The April 13, 1973, earthquake (MS = 6.5) in northwestern Costa Rica caused significant damage and the loss of 23 lives in a rural area of about 150 km2 centered south of Laguna de Arenal. Earthquake-triggered failures of mountain slopes were the cause of all deaths and injuries attributable to the earthquake. Incipient landslides and landslide-dammed lakes pose a continuing hazard to populated areas at Tronadora and Rio Chiquito during the 1973 rainy season.
The modest amount of shaking damage to structures is attributable to the moderate size of the earthquake and to the predominance of small wood-frame buildings with lightweight corrugated roofs that have good shear resistance to lateral forces. Most of the structural damage that did occur was to buildings made of concrete block with little or no reinforcing steel; none of these, however, involved total collapse. In some areas, damage to buildings and roads was also caused by foundation failures involving downslope slumping or sliding, lateral spreading, and differential compaction of unconsolidated or poorly consolidated surficial deposits including thick lateritic soil.
The location of the epicentral region and the nature of the earthquake and aftershock sequence suggest that the earthquake was of tectonic, rather than volcanic, origin. The shape and size of the damage area and the apparent absence of surface faulting are compatible with a moderately shallow source, possibly several kilometers in depth.