of seismic activity is observed in October-November and a second lower peak is detected in April.