Strong Earthquake Strikes Western Costa Rica: AIR Analysis

According to catastrophe modeling firm AIR Worldwide, on September 5 at 8:42 AM local time, a magnitude 7.6 earthquake struck a sparsely populated region of western Costa Rica approximately 150 km [94 miles] west of San JosĂ© , the nation’s capital and largest city (approximate population 365,800 as of 2006). The earthquake, initially estimated at a magnitude of 7.9, occurred at a depth of 40 kilometers [25 miles].

AIR explained that due to the strength of the quake the Pacific Tsunami Warning Center issued an alert for Pacific “coastlines of Costa Rica, Nicaragua and Panama, as well as for locations as far away as Mexico and Chile.” However, the alert was later cancelled when no tsunami formed. In addition AIR noted that the depth of the quake would probably limit the damages it could cause.

“A preliminary review of the event by Costa Rica’s National Commission of Risk Prevention and Emergency Attention revealed some structural damage near the epicenter and minor nonstructural damage beyond, though it is early in the aftermath of the event and the damage picture is still taking shape,” said the report.

According to AIR, “Costa Rica has considerable seismic risk. In the last 30 years, six damaging earthquakes with magnitudes greater than 6.0 have struck different regions of the country from east to west coasts. Today’s earthquake occurred in the Middle American Trench subduction zone where the Cocos plate subducts underneath the Caribbean plate at 8.5 centimeters per year. Preliminary earthquake faulting mechanisms indicate that today’s temblor occurred along the interface of these two plates—and that it is a subduction zone thrust event. Today’s earthquake was the biggest temblor in Costa Rica since a M7.6 quake in 1991.

“Although the plate convergence rate across the Middle American Trench is one of the fastest in the world, few earthquakes exceeding M8.0 have occurred here in known history. However, moderately large earthquakes, such as today’s event, do occur offshore of Costa Rica. Two earthquakes of approximately M7.5 occurred in Costa Rica’s southern offshore region in 1941 and 1983.

“More recently, in 1990, an earthquake of magnitude 7.0 occurred just south of Costa Rica’s Nicoya Peninsula, causing some damage in the province of Puntarenas. In the country’s northern offshore region, an earthquake between M7.7 and M7.9 occurred in 1950, causing damage in Costa Rica’s Central Valley and again in Puntarenas. Today’s event occurred between the epicenters of the 1950 and 1990 earthquakes, within 60 km of both, indicating that this area is quite seismically active.”

AIR explained that the “predominant construction type for insured residential buildings in Costa Rica is masonry; many masonry homes are either reinforced or confined masonry, both of which are expected to sustain the moderate shaking generated by today’s event with little damage. (Confined masonry, in particular, performs better than plain masonry under lateral loads due to the additional stability provided by confining elements.)

“Masonry is also the main construction type for low-rise commercial and industrial buildings, while high-rise commercial and industrial buildings, including those in the Costa Rican capital, are generally comprised of steel and reinforced concrete. Given the remote location and the depth of today’s quake, significant structural damage, including collapse, is not expected for a widespread location. Properties within a 20 to 30 kilometer radius of the epicenter of today’s earthquake may experience damage if they are constructed of unreinforced masonry, which is typical of uninsured properties in this region.”

So far the damage reports from the quake “have included mostly nonstructural damage, with some instances of structural damage,” AIR said. “Costa Rica’s history of damaging earthquakes in recent years has resulted in improvements to the building code. The first edition of the modern seismic code was published in 1974 as a result of the 1972 earthquake in Managua, Nicaragua, which caused tremendous structural damage.

“Using new seismic information and taking into account improved building technologies, Costa Rica updated the code again in 1986. The regulations were then updated in 2002 and again in 2004, making the Costa Rican code one of the most comprehensive in the region. Today, the building code is generally well enforced in cities.

“However, many buildings in rural locations (like that of today’s quake) are older; they typically do not adhere to the country’s building code and are characterized by poorer seismic performance than modern buildings. Furthermore, in some cases, buildings in these areas are built of adobe block and mud wall construction, which is very vulnerable to seismic shaking.”

Source: AIR Worldwide