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The February 2018 Seismic Swarm in the Island of São Miguel, Azores

Description: 

On February 2018, a seismic swarm occurred on the island of S. Miguel in the Azores archipelago. The Azores are located on the triple junction between the North American, Eurasian, and African plates and comprise 9 populated islands of volcanic origin. S. Miguel is the largest and most populated island and the one that is most seismically active. This work presents a detailed study of the February 2018 seismic swarm, the most recent unrest episode in S. Miguel. This work was funded by the Portuguese FCT – Fundação para a Ciência e a Tecnologia, I.P., within the scope of project PTDC/CTA-GEF/6674/2020.

We carried out an automated analysis of continuous waveform data. Our workflow includes detection and location using the back-projection of waveform-based characteristic functions, single-station magnitude estimation, clustering based on waveform similarity, and template matching to extend and complete the catalogue. We further computed moment tensor solutions for selected events.

We identified three clusters of earthquakes with similar waveforms. The first includes events that occurred during a 7-day precursory phase. The waveforms display high-frequency P- and S-waves, typical of brittle failure. A second cluster was activated on February 12, when the seismic rate increased abruptly, and the highest magnitude (M3.7) of the swarm was recorded. The waveforms of this cluster have a lower frequency content than those of the first one. Finally, a third cluster was activated, with a much lower number of events. These waveforms have a more harmonic character. From the first to the last cluster, earthquakes migrate slightly shallower (15 km to 7-10 km) and to the SE. Focal mechanisms indicate mostly normal faulting. We interpret the first cluster as brittle fracturing at depth, followed by the triggering of shallower structures (2^ndcluster), and finally by events that occur already in a fluid-rich environment (3^rd cluster). Interestingly, this seismic swarm marked the beginning of a period of aseismic surface deformation as observed by GPS over the subsequent 17 months.

Session: Seismology's Role in Assessing Volcanic Hazard at Multiple Time Scales

Type: Oral

Date: 4/18/2023

Presentation Time: 08:45 AM (local time)

Presenting Author: Susana Custodio

Student Presenter: No

Invited Presentation: 

Authors