Growing an ocean island: high-precision seismicity reveals a multi-faceted magma intrusion during the 2022 São Jorge, Azores seismic crisis

The central islands of the Azores Archipelago in the North Atlantic straddle a diffuse zone of dextral transtension between the African and Eurasian plates, providing an ideal setting for studying the interplay between tectonics and magmatism. São Jorge is a narrow island dominated by a westward progression of past basaltic fissure eruptions, where fault zones act as volcanic rifts. After two inland eruptions with significant socioeconomic impact in 1580 and 1808, the most recent probable eruption occurred offshore in 1964, after two years of seismic activity. In March 2022, a seismic crisis began on São Jorge (magnitudes up to ML 3.8). 

Our analyses of InSAR and GNSS data are consistent with a dike intrusion that stalled at 2 km depth below sea level. Here, we use seismicity to probe the space-time evolution of the intrusion. The unique geography and near-coastal position of seismicity yield inherently uncertain locations. To address this, we supplemented on-land stations with 6 ocean-bottom seismometers (OBSs) around the island later in the crisis. We use NLL-SSST-coherence, a location method ideal for changing station density, to exploit later OBS data to form robust source-specific station terms that allow precise relocation of the earlier part of the seismic sequence when coverage was sparser. In a final step, we combine waveform coherence and location uncertainty stacks to enhance hypocenter location precision to <100m. 

Relocations of ~12,000 earthquakes show precursory, weak seismicity that started ~6 months before, starting offshore, south of São Jorge before migrating to shallower depth beneath the centre of the island. The main seismic crisis on 19 March 2022 started at shallower (<8 km) depth and moved north-westward and deeper before concentrating in the central zone at ~10 km depth. Intriguingly, nearly all the seismicity is located west of and deeper than the modelled dike intrusion, suggesting the intrusion was largely aseismic. Nevertheless, the agreement between the strike of the dike and the seismicity lineations suggests that the pre-existing Pico do Carvão Fault Zone guided melt ascent in the crust. However, moment tensors from polarity and waveform inversion show double-couple left-lateral strike-slip faulting along planes striking obliquely (by ~20°) to the dike and seismicity lineation, evidencing high fluid/melt pressures. The overall b-value is high (~2).

Interpreting both the seismicity and near-field GNSS displacements, we discuss the intrusion’s evolution along the preexisting fault zone, particularly focussing on potential magmatic inflow and drainage beneath the main dike intrusion.

We are grateful to the UK Ocean Bottom Instrument Consortium (OBIC) and SEIS-UK teams for providing the instrumentation and installation services. This work was also supported by Portuguese FCT/MCTES through the project GEMMA (https://doi.org/10.54499/PTDC/CTA-GEO/2083/2021).