The Central Valley in Costa Rica has dozens of faults, some of which can generate earthquakes with magnitudes of up to 6.5, according to the Volcanological and Seismological Observatory (OVSICORI).
According to the director of OVSICORI, Esteban Chaves, in downtown San José, dense urbanization makes it difficult to easily observe geological features and makes complete mapping of these fractures difficult.
“We’re talking about dozens of faults that have been identified in the field and mapped. While many others are unknown because the infrastructure in the metropolitan area obscures the features, for example, those that caused the earthquake early Friday morning are unknown,” Chaves said.
He is referring to a strong, shallow 4.1 magnitude earthquake that woke residents of San José, Alajuela, and Heredia last Friday at 12:45 a.m. The epicenter was located in the Tournón neighborhood.
“So, the faults are completely obscured by the amount of infrastructure that has been developed. Outside the metropolitan area, when you start to see mountains, for example, it’s easier to follow the fault lines,” he added.
Chaves also explained that these small, local faults often move very slowly over time and the recurrence period is very long, even hundreds of years. “What happens is that in the end, our lifetime may not be long enough to know the location of these faults. They can break when we’re gone, and it will be the new generations who add the fault to the map,” he mentioned.
Known Faults
The Ovsicori has identified some relevant systems in the central region. Some of them are:
Alajuela Fault: extends from Santa Bárbara de Heredia to Grecia.
Desamparados–Tres Ríos: runs south of San José to the Cartago area.
Aserrí: a fault system that includes Patillos and other associated faults.
Escazú and Santa Ana: with the presence of local faults.
Río Azul and Cartago: associated with historical events such as the 1910 earthquake.
Chaves clarified that these faults vary in size and seismic potential. “There are faults that have the potential to generate events of up to 6.5 magnitude. And if that were to occur in a densely populated area like San José, the scenario would be very different from that of a magnitude 4.1 earthquake like the one early this morning (Friday),” he emphasized.
Why are there so many faults?
The origin of this large number of faults lies in the tectonic position of the Central Valley, where the Caribbean plate and the Panama microplate meet. “The central part of Costa Rica is a fractured shear zone, where the relative movement between the plates generates a large number of faults. It’s like hitting an egg with a spoon: the shell breaks in many directions. A very similar situation occurs in this region,” Chaves explained.
According to the specialist, this situation makes the Central Valley a seismically complex area. “The central region is highly fractured with many faults, but we lack specific research here because most studies have focused on areas like Nicoya, the Southern Caribbean, or the North Pacific, where powerful earthquakes have occurred,” he added.
Call for Investigation
“We need to do a lot of work in the Central region of Costa Rica, and this is the reality,” the expert added. “When you think about it in perspective, what would have happened in San José if, instead of a magnitude 4.1 earthquake, a magnitude 6.5 earthquake had occurred, perhaps the situation would be much worse,” he noted.
Given this, he asserted that work must be done to create a system to understand the seismic risks in highly urbanized areas, primarily in central Costa Rica. The central part of the country is home to around 2.5 million people, four orders of magnitude greater than the number of earthquakes that have occurred in the last 15 years, according to the director.
This difference in scale means that any medium-sized earthquake in the Central Valley can have a great impact, precisely because the people and infrastructure are concentrated in this area.
“We need to know what infrastructure we have, what infrastructure could be compromised by an earthquake of this magnitude, and what the effects could be. In general, a comprehensive study of the damaging effects that could be caused by an earthquake in the area is important,” Chaves emphasized.
Chaves ruled out the possibility of installing an early warning system for these cases, given that the faults are located directly beneath inhabited areas. “It wouldn’t make much sense because there isn’t enough time. The faults are directly beneath where people live, so there wouldn’t be enough time for an alert that would allow for a response before the shaking,” he stated.
As of Saturday morning, experts had counted 14 aftershocks from the capital’s earthquake, in areas such as Tibás and Zapote. Chaves acknowledged that the difference in epicenters is part of the existing seismic fault system. The Ovsicori (Operation of the Earthquakes in the National University) plans to deploy portable equipment to better study these faults and earthquakes.
Rumbling
The director of Ovsicori explained that the force with which the earthquake was felt and the rumbling reported by the population early Friday morning are due to its shallowness, as it occurred at a depth of just 4 kilometers.
“Because the hypocenter is so shallow, the energy doesn’t have time to attenuate or be absorbed by the earth. It is released and transmitted immediately, which is why the shaking was so strong,” Chaves stated.
Perception also depends on the type of construction; for example, high frequencies are felt more in the lower part of a tower, while in a larger earthquake with long periods, the effect would be stronger on upper floors.
In horizontal housing, which predominates in the Central Valley, the shaking was felt more intensely. Furthermore, the release of energy generated an additional phenomenon.
“When seismic waves propagate and couple with the atmosphere, they produce a sonic shock wave. That’s what people perceive as a rumble,” he explained. Chaves emphasized that this type of noise has occurred in other parts of the country, although it is not always reported as frequently. “In San José, the population is much larger, which is why the number of reports about the rumble was so high compared to other areas,” he noted.
