J. Eur. Opt. Society-Rapid Publ. 22, 11( 2026) 113
signal). This classification converts the qualitative inspection of the recordings into a metric that is comparable across events, highlighting the influence of source – receiver distance and magnitude on HDAS detectability. The results are displayed in Figure 3b as a function of distance and magnitude and are compared with the proposed reference threshold law. This comparison is important because it shows the extent to which HDAS matches( or exceeds) the sensitivity of the land network, particularly for offshore seismicity, where DAS is expected to offer clear advantages due to the proximity of the cable to the seismic sources.
4 Conclusion
Over five months, the DAS and OBS recorded several earthquakes, including a magnitude 2.7 event that fell on Faial Island. In addition to these earthquakes, other unidentified energy signals were detected. The two OBS stations show similar responses, with a time delay expected due to their spatial separation. During the same period, the IPMA network detected 229 local and regional earthquakes. The relationship between event magnitude and the minimum distance to the submarine cable was analyzed to assess the sensitivity and detection threshold in the area. This analysis defines the magnitude – distance limit beyond which events are no longer consistently recorded, providing a reference for comparing the performance of DAS / HDAS and OBS observations with that of the land-based network, especially for offshore seismicity.
Funding
This work is supported by the Portuguese Fundação para a Ciência e Tecnologia, FCT, I. P./ MCTES through national funds( PIDDAC): UID / 50019 / 2025 and LA / P / 0068 / 2020( https:// doi. org / 10.54499 / LA / P / 0068 / 2020), by the MODAS project 2022.02359. PTDC, and by EC project SUBMERSE project HORIZON-INFRA-2022-TECH-01-101095055. HFM acknowledges financial support from the MCIN / AEI / 10.13039 / 5011000 11033 and European Union « NextGenerationEU » / PRTR under grants RYC2021-035009-I, CPP2022-009772, PID2024- 162301OA-C22, CPP2024-011822. This work is co-funded by Component 5 – Capitalization and Business Innovation, integrated in the Resilience Dimension of the Recovery and Resilience Plan within the scope of the Recovery and Resilience Mechanism( MRR) of the European Union( EU), framed in the Next Generation EU, for the period 2021 – 2026, within project OBSERVA, with reference 2024.07610. IACDC( https:// doi. org / 10.54499 / 2024.07610. IACDC).
Conflicts of interest The authors have nothing to disclose.
Data availability statement Data are available upon reasonable request at the indicated e-mail.
Author contribution statement
Conceptualization, L. Matias and O. Frazão; Methodology, C. Corela and F. Carrilho; Software, A. Loureiro and R. Sousa; Validation, L. Matias and M. Niehus; Formal Analysis, H. Martins; Investigation, C. Corela, A. Loureiro, P. Robalinho, F. Carrilho and R. Omira; Writing – Original Draft Preparation, O. Frazão and S. Silva; Writing – Review & Editing, L. Matias, M. Niehus and O. Frazão; Visualization, H. Martins; Supervision, L. Matias; Project Administration, L. Matias.
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