The Volcanological and Seismological Observatory of Costa Rica at the National University (OVSICORI-UNA) has revised Friday night’s Cartago earthquake to magnitude 5.0 at a depth of 8 kilometers (5 miles), and counted 60 aftershocks in the nine hours that followed. The observatory placed the epicenter roughly 4 kilometers (2.5 miles) from Orosi, in the canton of Paraíso, with the main shock recorded at 8:16 p.m.
The first automatic reading put the event at magnitude 4.5 and a depth of 7.5 kilometers (4.7 miles). Revisions of that kind are routine. Automatic solutions are generated within seconds from a handful of nearby stations, and analysts adjust them once data from the full network has been reviewed.
Between 8:16 p.m. Friday and 5:00 a.m. Saturday, OVSICORI counted 60 aftershocks in the Orosi area, ranging from magnitude 0 to magnitude 3.1. Only two were strong enough to be felt by residents, one at 9:09 p.m. Friday and one at 12:13 a.m. Saturday.
That ratio is normal. Most aftershocks in a sequence like this one are small enough that only instruments register them. Sequences also decay predictably: the events grow less frequent and smaller over hours and days, though the tail can run for weeks. OVSICORI said monitoring of the area continues.
Costa Rica has two independent seismic monitoring bodies, and their figures for this earthquake do not match exactly. OVSICORI-UNA reported magnitude 5.0 at 8 kilometers (5 miles) depth, about 4 kilometers (2.5 miles) from Orosi. The National Seismological Network (RSN), operated jointly by the University of Costa Rica and ICE, reported magnitude 4.9 Mw at 9 kilometers (5.6 miles) depth, 3 kilometers (1.9 miles) east of Orosi.
Neither figure is a correction of the other. The two networks use different station arrays and different calculation methods, and gaps of a tenth of a magnitude unit and a kilometer or two of depth are ordinary between them. Readers who saw other depths circulating on Friday night, including figures as deep as 12 kilometers (7.5 miles), were looking at early automatic solutions that were later refined. The RSN attributed the origin to faulting within the crust of the Caribbean and Panama plates, in other words a local, shallow fault rather than the offshore subduction zone.
Why a shallow earthquake shakes harder
Magnitude and shaking are two different things. Magnitude measures the total energy released at the source. Intensity measures how hard the ground actually moves at a given spot, and depth is one of the biggest factors in the gap between the two.
An earthquake at 8 kilometers has very little rock between the rupture and the surface to absorb its energy. What arrives is a short, sharp, high-frequency jolt concentrated close to the epicenter, which is why a moderate magnitude 5.0 rattled dishes and cracked glass in Paraíso and Oreamuno. The trade-off is reach: shallow quakes fade quickly with distance.
Deeper events behave the opposite way. A subduction earthquake at 30 to 60 kilometers (19 to 37 miles) sends energy through far more rock, which filters out the sharp high-frequency component. Those quakes are felt across a much wider area, but they arrive as a slower rolling motion and rarely produce the same violent shaking at any single point. Friday’s event still registered as far away as Miramar de Puntarenas, but the hardest shaking stayed within the Cartago valley.
OVSICORI seismologist Marino Protti attributed the earthquake to a system of faults south of the Cartago valley and said the observatory would keep watching the region for further activity. That distinction matters more than the magnitude number does. The Central Valley, home to most of Costa Rica’s population, sits above a dense network of shallow local faults that are entirely separate from the Cocos-Caribbean subduction zone off the Pacific coast.
Those faults produce smaller earthquakes than the offshore zone, but because they are shallow and directly beneath populated ground, they have historically caused more damage. The clearest example is the earthquake of May 4, 1910, which originated on a shallow fault near Cartago and destroyed the city. The Orosi and Tobosi area has produced clusters of felt earthquakes on several occasions since.
What has been reported on the ground
All damage information at this stage is preliminary. The National Emergency Commission (CNE) registered no incidents in its initial assessment after the earthquake and had not issued an updated damage report as of Saturday afternoon.
Separately, the Costa Rican Red Cross reported that a 38-year-old man in Cot de Oreamuno was injured when glass fell on him and was transported in yellow condition to Hospital Max Peralta. JASEC, the municipal electricity utility for Cartago, reported outages and damage to power lines across a considerable area, listing affected communities that included Birrisito, La Flor, El Yas, Santiago, Cot, Cervantes, Pacayas, Capellades and parts of Oreamuno. Crews worked overnight to restore service.
The earthquake struck while pilgrims were already walking toward the Basílica de Nuestra Señora de los Ángeles in Cartago for the August 2 Romería, and the shaking was captured on a live broadcast of the pilgrimage. Cartago is both the epicenter zone for this sequence and the destination for hundreds of thousands of walkers this weekend, which is why the event drew immediate national attention despite its moderate size.
If aftershocks continue
The standard response during shaking is to drop, take cover under something solid and hold on until the motion stops, rather than running for a doorway or outside. Glass is the most common source of injury in moderate earthquakes, as Friday night demonstrated, so moving away from windows, mirrors and glass display cases is worth doing as soon as shaking begins. It is also worth checking shelves and wall fixtures afterward, since a second jolt can bring down objects that the first one loosened.
The Central Valley sits on a dense network of shallow faults, and a sequence like this one is a normal expression of that geology rather than a sign that something larger is building. Costa Rica records moderate earthquakes regularly, from the magnitude 5.4 event near Jacó earlier this month to the ongoing alert at Rincón de la Vieja. OVSICORI said it will continue monitoring the Orosi area, and the figures for magnitude, depth and aftershock totals may be refined again as analysts work through the full record.
