The shock originated in the Great Northern Puerto Rico fault zone (GNPRfz) in the central eastern mountainous region of the island between the Juan Asencio and Sonadora barrios of Aguas Buenas, about 7 miles (11 km) from Caguas and 14 miles (23 km) from San Juan. This fault zone is located in the boundary between the North American and Caribbean plate tectonics, on a complex geologic zone with both subduction and strike faults. It was initially reported as measuring 5.4 on the Richter magnitude scale. The main shock was followed by two aftershocks (4.8 and 4.7) about 30 minutes after.[4] Given its moderate size and its epicenter being located on land, the earthquake did not cause any tsunami. The strongest shaking was felt in the municipalities of Aguas Buenas, Bayamón, San Lorenzo and Caguas, and moderate to strong shaking was felt throughout the San Juanmetropolitan area. There were more than 4,000 initial entries on the USGS and the Puerto Rico Seismic Network "Did you feel it? reports".[5]
The earthquake was felt by most due to its epicenter on land close to some of the major urban areas of Puerto Rico and the fact that it hit during Christmas Eve celebrations when people are usually gathered in large family groups, often indoors or attending church services.[6] There were more than 50 emergency calls in Bayamón and Guaynabo.[7]
