Focal mechanisms and bouguer-gravity anomalies of the 2025 earthquake cluster in the Santorini-Amorgos region (Southern Aegean Sea, Greece): evidence for shallow extensional magmatism

Abstract

Understanding clustered earthquake sequences is essential for seismic hazard assessment, as it involves constraining faulting styles and nodal planes of potential ruptures. This study investigates the nature of a dense earthquake sequence (~ 3000 events) initiated on January 27, 2025, in the Santorini-Amorgos region of the Southern Aegean Sea (SAS), a tectonically active Volcanic Island Arc (VIA). We analyzed 23 shallow crustal earthquakes (Mw ≥ 4.5, depth ≤ 10 km) that occurred between February 2–9, 2025, using full-waveform, low-frequency Centroid Moment Tensor (CMT) inversion from regional seismograms. The inversion was complemented by high-resolution Bouguer gravity anomaly data derived from the EIGEN-6C4 satellite gravity model to assess subsurface density variations. The focal mechanisms consistently indicate NE-SW striking, high-angle (≥ 45°) normal faults with NW- and SE-dipping planes and centroid depths ≤ 10 km. Integration of CMT results with gravity anomalies (90–100 mgal) suggests a migrating zone of shallower extensional magmatism (SEM) driving the sequence. These findings reveal a Precursory Seismic Cluster (PSC) and provide new constraints on the seismotectonic and magmatic processes shaping seismic hazard in the region.