Volcanic or tectonic origin? A case study of the 2025 Santorini-Amorgos sequence

Xing Tan, William L. Ellsworth, Gregory C. Beroza, Stephanie G. Prejean, & Jeremy D. Pesicek

Submitted September 7, 2025, SCEC Contribution #14778, 2025 SCEC Annual Meeting Poster #TBD

A key question during the intense 2025 Santorini-Amorgos sequence was whether the earthquakes were tectonic or volcanic. When seismicity in an area behaves unusually, both historical global analogs and modern high-precision analyses can help assess the situation. We applied a machine learning-based workflow to the Santorini–Amorgos seismic sequence (1 Jan 2025 – 08 Mar 2025) and detected over 50,000 earthquakes, including 312 events with magnitudes greater than 4. While most earthquakes show brittle-failure characteristics, we also identified long-period earthquakes. We found that seismicity is concentrated between 5 and 10 km depth, with localized activity reaching depths of up to 15 km. The resulting high-resolution earthquake catalog, combined with focal mechanism solutions reported by GFZ Helmholtz Centre for Geosciences, National Observatory of Athens, and Observatoire de la Côte d’Azur, reveals a sharply defined network of northeast–southwest-trending normal faults extending from northeast of Thera (Santorini) to southwest of Amorgos, and extending directly beneath Anydros Island. The sequence initiated near the southwestern end of the fault system on January 27, followed by rapid intensification and a northeastward propagation beginning on February 1. Subsequent episodes of high earthquake activity propagated both farther northeast and intermittently backtracked to the southwest, reactivating previously active zones. Throughout the sequence, we observed recurring seismic bursts—each lasting 1–2 hours with hundreds of events, including many of the largest, that occurred episodically over an ~11-day period. Within some seismic bursts, we observed rapid migration of seismicity along the fault zones.

The large and sustained number of moderate magnitude earthquakes during the Santorini-Amorgos sequence, combined with the repeating pulse-like nature of the seismicity, is remarkable. We searched for global analogs by comparing the evolving count of 𝑀 ≥ 4 earthquakes against multi-decadal global catalogs to find sequences with equal or higher rates over matched durations. Only a handful of historical analogs exist, all of which were related to magmatic intrusion or eruption.

Key Words
Santorini, swarms, quakeflow, volcano

Citation
Tan, X., Ellsworth, W. L., Beroza, G. C., Prejean, S. G., & Pesicek, J. D. (2025, 09). Volcanic or tectonic origin? A case study of the 2025 Santorini-Amorgos sequence. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology