Historic Earthquakes

1972 December 23 06:29:42 UTC
Magnitude 6.2

One of the worst seismic disasters of the year - and the most lethal of record for the western hemisphere above South America - occurred on December 23 when a strong shock destroyed most of the Nicaraguan capital of Managua.

Although hundreds of aftershocks were reported, only two exceeded magnitude 5, and these occurred within an hour of the main shock.

Managua suffered a disastrous earthquake in 1931 when some 2,000 were killed. A magnitude 4.5 earthquake damaged several hundred homes in 1968.

Abridged from Earthquake Information Bulletin, January-Febuary 1973, Volume 5, Number 1.

Damage photo
From the air, Managua seemed a city under siege. Flames poured from its shattered downtown area, people and traffic scrambled through whole neighborhoods of rubble, searching out the remnants of lives and belongings. For those who could remember March 31, 1931, this was another visit by an old enemy, the first recurrence of a seismic nightmare 41 years past.

The earthquake which struck the Nicaraguan capital near midnight on December 23 broke ground, streets, and structures with its earth-heaving blows from below, and transformed the Central American nation's only major city into a ruined graveyard. Although not a large earthquake at magnitude 6.2, the Nicaraguan shock appeared to have had a shallow focus virtually beneath the city, so that much of the seismic energy released had to be absorbed by Managua.

For almost a day, the crippled city lay in dreadful silence, cut off from the world except for occasional broadcasts received from ham radio operators before Managua's power failed. Then, as relief agencies in the United States and in nations around the Americas and the world went to work, grim reports began to come in. Two-thirds of Managua's 325,000 residents were displaced, and three-quarters of the city shaken to rubble. Worse, dry-season winds had readied the city for fire, and the tremor torched it off with broken pilot lights and gas lines and severed electrical cables. Famine and disease stalked the survivors.

Between 3,000 and 7,000 persons were killed by the earthquake, and some 15,000 were injured. The full figure may never be known, for fear of epidemic disease from rubble-entombed, decomposing bodies led authorities to level the worst areas into mass graveyards. In terms of deaths, this appears to have been the worst seismic disaster ever recorded for this hemisphere above South America. Until the Managua event, the worst earthquake of record for this part of the world was a tremor at Kingston, Jamaica, in 1692, which killed more than 2,000 persons.

Abridged from Earthquake Information Bulletin, January-Febuary 1973, Volume 5, Number 1.

The Managua, Nicaragua Earthquakes of December 23, 1972

by Christopher Rojahn

At 12:29 a.m. (local time) on Saturday, December 23, 1972, an earthquake of magnitude 6.2 occurred at a depth of about 5 kilometers beneath the center of Managua, the capital of Nicaragua. Within an hour after the main shock, two aftershocks, one of magnitude 5.0 and the other 5.2, occurred. The earthquake caused widespread damage and a multitude of deaths and injuries. Approximately 5,000 of the 400,000 residents of Managua were killed while 20,000 were injured and some 250,000 left homeless.

Damage photo
The city of Managua is located on the southern shore of Lake Managua near the western coast of Nicaragua. Situated within an active volcanic zone - the Central American Volcanic Chain - the city has been struck by a series of moderate earthquakes in recent history (1844, 1858, 1881, 1898, 1913, 1918, 1928, 1931, and 1968). Both the volcanic and seismic activity are believed to be related to the relative movements of two crustal plates intersecting near the southwestern border of Central America. Modern global tectonic theory postulates that the Cocos plate, located east of the East Pacific Rise, is moving northeastward and is slowly being driven beneath the Caribbean plate. The dipping zone, as defined by earthquake activity, begins near the surface at the Middle American Trench, a major geologic feature 4 to 5 kilometers deep extending along the Pacific Coast from Mexico to Costa Rica, and extending downward at about an angle of 45° towards the northeast to a depth of approximately 170 kilometers. The zone lies 100 to 200 kilometers beneath the city of Managua. Because of the shallow focus of the December, 1972 events, it seems unlikely that the quakes were the simple result of crustal movement between the two plates. Rather, they were more likely caused by a relatively shallow adjustment to accumulating strain within the southwestern-most part of the Caribbean plate.

Geologic effects of the quake included surface faulting, landslides and local ground subsidence. By far the most significant of these effects was surface faulting. At least four major faults have been identified with the December 1972 events, and all consistently show left lateral motion trending in a northeasterly direction. Aftershock data indicates that at last one of the faults extends from the surface to a depth of 8 to 10 kilometers and that faulting extends at least 6 kilometers northeast of the city beneath Managua.

A preliminary summary of the extent of damage indicated that earthquakes seriously affected an area of 27 square kilometers and destroyed 13 square kilometers in the heart of the city.

At the time of the earthquake, Managua had one 17-story building, one 15-story building, approximately 5 buildings in the 7- to 9-story range, several dozen buildings in the 3- to 6-story range and a multitude of 1 and 2 story buildings. Most of these buildings sustained significant structural damage. Although a number of buildings were damaged by permanent ground displacement, most of the building damage was caused by strong seismic ground shaking (10-15 seconds during the main shock). Because all the fire-fighting equipment had been demolished, fires raged out of control in the downtown area for several days. Some 750 school rooms were affected by the earthquake, all four main hospitals (1650 beds) were rendered unserviceable, and 53,000 units of family housing (mostly low and middle income) were lost or seriously damaged.

The extensive damage to low and middle include housing units, small shops, and factories can be attributed to the type of construction. Most of these structures were of taquezal construction and, in many cases, at least 40 years old. Taquezal construction means walls composed of wood post and beam construction using horizontal wood strips with adobe or stone fill and plaster on both the interior and exterior face. Typically, the roofs of such structures are of unanchored curved clay tile. Because of termite damage and other erosive processes, the older taquezal buildings were damaged more severely than newer ones.

Both the water and electrical power distribution systems were severely impaired. A large number of breaks occurred in the water mains and distribution pipes because of ground disturbance. One week after the earthquake only 10% of the population had water service. Similarly, the electrical distribution system in the downtown area was severely damaged. Overall damage to the power plant in the eastern part of the city, however, was light.

The factors most influential in causing the extensive destruction were: strong seismic shaking (depth of focus of the seismic event was shallow); permanent ground displacement along at least four major faults; and non-earthquake resistant characteristics of many of the structures (Taquezal construction was vulnerable).

Abridged from Earthquake Information Bulletin, September-October 1973, Volume 5, Number 5.