UNPRECEDENTED WELL-LOCALIZED SHALLOW SEISMICITY BENEATH A CAPITAL CITY: RELOCATIONS, FOCAL MECHANISMS AND EARTHQUAKE SOURCE PARAMETERS OF THE 2025–2026 SAN JOSÉ, COSTA RICA, EARTHQUAKE SEQUENCE
Between 22 August 2025 and 28 June 2026, a sequence of 45 shallow earthquakes (ML 0.6–4.4) nucleated directly beneath the capital of Costa Rica, San José. Only one sequence with very similar characteristics, although poorly recorded, had been previously documented, occurring in 1933 at a time with very limited instrumental data. The activity initiated with an ML 4.1 event on 22 August 2025 and culminated with an ML 4.4 earthquake on 19 January 2026, both at ~5–6 km depth and widely felt across the metropolitan area. We combine the permanent OVSICORI-UNA network with a rapid response deployment of up to eleven 250-Hz seismic nodes to characterize the sequence. Double-difference relocations collapse the seismicity into a compact, ≤ 2-km-wide cluster between ~2 and 7 km depth. Well-constrained first-motion focal mechanisms reveal heterogeneous faulting styles, including oblique strike-slip, reverse, and normal solutions, that nevertheless share consistently SSW- to SW-trending P axes, in agreement with transpressional deformation across the Central Costa Rica Deformed Belt. Spectral-ratio analysis using empirical Green’s functions (EGF) for twelve events yields well-resolved source parameters for six (Mw 2.35–4.43). Five of these, including the Mw 4.1 August mainshock, follow self-similar scaling with stress drops of ~4–12 MPa, whereas the 19 January 2026 mainshock is anomalous: M0 = 5.56 × 10 15 N·m (M w 4.4), corner frequency 4.32 ± 0.34 Hz, and stress drop 83 ± 21 MPa, nearly an order of magnitude above its co-located neighbors. For this event, directivity analysis and EGF-normalised P-wave amplitudes jointly resolve statistically significant unilateral rupture toward the ESE (~100°; p =0.014), marginally favoring the north-striking, east-dipping nodal plane, and the August mainshock shows a consistent, marginally significant eastward pattern. The sequence illuminates a previously unrecognized, seismically capable blind structure beneath Costa Rica capital, that concentrates more than half of Costa Rica’s population, with direct implications for its seismic hazard.