Ambient noise interferometry to investigate temporal changes in the São Jorge Island (Azores) subsurface structure associated with the 2022 seismic crisis.

In March 2022, a seismic crisis was declared in São Jorge Island. Despite the regular seismotectonic activity observed in the Azores Central Group, São Jorge has not exhibited significant activity since the crisis associated with an eruption in 1964. Between the fall of 2021 and the end of 2022, approximately 12,000 earthquakes (magnitudes up to ML 3.8) have been recorded, with the seismicity and geodetic modelling pointing to a magmatic intrusion. Intrusions cause gas release, fluid circulation, and pressure perturbations in the subsurface volcanic system that often induce changes in seismic velocity. Here, we probe spatial-temporal changes in the seismic velocity structure beneath São Jorge using ambient noise interferometry.

In this study, we analyzed data continuously recorded between January 2021 and December 2022 by two permanent stations (PMAN and ROSA) operated by the Instituto Português do Mar e da Atmosfera (IPMA) to investigate the presence of subsurface structural changes in response to the seismic crisis. Data were cut into 1-hr length files and filtered between 1 and 3 Hz for autocorrelation, and between 0.1 and 1.0 Hz for cross-correlation. We applied the Phase Auto- and Cross-Correlation (PAC and PCC) method to the filtered data. This method is based on phase coherence and is amplitude-unbiased. PAC and PCC functions were then linearly stacked over three days to achieve a stable noise response. To infer changes in the velocity structure, we analyzed the waveform similarity values for different time lag windows. We compared the waveform similarity results with meteorological data and ground deformation inferred from GPS. Additionally, relative velocity changes have been estimated. 

The two analyzed stations exhibit different waveform-similarity results. Preliminary interpretation of PMAN results (closer to the island center) show, in the second half of 2022, a very slight recovery of the waveform similarity at shorter lag times (shallower depths) that decreases again in the fall of the same year. Globally, data from this station exhibits a more systematic decorrelation when the crisis was declared, most likely due to perturbations in the seismic structure between < 8 - 10 km and deeper than 15 km. 

This work is a contribution to RESTLESS (DOI:10.54499/PTDC/CTA-GEF/6674/2020) and GEMMA (DOI:10.54499/PTDC/CTA-GEO/2083/2021). It was also funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UIDB/50019/2020 (https://doi.org/10.54499/UIDB/50019/2020), UIDP/50019/2020 (https://doi.org/10.54499/UIDP/50019/2020) and LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020).