On May 21, 1950, an earthquake seriously damaged more than half the buildings of Cusco, Peru. It occurred without warning of foreshocks and was followed by only a few weak aftershocks. The epicentral area, within which buildings were extensively damaged, covered only about 12 square kilometers within the Cusco Basin, and minor damage was limited to a surrounding area of less than 500 square kilometers. The earthquake was felt over a total area of only 15,000 to 16,000 square kilometers and therefore may be considered local in effect. The depth of focus probably was between 8 and 9 kilometers.
The maximum intensity in the epicentral area was estimated to be VII on the Modified Mercalli scale, and the acceleration was less than 300 gals. The extensive damage in the epicentral area is attributed largely to poor construction and the age of buildings. The buildings that were most seriously damaged were colonial churches 250 to 350 years old, old adobe houses, and new houses of combined adobe and rock or brick construction. Many of the buildings that were destroyed in this earthquake had previously been damaged during an earthquake in 1941.
From a summary of the geology of the Cusco Basin it is concluded that the most extensive damage was to buildings resting on thick water-saturated alluvial gravels.
The earthquake was probably of tectonic origin, but there is no positive evidence of active faulting. However, an extensive fissure zone in unconsolidated sediments at the south side of the Cusco Basin may be related to a bedrock fault beneath these sediments.