Managua lies along the "circle of fire," a ring of volcanoes and seismic fault lines that encircle the Pacific from the Aleutians down through the western rim of the Americas to New Zealand and up through Japan. Central America is often shaken by geologic turbulence. Managua, situated on top of a volcanic belt in a highly active area (an active volcano lies within sight of Managua), is especially vulnerable. As seismologists gauge earthquakes, the pre-Christmas tremors were not particularly severe, measuring just 6.5 on the Richter scale (only quakes that hit 7.5 or higher are considered "'major"). Evidently Managua's fatal misfortune was that it sat on or near the exact epicenter of an unusually "shallow" quake: one that occurred only about nine miles below the surface of the earth. The city, built on top of compacted volcanic debris rather than solid rock, was especially susceptible to the force of the tremors. Somoza speculated on an additional factor: "Earthquakes may have something to do with the water table--they seem to come at times of drought."
Scientists of the National Center for Earthquake Research, a branch of the U.S. Department of the Interior's Geological Survey, reported that intense shaking, with extremely high vertical and horizontal ground accelerations, appeared to have been the primary causes of the heavy damage in Managua. The scientists said the violent shaking was "unusual for an earthquake of 6.5" and was "a function of shallow focus ... that confined major energy to a relatively small area."
The scientists reported that there was movement on at least four parallel active faults under Managua. "The faults," they said, "cross Managua in a northeast-trending direction, and span the central part of the city where there was near total destruction of buildings. A fifth fault, on which movement was not observed but which was active during the destructive. Managua earthquake of 1931, also cuts through the city and is parallel to the other faults."
(The existence of the five faults under the city is a factor which weighs heavily on studies to determine where the city is to be reconstructed.)
Another factor in Managua's vulnerability was the type of construction of many homes. These were made of rock plastered over with adobe and therefore were particularly brittle--and thus dangerous. Wooden walls stand up better during earthquakes; even if they collapse, victims caught beneath have a better chance of survival than if they are crushed beneath bricks or concrete.
The 1972 earthquake was not the first to level Managua, nor was it the only disaster to strike the city. The Miami Herold published the following listing:
1876--City ruined by floodwaters from LakeManagua.
1885--Leveled by earthquake.
1902--Heavily damaged by explosion of military arsenal.
1912--Ravaged by civil war.
1928-33--Order threatened throughout country by leftist insurgents; country occupied by U.S. marines in the attempt to keep order.
1931--Earthquake and fire on March 31 destroy much of the city and kill 2,000 persons.
1971--Dozens of infants struck down by polio; city asks medical help from all over the world.