Q COSTA RICA — Costa Rica’s geology places a natural limit on the size of earthquakes the country can generate, according to seismologist Marino Protti of the Observatorio Vulcanológico y Sismológico de Costa Rica (Ovsicori)—Costa Rican Volcanological and Seismological Observatory.
An earthquake measuring magnitude 8.0 would generally require a fault capable of rupturing across an area roughly 200 kilometers long and 50 to 60 kilometers wide. No known geological structure within Costa Rica reaches those dimensions, Protti told El Observador.
The country’s largest identified fault lies beneath the Nicoya Peninsula. It extends for about 100 kilometers and is approximately 60 kilometers wide—substantial enough to produce a major earthquake, but not large enough to generate a magnitude-8.0 event under the estimates cited in the report.
That does not mean Costa Rica is protected from powerful earthquakes. Regions including Guanacaste, the southern Pacific coast and the southern Caribbean could experience earthquakes above magnitude 7.0. The potential depends on the length and width of the rupture zone, as well as the amount of energy accumulated beneath the surface.
Smaller faults can still cause serious damage
The faults beneath the Central Valley and around Cartago are considerably smaller. Many measure between five and 10 kilometers, although some extend to roughly 15 kilometers. Their estimated maximum magnitudes generally fall between 6.0 and 6.5.
Those numbers should not be mistaken for a measure of safety. A magnitude-6 earthquake occurring near a city and at shallow depth can cause severe damage, particularly when buildings are vulnerable or local soils amplify the shaking.
This is one of the most important points in Costa Rica’s earthquake risk. Magnitude measures the energy released by an earthquake, but it does not describe the full extent of its consequences. Depth, distance from populated areas, ground conditions and construction quality can all determine whether an earthquake becomes a disaster.
A smaller rupture close to an urban center may therefore be more destructive locally than a larger, deeper earthquake farther away.
Osa Peninsula remains a closely watched area
The Osa Peninsula is among the regions receiving particular attention from seismologists. Geodetic measurements have identified a section beneath the peninsula that appears to be accumulating strain. The area should be moving, but remains largely locked.
Based on scientific research, that zone could produce an earthquake between magnitude 7.2 and 7.4. Ovsicori uses satellite-based monitoring stations to measure ground movement, map areas of deformation and estimate the size of a possible rupture.
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The findings can indicate where energy is accumulating and how large an earthquake might be. They cannot determine the date or time of the event.
That distinction matters. Seismologists may identify a zone with elevated earthquake potential, but no monitoring system can currently provide an exact prediction of when a particular earthquake will occur.
The lesson from Nicoya
The 2012 Nicoya earthquake reached magnitude 7.6 but caused no deaths, according to the report. Protti attributed that outcome in part to local soil conditions and Costa Rica’s earthquake-resistant construction practices.
The event illustrates why magnitude alone is a poor measure of human impact. The same numerical magnitude can produce very different results depending on where the rupture occurs, how deep it is, how close it lies to populated areas and how well buildings withstand shaking.
Costa Rica’s apparent inability to generate a magnitude-8.0 earthquake should therefore not be interpreted as an absence of seismic danger. The country’s geological structures may limit the maximum size of an earthquake, but the faults that do exist are capable of producing destructive events—especially shallow earthquakes near communities.
The practical implication is straightforward: earthquake preparedness should focus less on the pursuit of a single headline number and more on local exposure. Monitoring active faults, enforcing resilient construction standards and preparing communities in areas such as Osa, Nicoya, Guanacaste and the Central Valley remain essential.
Costa Rica may not possess a fault large enough to produce a magnitude-8.0 earthquake. It still has more than enough seismic activity to demand serious preparation.