Profiling Earthquake Risk in Costa Rica

Costa Rica has experienced a considerable amount of violent earthquakes during the last four decades. The 1991 Limón earthquake, the greatest recorded event in the country of Mw 7.6, left a death toll of 48 and economic losses reaching $100 million USD. More recently, the Mw 6.1 Cinchona earthquake caused 40 fatalities and more than $500 million USD in damage.

The 1991 earthquake damaged residences and lifelines of national importance
The 2009 Cichona earthquake caused landslides and soil instability near the epicenter, affecting more than 800 families

Despite these events, loss estimation models at a national scale have not been updated since the seventies. In this context, the GEM Foundation in collaboration with local experts have developed a model that could be used to support decision makers towards effective risk reduction in the future.

Achieving this goal requires revision of the three components of risk assessment: hazard, exposure and vulnerability. Hazard is possibly the most thoroughly reviewed of the components by national experts. The expected PGA in the capital of Costa Rica (San José) for a return period of 500 years has evolved from 0.3 g in the seventies, up to 0.5 g at present. Recent studies suggest that this value is not lower than 0.2g for the whole country, indicating that the entire population and capital stock of the country are under earthquake threat.

Recent exposure models have been developed for potentially vulnerable regions and cities. However, studies at a national scale for risk assessment are significantly outdated. The study presented herein focused on the development of a model for the residential building stock. Using Housing Censuses from the National Institute of Censuses (INEC) and data from the Costa Rica Central Bank, it was possible to identify the main building classes and their respective structural attributes (e.g. load resisting systems, height, approximate date of construction, expected ductility and replacement cost). This evaluation identified a total of 34 building classes and their distribution for each of the 472 districts in the country, concluding that more than 92% of the buildings are located in the reinforced masonry, precast concrete and wood building classes.

Costa Rica hazard map (left), and building distribution per district (right)

Considering the results from the exposure model, a number of analytical fragility functions were derived for the most common building classes: reinforced masonry and precast concrete buildings. For the remaining classes, existing fragility functions developed within the South American Risk Assessment (SARA) Project complemented the fragility catalogue. These functions were combined with a damage-to-loss model calibrated for Costa Rica, in order to derive a set of vulnerability functions.

One and two storey confined and reinforced masonry buildings, constructed post 1980, were identified amongst the most common building classes in Costa Rica

With all the necessary models complete, a probabilistic event-based risk analysis was performed to calculate earthquake losses. A stochastic event set of a 100,000 years and half a million ruptures were generated using the OpenQuake-engine, the open-source software for seismic hazard and risk analysis supported by GEM. As a result, average annual losses and aggregated loss exceedance curves for the country were obtained.

Aggregated Annual Loss (AAL) distribution at a district scale for Costa Rica

Results for Costa Rica indicate that the gross of the losses are concentrated in the Great Metropolitan Area of the country. The capital districts of San Jose, Pavas, Alajuela, San Francisco and Curridabat exhibit the largest loss potential, which can be attributed to high the population density and existence of soft soils in the Central Valley. However, there is a strip of districts along the southern coast of the country where losses are also particularly high. Liberia, Santa Cruz, Parrita, Quepos, San Isidro Del General, Canoas, Guaicará and Corredor are zones that register losses comparable to the mean found in the Central Valley. Even though population in some of them is lower, the estimated significant risk indices are the product of a higher exposure to seismic hazard combined with effects of soft soils and concentration of low quality construction. Along the north Caribbean Coast and the tropical rainforest in the east, losses are much smaller. Districts like Limón, Sarapiquí and Guapiles show appreciable risk levels mainly because they are highly populated centers.

Referencing the magnitude of the losses, a total mean of $105 USD million annually is the final estimation. To put it in context, according the Ministry of Treasury of Costa Rica, in the year of 2015, the budget for road infrastructure development and maintenance was nearly $450 USD million. In other words, with loss estimation correct within the order of magnitude, the country potentially losses yearly due to earthquakes more than a fifth of the sources directed to construct and maintain lifelines.

Probable Maximum Loss (PML) curve for Costa Rica

The probable maximum loss curve indicates for Costa Rica a mean value of U$1,839 million for a return period of 200 years, nearly 4% of its yearly gross domestic product. The maximum estimated loss was generated by a Mw 7.9 earthquake on the Central Valley, with an associated loss of U$8,180 million (around 16% of the national GDP).

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