A magnitude 5.1 earthquake struck near Golfito on Friday night, the strongest of four events above magnitude 4 recorded in Costa Rica’s Southern area in roughly 32 hours. The Red Sismológica Nacional (RSN) of the University of Costa Rica placed the quake at 7:59 p.m. on September 11, with an epicenter 10 kilometers (6 miles) east of Golfito in the province of Puntarenas. The RSN described its figures as preliminary and said the magnitude, location and depth could change as analysts finish processing the data.
Thirty minutes later, at 8:29 p.m., a magnitude 4.0 movement was recorded southwest of Quepos at a depth of 10 kilometers (6 miles). Whether it was an aftershock of the Golfito event or a separate rupture has not been established. Both quakes followed a pair of movements on Thursday afternoon off the Osa Peninsula.
The Observatorio Vulcanológico y Sismológico de Costa Rica (OVSICORI) at the Universidad Nacional registered a magnitude 4.8 at 12:03 p.m., some 48.2 kilometers (30 miles) southwest of Cambutal de Osa, and a magnitude 4.2 four minutes later at 41.4 kilometers (25.7 miles) from the same reference point. Both were placed at a depth of 10 kilometers (6 miles).
No damage or injuries have been reported from any of the four events. Shaking from the Golfito quake was described well beyond the Southern area, including in Grecia and San Pedro de Montes de Oca. Those accounts come from residents posting on social media and do not constitute an official measurement of intensity, which the RSN calculates separately.
The southern Pacific margin is among the most seismically active parts of Costa Rica. The Cocos Plate is being forced beneath the Caribbean Plate along the Middle America Trench, and off the Osa Peninsula that process is complicated by the Cocos Ridge, a thickened stretch of seafloor grinding into the margin. Researchers have identified a segment beneath the Osa area where strain is accumulating and that could, in principle, generate an earthquake of magnitude 7.4. Studies of that kind describe hazard over long periods and do not forecast when a rupture will occur.
The seismic activity coincides with a green alert covering the South Pacific, issued by the Comisión Nacional de Emergencias ahead of the passage of tropical waves through midweek. The commission has warned that soils across the alerted regions are heavily saturated after days of rain, a condition that raises the risk of landslides. Shaking can dislodge material on slopes that have already lost cohesion, and the two hazards overlap across much of the same terrain.
The RSN and OVSICORI revise their parameters as processing continues and publish updated reports through their official channels. Emergencies should be reported to 9-1-1.
